A dichroic mirror for reflecting red light for optical separating and a dichroic mirror for reflecting red light for optical synthesizing are installed on the same plane in an image generating apparatus of the present invention. In addition, a dichroic mirror for reflecting blue light for optical separating and a dichroic mirror for reflecting blue light for optical synthesizing are installed on the same plane. Accordingly, a compact configuration can be realized. Since a main reflecting mirror is a rotating elliptic mirror and a lamp is located on a focus or around the focus, a lighting equipment which irradiates conic luminous flux can be realized.

1. Field of the Invention
The present invention relates to a color image generation apparatus such as a screen projector and a projection television. The present invention also relates to parts, such as a lighting equipment, an optical mixer and a dichroic filter, used in the color image generation apparatus.
An image generation apparatus, wherein an optical modulation element such as a liquid crystal panel is used, can be classified as follows.
(a) direct-view type display
An image formed by an optical modulation effect of the liquid crystal panel and a polarization film is viewed directly without being magnified. Display of this type is used for watches, notebook type personal computers and various electric appliances.
(b) projecting type display
An image formed by projecting light on the liquid crystal panel and a polarization element, which have an optical modulation effect, is projected on a screen with being magnified. Depending upon the number of the liquid crystal panels, there are two methods in the projecting type display. One is a single-panel-method having only one panel and the other is a plural-panels-method having more than two or three panels.
The present invention relates to the plural-panels-method of the projecting type display. The present invention also relates to a lighting equipment for irradiating the liquid crystal panel. Particularly, the present invention relates to an image generation apparatus wherein a basis that human visual characteristic has a high resolution for luminance (bright or dark) and a low resolution for color is applied.
2. Description of the Related Art
Related Art 1.
FIGS. 121 and 122 show configurations of optical systems of conventional liquid crystal projecting apparatus in which the single-panel-method is applied. In this single-panel-method, one liquid crystal panel having a polarization film is used as optical modulation means.
There are two methods of projecting: one is a rear projection and the other is a front projection. FIG. 121 shows the rear projection method. FIG. 122 shows the front projection method. (Reference: "Everything of Liquid Crystal Display" by Akio Sasaki and Shouhei Naemura, Kougyou Cyousakai Company, Apr. 22, 1994 p. 210)
In FIGS. 121 and 122, a lamp (light source) 10, a main reflecting mirror 11, a condenser lens 12, a light source system 1 including the lamp 10 and the main reflecting mirror 11, a color filter CF formed like mosaic on a liquid crystal panel (LCD: Liquid Crystal Display) 300, a projection lens 50 for magnifying and projecting an image of the liquid crystal panel passed through the color filter CF onto a screen 60, are shown.
Although display luminance is lessened to one-third by the color filter CF, the color filter is utilized because of its simplicity.
Related Art 2.
FIG. 123 shows a three-panels-method which is currently used. The present invention relates to the three-panels-method. FIG. 123 shows a configuration of optical system in a conventional liquid crystal projecting apparatus wherein three liquid crystal panels are used. The three liquid crystal panels of three primary colors, red (R), green (G) and blue (B), are used as the optical modulation means in order to avoid luminance reduction caused by the color filter in the single-panel-method.
The followings are shown in FIG. 123. The lamp 10, the main reflecting mirror 11, the light source system 1 including the lamp 10 and the main reflecting mirror 11, and a mirror 2 for spectral-separation and reflection are provided. In addition, a first dichroic mirror 21 which transmits blue components and reflects red components and green components, in the white light of the lamp 10 is shown. A second dichroic mirror 22 reflects only the green components in the red light and the green light which have passed through the first dichroic mirror 21. Then the second dichroic mirror 22 supplies the green components to a liquid crystal panel 31 for green. Reflecting mirrors 23 and 24 supply the red light and blue light spectral-separated in the first and second dichroic mirrors, to a liquid crystal panel 30 for red and a liquid crystal panel 32 for blue. A third dichroic mirror 40f and a fourth dichroic mirror 41f synthesize light passed through one of liquid crystal panels 30, 31 and 32 for red, green and blue. The projection lens 50 magnifies and projects an image synthesized in the third dichroic mirror 40f onto the screen 60. The liquid crystal panels 30, 31 and 32 for red, green and blue are located at places whose optical path lengths from the lamp 10 are equal and optical path lengths to the projection lens 50 are also equal. Namely, optical path lengths from the lamp 10 to the projection lens 50 through each of the liquid crystal panels 30, 31 and 32 are equal.
Liquid crystal panels having seventy thousand pixels through three hundred thousand pixels are used for red, green and blue in the liquid crystal projector of the prevailing three-panels-method.
Operation of the liquid crystal projector of the three-panels-method will now be explained.
The light source system 1 is composed of the lamp 10 and the main reflecting mirror 11. A subsidiary reflecting mirror, not shown in FIG. 123, is sometimes provided for returning light irradiated forward, to the main reflecting mirror. This is a reason for naming the mirror 11 the main reflecting mirror. Almost parallel white output light 100 is output from the light source system 1.
A white light source, such as a metal halide lamp, xenon lamp and halogen lamp, is used as the lamp 10. A reflecting surface of the main reflecting mirror 11 is used as light collecting means. A rotating parabolic surface is usually used for the reflecting surface of the main reflecting mirror 11. By locating a center of emission of the lamp 10 at about a focal point of the rotating parabolic surface, almost parallel output light 100 can be obtained.
The output light 100 is separated into three monochromatic lights by the first dichroic mirror 21 and the second dichroic mirror 22. The first dichroic mirror 21 transmits the blue light and reflects the green light and the red light. The second dichroic mirror 22 reflects the green light and transmits the red light. The three monochromatic lights are a red monochromatic light 100R, a green monochromatic light 100G and a blue monochromatic light 100B. An optical path of the red monochromatic light 100R is turned at the reflecting mirror 24 and input into the liquid crystal panel 30 for red. An optical path of the blue monochromatic light 100B is turned at the reflecting mirror 23 and input into the liquid crystal panel 32 for blue. An optical path of the green monochromatic light 100G is turned at the second dichroic mirror 22 and input into the liquid crystal panel 31 for green.
Each of the liquid crystal panels 30, 31 and 32 generates a monochromatic image corresponding to each of R signal, G signal and B signal. The R, G and B signals are monochromatic picture signals decoded from color signals in picture signals. Each of monochromatic lights, red, green and blue input into the liquid crystal panels 30, 31 and 32 is controlled its strength of the light by passing through the liquid crystal panels 30, 31 and 32. Namely, each of monochromatic lights is modulated optically.
Then, the light optically modulated is synthesized to luminous flux again at the third and fourth dichroic mirrors 40f and 41f. The third and fourth dichroic mirrors 40f and 41f are means for synthesizing each monochromatic light. The synthesized luminous flux is input into the projection lens 50 and magnified by specific magnification. Then, the magnified luminous flux is projected onto the screen 60.
Compared with the single-panel-method in which three primary colors are realized by using one liquid crystal panel, the three-panels-method wherein three plates (liquid crystal panels) are used has an advantage that high precise picture quality can be obtained.
The followings are some disadvantages of the three-panels-method.
(1) configuration is complicated PA1 (2) a large number of components are indispensable (ex. lens, dichroic mirror) PA1 (3) accuracy of optical axis should be enhanced PA1 (a) a wavelength separating module PA1 (b) a first, a second and a third optical modulators for respectively modulating the three lights output from the wavelength separating module, and for respectively outputting the three lights modulated by the first, the second and the third optical modulators; PA1 (c) a wavelength synthesizing module PA1 (d) a first optical path connecting module for setting a first optical path between the wavelength separating module and the wavelength synthesizing module. PA1 (e) a polarization separating module for inputting a light, separating the light into a first light and a second light, whose polarization directions are different, and outputting the first light to the wavelength separating module; PA1 (f) a fourth optical modulator for inputting the second light and an image signal, modulating polarization state of the input second light based on the image signal, and outputting a modulated light made by modulating the polarization state of the input second light based on the image signal; PA1 (g) a polarization synthesizing module for inputting the synthesized light output from the wavelength synthesizing module and the modulated light output from the fourth optical modulator, and for synthesizing the lights to generate an image; and PA1 (h) a second optical path connecting module, installed between the polarization separating module and the polarization synthesizing module, for setting a second optical path in order to lead the second light from the polarization separating module to the polarization synthesizing module through the fourth optical modulator. PA1 (a) a wavelength separating module PA1 (b) a first, a second and a third optical modulators for respectively modulating the three lights output from the wavelength separating module, and for respectively outputting the three lights modulated by the first, the second and the third optical modulators; PA1 (c) a wavelength synthesizing module PA1 (d) at least one collective lens, provided between the wavelength separating module and the wavelength synthesizing module; and PA1 (e) a first optical path connecting module for setting an optical path including the at least one collective lens, between the wavelength separating module and the wavelength synthesizing module. PA1 (f) a polarization separating module for inputting a light, separating the light into a first light and a second light, whose polarization directions are different, and outputting the first light to the wavelength separating module; PA1 (g) a fourth optical modulator for inputting the second light and an image signal, modulating polarization state of the input second light based on the image signal, and outputting a modulated light made by modulating the polarization state of the input second light based on the image signal; PA1 (h) a polarization synthesizing module for inputting the synthesized light output from the wavelength synthesizing module and the modulated light output from the fourth optical modulator, and for synthesizing the lights to generate an image; and PA1 (i) a second optical path connecting module, installed between the polarization separating module and the polarization synthesizing module, for setting a second optical path in order to lead the second light from the polarization separating module to the polarization synthesizing module through the fourth optical modulator. PA1 (a) a first, a second and a third optical modulators for displaying color; PA1 (b) a wavelength separating module, installed at light-input-side of the first, the second and the third optical modulators, for inputting a light, separating the light to three lights of a first light, a second light, and a third light, and outputting the three lights to the first, the second and the third optical modulators; PA1 (c) a wavelength synthesizing module, installed at light-output-side of the first, the second and the third optical modulators, for inputting the three lights from the first, the second and the third optical modulators, for synthesizing the three lights to generate a synthesized light as the image, and for outputting the image; and PA1 (d) a first optical path connecting module including a reflecting mirror, for setting optical paths between the wavelength separating module and the first, the second and the third optical modulators, and between the wavelength synthesizing module and the first, the second and the third optical modulators. PA1 (e) a polarization separating module for inputting a light, separating the light into a first light and a second light, whose polarization directions are different, and outputting the first light to the wavelength separating module; PA1 (f) a fourth optical modulator for inputting the second light and an image signal, modulating polarization state of the input second light based on the image signal, and outputting a modulated light made by modulating the polarization state of the input second light based on the image signal; PA1 (g) a polarization synthesizing module for inputting the synthesized light output from the wavelength synthesizing module and the modulated light output from the fourth optical modulator, and for synthesizing the lights to generate an image; and PA1 (h) a second optical path connecting module, installed between the polarization separating module and the polarization synthesizing module, for setting a second optical path in order to lead the second light from the polarization separating module to the polarization synthesizing module through the fourth optical modulator, PA1 (a) a first filter for reflecting a light having a first wavelength; PA1 (b) a second filter, crossing the first filter, for reflecting a light having a second wavelength; and PA1 (c) a third filter, diagonally crossing both the first filter and the second filter, for reflecting a light having a specific polarization-direction and transmitting a light having other polarization-direction. PA1 (a) a wavelength separating module PA1 (b) a first, a second and a third optical modulators for respectively modulating the three lights output from the wavelength separating module and for respectively outputting the three lights modulated by the first, the second and the third optical modulators; PA1 (c) a wavelength synthesizing module PA1 (d) an optical path connecting module for setting an optical path between the wavelength separating module and the wavelength synthesizing module.
Related Art 3.
A configuration in FIG. 124 is devised for improving the disadvantages of FIG. 123. The main optical system is composed of the lamp 10 (metal halide lamp), a condenser lens 12, dichroic mirrors 36 and 37 (since crossly located, commonly called a cross dichro), three liquid crystal panels 30, 31 and 32 for red, green and blue, a dichroic prism 45, the projection lens 50, field lenses 12a and so forth.
FIGS. 125 and 126 show configurations of the cross dichro. The dichroic mirrors 36 and 37 are jointed crossly at joints 36t and 37t. One side of the dichroic mirror 36 is coated with multilayer thin film element 36m and that of the dichroic mirror 37 is with multilayer thin film element 37m. The multilayer thin film elements 36m and 37m are for separating or synthesizing the light. Since it is possible that components for separating or synthesizing the light become one module by using the cross dichro and the dichroic prism 45, the configuration in FIG. 124 is simpler than that of FIG. 123. Namely, comparing with the configuration of FIG. 123, the number of the components is less and the accuracy of the optical axis can be higher in FIG. 124.
Now, operation of the liquid crystal projector shown in FIG. 124 will be explained.
The output light 100 from the metal halide lamp used as the lamp 10 is separated into the monochromatic lights 100R, 100G and 100B by the cross dichro made of the dichroic mirrors 36 and 37. The lights 100R, 100G and 100B are monochromatic lights of the three primary colors R, G and B. The lights of R and B are turned 90 degrees at reflecting mirrors 23, 24, 25 and 26 and sent to the dichroic prism 45. On the other hand, the light of G is sent to the dichroic prism 45 directly passing an optical path shorter than that of R or B, without passing any reflecting mirror. The three liquid crystal panels 30, 31 and 32 for displaying each color image of R, G and B are installed at entrances of the dichroic prism 45. The light of R, G or B is input into each of the three liquid crystal panels 30, 31 and 32. The lights of R, G and B are modulated and transmitted respectively by one of the three liquid crystal panels, and are synthesized at the dichroic prism 45. Then the synthesized light is magnified and projected onto the screen by the projection lens 50.
A disadvantage of optical transmission systems in the Related Arts 2 and 3 is that the dichroic prism, which is expensive, is necessary for optical synthesizing instead of the cross dichro when the distance between the liquid crystal panel and the dichroic prism is short. The reason for this is that if the cross dichro is used for optical synthesizing instead of the dichroic prism, joints 36t and 37t of the cross dichro are projected on the screen.
A technique for solving the above problem by soaking the cross dichro in liquid is disclosed in Unexamined Japanese Patent Publications 1-214801 and 5-241018.
In addition, the optical path lengths of red and blue between the cross dichro and the dichroic prism are longer than that of green. Since there is inconsistency in the optical path lengths, it is difficult to execute various optical systematic adjustments. This is also the problem of the optical transmission system of the Related Art 3 shown in FIG. 124.
In the optical transmission system of the Related Art 2 shown in FIG. 123, the optical path lengths are equal. Although the optical transmission system of the Related Art 3 shown in FIG. 124 has an advantage of simplifying the configuration by using a small number of components, it has the disadvantage that its optical path lengths are not equal.
Related Art 4.
FIG. 127 shows a perspective illustration of one example of a conventional liquid crystal color projecting apparatus disclosed in Unexamined Japanese Patent Publication 6-281881.
The light source system 1 including the main reflecting mirror 11, an optical separating/synthesizing apparatus 81, a liquid crystal light valve 51 including a polarizing plate, and a projecting optical system 19 are provided in a liquid crystal color projecting apparatus 80. The optical separating/synthesizing apparatus 81 separates light from the light source system 1 into three primary colors and synthesizes the light serially.
The optical separating/synthesizing apparatus 81 includes the followings. Color-separation optical means 82 separates the light from the light source system 1 into three primary color lights of red, green and blue. Each of optical path opening/closing optical means 83, 84 and 85 respectively transmits or shuts down the primary color light separated by the color-separation optical means 82, in order of red light, green light and blue light at a desired timing.
Color-synthesis optical means 86 serially synthesizes the primary color lights of red, green and blue transmitted through the optical path opening/closing optical means 83, 84 and 85 in order of red, green and blue. Right-angle prisms 87, 88 and 89 lead the primary color lights transmitted through the color-separation optical means 82, to the optical path opening/closing optical means 83, 84 and 85 as a light leading system.
The color-separation optical means 82 is located on the color-synthesis optical means 86. The light source system 1 is located near a light input surface 82a of the color-separation optical means 82, facing the light input surface 82a. The liquid crystal light valve 51 is located near a light output surface 86a of the color-synthesis optical means 86, facing the light output surface 86a. Namely, the liquid crystal light valve 51 is located under the light source system 1.
The color-separation optical means 82 and the color-synthesis optical means 86 have identical configurations. In the above configurations, a dichroic prism wherein a dichroic multilayer film 892 for reflecting red and a dichroic multilayer film 893 for reflecting blue are crossed, is used. Each optical path opening/closing optical means 83, 84 or 85 is located facing each of three surfaces, one of which, denoted by 86b, faces the light output surface 86a and two of which cross the surfaces 86a and 86b orthogonally. The optical path opening/closing optical means 83, 84 and 85 transmit or shut down the red light, green light and blue light separated by the color-separation optical means 82, by being controlled on or off in turn at the desired timing. The red light, green light and the blue light phase-converted to light of S polarization are input into the color-synthesis optical means 86 again serially in order of red light, green light and blue light.
The right-angle prisms 87, 88 and 89 are used as the light leading optical system. Bottoms of the right-angle prisms 87, 88 and 89 are located on three sides of the color-separation optical means 82 and the color-synthesis optical means 86, excepting the side which faces the light source. Accordingly, each slope of the right-angle prisms 87, 88 and 89 slopes 45 degrees with respect to the surface facing the color-separation optical means 82 and the color-synthesis optical means 86. The red light, green light and the blue light output from the color-separation optical means 82 hit the upper slopes 1104 of the right-angle prisms 87, 88, 89, and are reflected totally, downward vertically. Then, the reflected lights hit the lower slopes 105 and are reflected totally and horizontally. The reflected lights are input into each of the optical path opening/closing optical means 83, 84 and 85.
Similar configurations to the configuration of FIG. 127 are disclosed in Unexamined Japanese Patent Publications 1-314289, 3-245136, 6-242414, 5-244616 and "46.4: Avionics Color Display Using LCD Projection Technology" by J. A. Fowler and R. Blanchard, Hughes Aircraft Co., Carlsbad, Calif. 912 SID 92 DIGEST.
Related Art 5.
FIG. 128 shows a conventional liquid crystal projector disclosed in Unexamined Japanese Patent Publication 6-214208.
The liquid crystal projector has two-layers structure of optical separation system and optical synthesis system. In the liquid crystal projector, the number of installed dichroic filters is reduced by extending two dichroic filters of the above two optical systems to the both layers and unifying them. The two layers, first layer and second layer, are arranged top and bottom. The two dichroic filters 20 and 21 are arranged parallel or orthogonally by being extended from the first layer to the second layer. An optical separation system 120 separates light from the light source 1 into plural color lights in order. An optical synthesis system 160 synthesizes each color image. The optical separation system 120 is provided in the first layer and the optical synthesis system 160 are provided in the second layer. Liquid crystal display panels 13, 14 and 15 which display images corresponding to each color and a projection lens 18 located overlapping the light source are provided in the second layer. The reflecting mirrors 23, 24 and 25 reflecting optical path of each color, from the first layer to the second layer are also provided.
Characteristics and specifications of LCD used in the stated Related Arts will now be described with reference to FIG. 129.
In the Related Art, a multi color image of diagonally 38 inches can be displayed in a cabinet of 650 mm deep by applying the rear projection method using three reflecting mirrors. Panel size of ferroelectricity LCD for Black/White (B/W) light shutter is 3.34 inches diagonally. Electrode dot structure is 2000.times.2000. Resolution of a projected image is 3 lines/mm. Maximum luminance 40 ft-L, maximum contrast 100:1 can be obtained by using a metal halide lamp of 250 W. (Reference: "Color Liquid Crystal Display" by Shunsuke Kobayashi, Sangyo Tosho Company, Dec. 14, 1990, pp. 111-113)
Related Art 6.
The liquid crystal panel used for optical modulation will now be described.
FIGS. 130 and 131 illustrate operations of a liquid crystal panel of 90 degrees twisted nematic (TN). (Reference: "Color Liquid Crystal Display" by Shunsuke Kobayashi, Sangyo Tosho Company, Dec. 14, 1990, p. 1)
FIG. 130 illustrates how a polarization light of input light, in the rubbing direction P of a polarizing plate (or polarizer), is transmitted when applied voltage of the liquid crystal panel is off (=0). FIG. 130 also illustrates how the polarization light of the input light passes through the polarizing plate (or polarizer, analyzer) at the output side, whose rubbing direction A is twisted 90 degrees with respect to the rubbing direction P. On the other hand, FIG. 131 shows that the input light is blocked by the polarizing plate (or polarizer, analyzer) at the output side when the applied voltage of the liquid crystal panel is on. This explanation for TN describes one example of controlling a polarization direction with keeping the state of direct polarizing. There are simple nematic, vertical array nematic, super twisted nematic (STN) and so forth in the nematic liquid crystal.
Polarizer is needed for obtaining a polarization light necessary as an incident light of TN type liquid crystal. A polarization film is most generally used as the polarizer. However, since the polarization film absorbs a wave P or a wave S from the light source, quantity of light used effectively for image forming of optical modulation means of the liquid crystal is less than 50%. Namely, there is the problem that it is difficult to obtain a bright image. In addition, there is also the problem that the polarizer of the polarization film easily deteriorates since absorbed light by the polarizer is converted to heat and makes temperature of the polarization film increase. Concretely, the polarizer can be tolerant of only one million lux though the liquid crystal panel is tolerant of five million lux. It is desirable to irradiate light up to the limit of the liquid crystal panel in order to improve image quality. However, heat resistance performance of the polarization film prevents from irradiating the light up to the limit of the liquid crystal panel.
So as to solve the above problems, a technique of cooling the liquid crystal panel by soaking it in liquid is disclosed in Unexamined Japanese Patent Publications 4-31847 and 6-118371.
Related Art 7.
A polymer dispersed liquid crystal panel does not need the polarizer having the above problems. In the method of the polymer dispersed liquid crystal panel, a plurality of sphere small drops of the nematic liquid crystal are dispersed into polymer. An array of liquid crystal molecule in each of the sphere small drops is changed depending upon an electric field. Then, a refractive index change caused by the array change is applied. When applied voltage in the electric field is off, optic axes of the liquid crystal are irregularly directed as shown in FIG. 132, and transmitted light is irregularly reflected to be opaque white. When the applied voltage in the electric field is on, the optic axes of the small drop are arranged in the direction of the electric field as shown in FIG. 133. Then, the refractive index of the light is almost coincident with the refractive index of the polymer, which makes the dispersion decrease and the transmitted light becomes almost transparent. (Reference: "Everything of Liquid Crystal Display" by Akio Sasaki and Shouhei Naemura, Kougyou Chousakai Company, Apr. 22, 1994 p. 32)
When degree of parallelization of a projected light is high, a remarkable contrasted modulated light can be obtained in the configuration of FIG. 134. (Reference "Liquid Crystal Display Technique in Next Generation" by Tatsuo Uchida, Kogyou Chousakai Company, Nov. 1, 1994, p 229)
Related Art 8.
In order to overcome the problem of the polarizer absorbing the light from the light source more than 50% and converting it to the heat, a polarization converting element shown in FIG. 135 has been considered. Light from the light source is separated into the wave P and the wave S by a polarization beam splitter 952. The waves P and S are direct polarization light and polarization surfaces of the waves P and S go orthogonally. Polarization direction of the wave S is turned 90 degrees at a 1/2 wavelength plate 953, and so all the waves become the wave P. A disadvantage of the configuration is that a section shape of the optical path after being polarized becomes a rectangle whose ratio of length to breadth is 1:2 as shown in FIG. 135. Therefore, it sometimes has difficulty in improving light utilization efficiency.
Related Art 9.
Now, the light source will be described. It is necessary to have enough brightness in the projecting type liquid crystal projector. In order to realize the brightness, a bright light source is needed.
Requiring the enough brightness of the light source indicates that the light source size becomes large. According to Boltzman law and Wien law, it is impossible to downsize the light source size below a specific amount. Life time of the light source and color temperature are also factors of not allowing the downsizing of the light source. In addition, it is inevitably necessary for the light source to have some length when long life time of the light source is required.
FIG. 136 shows characteristics of various light sources. When long life time of thousands of hours is required in the case of metal halide lamp having adequate temperature, an arc length decided by an electrode distance of discharging, that is the light source length, is 5 mm. FIG. 137 shows space distribution of emission luminance which is 250 W and whose arc length is 5 mm.
Assuming that the brightest emission light is 1.0 in FIG. 137, values less than 1.0 are weighted based on 1.0 as a standard.
FIG. 138 shows a metal halide lamp of 250 W used in a conventional liquid crystal projector.
The metal halide lamp is composed of the lamp 10 and the main reflecting mirror 11. The main reflecting mirror 11 is a rotating parabolic mirror, for instance. The metal halide lamp includes a filter 149 for blocking light in a band wherein unnecessary heat for the optical system, such as infrared radiation, is generated. The infrared radiation heats the main reflecting mirror. In order to treat (cool) the heat, it is necessary for the main reflecting mirror to be a specific size. A diameter of the lamp shown in FIG. 138 is 80 mm (8 cm).
FIG. 139 shows an example of the light source system used in a conventional liquid crystal projecting apparatus. The lamp 10, the main reflecting mirror 11 and a subsidiary reflecting mirror 16 made of the rotating parabolic mirror having the same focus with the main reflecting mirror 11, are provided in the liquid crystal projecting apparatus. By installing the subsidiary reflecting mirror 16, luminous flux dispersed forward, which has not been treated in the case of the subsidiary reflecting mirror being not provided, can be effectively utilized.
Related Art 10.
The dichroic mirror will now be described. The dichroic mirror is a glass plate whose surface is coated with a multilayer thin film. Owing to an optical interference function of the thin film, some light whose wavelength is more than a fixed value, in the light input at a specific incident angle, is reflected and other light is transmitted. Namely, the dichroic mirror has a function of optical filter. The dichroic mirror is sometimes called a dichroic filter.
The function is greatly various depending upon the incident angle of input light, which is a problem. FIGS. 140 and 141 show examples of characteristic of the dichroic mirror being dependent upon the incident angle. FIG. 140 shows the characteristic of green being dependent upon the incident angle, by the quadrature axis of wavelength of the input light and the vertical axis of transmittance of the wave P. FIG. 141 shows the characteristic of red being dependent upon the incident angle, by the above-mentioned quadrature axis and the vertical axis. When the incident angle changes .+-.5 degrees, the characteristic changes largely.
A center value of the incident angle can be freely changed to some extent depending upon the configuration of the thin film. However, a filter function decline caused by large dispersion of the incident angle depends upon substance of the thin film configuration, which can not be improved. This problem is characteristic and substantial for not only the dichroic mirror but also an element wherein the multilayer thin film is used.
The polarization beam splitter is also the element wherein the multilayer thin film is used. The polarization beam splitter has a function of polarization separation realized by the thin film effect. Accordingly, a separation characteristic depends upon the incident angle.
FIGS. 142, 143 and 144 show the characteristic of the polarization beam splitter, that is being dependent upon the incident angle, by the quadrature axis of wavelength of the incident light and the vertical axis of transmittance of the waves P and S.
In FIGS. 142, 143 and 144, TP denotes the transmittance of the wave P and TS denotes that of the wave S. TH indicates an average of TP and TS.
FIG. 142 shows the case of the incident angle being 45 degrees .+-.0 degrees. FIG. 143 shows the case of the incident angle being 45 degrees-2 degrees=43 degrees. FIG. 144 shows the case of the incident angle being 45 degrees+4 degrees=49 degrees. Similar to the dichroic mirror, the characteristic of the polarization beam splitter considerably changes depending upon the incident angle change of around .+-.5 degrees.
Compared with the element wherein the multilayer thin film is used, the characteristic depending upon the incident angle is not so conspicuous in the TN type liquid crystal panel because the TN type liquid crystal panel is not the element made of multilayer thin film. However, it is necessary to take the following two phenomena (a) and (b) into consideration.
(a) Phenomenon caused by product of refractive index anisotropy .DELTA.n of liquid crystal and thickness d of liquid crystal
The phenomenon is dependent upon the product .DELTA.n.times.d. The product .DELTA.n.times.d is made by multiplying refractive index anisotropy An of liquid crystal and the depth d of liquid crystal. (Reference: "Everything of Liquid Crystal Display" by Akio Sasaki and Shouhei Naemura, Kougyou Cyousakai Company, Apr. 22, 1994 p. 143) As shown in FIG. 145, the characteristic changes depending upon the incident angle (angle of view). The characteristic depending upon the incident angle changes based on a direction of the incident angle. FIG. 146 shows one example of this characteristic. FIG. 146 shows a luminance characteristic, being dependent upon the angle of view, of a liquid crystal panel of 1.3 inches. Each circle indicates the angle of view being 10 degrees, and the case of the luminance being various depending upon the direction of the incident angle is shown. In FIG. 146, the smaller the value is, the higher the luminance is. These values are measured in the case that the rubbing directions of the liquid crystal panel are up-and-down and right-and-left. In FIG. 146 case, for instance, the phenomenon of the characteristic being dependent upon the incident angle becomes a problem when spread of the incident angle exceeds .+-.10 to 15 degrees. (Reference: "Basis of Liquid Crystal and Display Application" by Katsumi Yoshino and Masanori Ozaki, Korona Company, 1994)
(b) Phenomenon caused by crosstalk
The phenomenon is generated when light of high luminance, the incident angle of which is more than a specific value, is input since a glass plate for enclosing the liquid crystal has some thickness. FIG. 147 shows a basis of the phenomenon. Generally, in the case such as the projecting type, when a TFT (Thin Film Transistor) for driving the liquid crystal is directly projected by the light of high luminance, dark current flows in a transistor circuit of the TFT. Consequently, a condenser which has a memory function formed in the TFT, discharges early and contrast of the liquid crystal declines. In order to prevent this phenomenon, a surface of the liquid crystal of the TFT is coated with chrome mask for cutting the light off, in the projecting type liquid crystal panel. However, as shown in FIG. 147, when the incident angle is more than a specific value, slanting light is reflected at one of glass plates for enclosing the liquid crystal, that is, at opposite side with respect to the liquid crystal. Therefore, the TFT is projected from the rear side having no chrome mask. Although reflectance of the glass is about 4%, the above phenomenon becomes a problem when light of high-density is projected on a high light-resistant panel, such as polycrystalline silicon (poly. Si) liquid crystal panel. It is necessary to take measures of antireflection for the side which reflects the problematic light.
The polymer dispersed liquid crystal will now be described.
Generally, it is possible to make the light whose solid angle is corresponding with angular dispersion .+-.d.theta. of light slanting by providing a convex lens and an adequate aperture diaphragm on an optical path wherein light is collimated.
If the polymer dispersed liquid crystal is provided between the lens and the light source, all the parallel beam whose maximum dispersion is .+-.d.theta. can be transmitted. However, in light dispersed at the polymer dispersed liquid crystal panel, only the light, which is in the solid angle of d.theta. in dispersion solid angle .OMEGA., can be transmitted. Accordingly, contrast of the dispersion type liquid crystal is expressed in the following expression. ##EQU1##
On the other hand, d.theta. can be expressed in the following expression if contrast of value Co is required. EQU d.theta.=(.OMEGA./.pi./Co).sup.1/2
Supposing that Co is 100, d.theta. can be expressed as follows. EQU d.theta.=3.24.OMEGA..sup.1/2 degrees
Even if the dispersion type liquid crystal shows an ideal characteristic, a permissible value of the dispersion is 8 degrees.
Since the dispersion solid angle .OMEGA. is actually smaller, the polymer dispersed liquid crystal allows only a dispersion angle value which is close to the permissible value of dispersion angle of an element in which thin film is used, such as the cross dichro and the polarization beam splitter.
Because of the above reason, the following has been considered to be necessary in a configuration of the optical transmission system, which mainly includes the main reflecting mirror 11, the condenser lens 12 and the projection lens 50 shown in FIG. 122 for instance, in the conventional projecting type liquid crystal projector. It has been considered to be necessary to make the optical transmission system close to a telecentric system using the main reflecting mirror as a rotating parabolic mirror and the condenser lens as a long focus lens, and to make the light be parallel light as much as possible.
Even the telecentric system has some problems. Although the light irradiated from the part at the focus of the main reflecting mirror 11 in FIG. 138 become parallel, other light irradiated from the other part is slanting and not parallel. The maximum value d.theta. of the slant is in proportion to an arc length is located at the optical axis, and in inverse proportion to a diameter Dm of the main reflecting mirror and aspect ratio asr of the main reflecting mirror. The above can be easily proved and explained with reference to FIGS. 148 to 150 and mathematical expressions in FIGS. 161 to 172.
As long as the arc length ls is finite, it is necessary even for the telecentric system to make the diameter Dm of the main reflecting mirror comparatively large in order to control the maximum value d.theta. of the slant to be within a tolerance of the thin film element. Supposing that the tolerance of the thin film element such as a dichroic mirror and polarization separation element is 6 degrees, the value of the diameter of the main reflecting mirror can not be less than 7.5 cm (3 inches) on condition that the light source whose arc length is 5 mm and the conventional main reflecting mirror whose aspect ratio is less than 2 is used. The aspect ratio is a value expressed by the diameter Dm of the main reflecting mirror/length Lm of the main reflecting mirror. In the configuration of the conventional lamp as shown in FIG. 138, a front end of the main reflecting mirror is located forward with respect to the light source, which increases light collecting efficiency and then the aspect ratio is around 2.
Accordingly, each optical component in the telecentric system is large, which makes the cost high.
Especially when size of the liquid crystal panel is large, there is a possibility of not only the cost of the TFT but also the cost of peripheral equipment becoming rather high. Regarding the material of the TFT, it is ideal to use polycrystalline silicon (poly. Si) having high mobility. However, it is necessary to use quartz glass for the poly. Si because of high temperature treatment. The cost of the quartz glass is much higher than that of other material. Therefore, amorphous silicon (a-Si), which does not need to use the quartz glass, is currently used in a large size liquid crystal panel. The size of the liquid crystal panel used for projecting is mainly 3 inches (7.62 cm) or 3.26 inches (8.28 cm), for instance.
FIG. 151 shows a relation between the light source and the liquid crystal panel in the case that the light source of the telecentric system shown in FIG. 138 is applied in the liquid crystal panel of 3 inches or 3.26 inches or in a filter system of the same size. The main reflecting mirror, whose diameter is 8 cm or 9 cm, projects parallel light on a panel of 7.62 cm to 8.28 cm with an angular dispersion of .+-. around 8 degrees.
Now, the optical transmission system of a projecting apparatus wherein the liquid crystal panel whose poly. Si is small to be around 1.3 inches is used, will be described. In this case, it is also necessary for the diameter of the main reflecting mirror to be around 3 inches, based on the efficiency of a thin film element in use. Therefore, as shown in FIG. 152, only a part of the parallel light from the light source is projected on the liquid crystal panel, which brings about a decline of the luminance on the screen. However, comparing with the case of the amorphous silicon of 3 inches, it is possible to prevent the luminance decline to some extent in the case of the poly. Si by increasing an aperture efficiency more than twice, because of a high mobility of the poly. Si. Since an optical path diameter and an optical path length can be small, the apparatus can be compact. In this respect, the apparatus of the case can be a meritorious product. In proportion to downsizing and decreasing weight of a telecommunication apparatus caused by progress of semi-conductor technique, it is required for the projector as an information display apparatus to be compact.
Related Art 11.
Image signals input into Cathode-Ray Tube (CRT) and the liquid crystal panel will now be described. Taking a response time characteristic of human vision into consideration, an image frame is transmitted every 30 ms in a common TV broadcast. In this case, the image signals are transmitted as forms of luminance signal Y and color difference signals U and V. The followings show relations between the signals of luminance and color difference, and values of the color signals R, G and B of red, green and blue. The color signals R, G and B can be easily reproduced by calculating the following expressions. EQU Y=0.30R+0.59G+0.11B (1) EQU U=R-Y (2) EQU V=B-Y (3)
A color television broadcast system of National Television System Committee (NTSC) is a typical example of narrowing a band of electric wave for broadcasting, by signal compression applying the above basis and a human visual characteristic for chromaticity recognition function.
The electric wave for broadcasting is sent to each house by applying a characteristic of human vision wherein a recognizable resolution of the color signal is low comparing with one of the luminance signal. In this case, bandwidths of the color difference signals U and V, in which the resolution can be low, are 1.5 MHz and 0.5 MHz, and the bandwidth of the luminance signal, in which the resolution should be high, is enhanced to be 4.5 MHz.
Coding of the image signal will now be explained. A method that the luminance signal Y and the color difference signals U and V, which compose the image signal, are respectively converted into digital codes, is called component coding. An input image signal in this coding method is called a component image signal.
It is a respect to be considered how a transmission rate (transmittable bit in a unit time, Mb/second) or a data amount of referred pixel is decreased, when the component image signals Y, U and V are transmitted or memory process is performed, by the component coding method.
Supposing that the luminance signal Y and the color difference signals U and V are digital signals in the coding method, the following three component coding methods can be expressed, for instance. The three component coding methods are different depending upon taken consideration for human visual characteristic. EQU (A) Y:U:V=4:4:4 (4) EQU (B) Y:U:V=4:2:2 (5) EQU (C) Y:U:V=4:1:1 (6)
In the above method (A), which is a basic method, all of the transmission rates (or the number of data of referred pixel) of digital data of the luminance signal Y and the color difference signals U and V are equal, and the human visual characteristic has not been considered. In the methods (B) and (C), the human visual characteristic has been considered, and the color difference signals are processed (transmitted or memory processed) in the transmission rate (or data amount of referred pixel) corresponding to 1/2, 1/4 of the transmission rate (or data amount of referred pixel) of the digital data of the luminance signal Y.
It is natural that image quality of the color difference signal declines in order of (A), (B) and (C). Since human visual characteristic is that color resolution is low for the luminance signal, the above methods (B) and (C) are practically used without any problem.
FIG. 153 shows a typical example of 4:2:2. (Reference: "Digital Circuit of Television Signal" by Etoh and Achiba, Korona Company, Sep. 25, 1989 pp. 8-10.) According to parameter of FIG. 153, the transmission rate (Mb/s) is 216 Mb/s (=8 bits.times.13.5M+8 bits.times.6.75M.times.2) in a serial transmission method, and 27 Mb/s (13.5M+6.75M.times.2) per transmission line in a parallel transmission method having nine transmission lines of eight data lines and one clock. Practically, redundant bit is needed in the above. Supposing that the redundant bit is 1 bit for data of 8 bits, the transmission rate is 243 Mb/s (=9.times.13.5M+9.times.6.75.times.2) in the serial transmission method.
In the case that the transmission lines of the luminance signal and the color difference signal are separated into two lines in parallel (the redundant bit is omitted), each of the luminance signal and the color difference signals U and V can be 108 Mb/s by using the above calculation. Then, it is supposed to add one clock transmission line of 108 Mb/s. In any case, the transmission line tolerant of the transmission rate of around 100 Mb/s is needed. The coding method of 4:2:2 shown in the above (B) is a standard recommended by General Meeting of CCIR (CONSULTIVE COMMITTEE INTERNATIONAL RADIO) used in a studio for a digital television and used for DI format of a digital VTR.
As a reference, FIG. 154 shows a proposed HDTV coding method described in the "Digital Circuit of Television Signal" (p. 10). In the case of the High Definition Television (HDTV) coding method, 148.5 Mb/s (74.25M+37.125M.times.2) is necessary in the parallel method, and 1188 Mb/s (148.5M.times.8) in the serial method.
In the case of 4:1:1 of the method (C), the luminance signal is 108 Mb/s and the color difference signals U and V are 54 Mb/s when the transmission lines are separated into two lines in parallel as the above (the redundant bit is omitted).
If the transmission rate for the color difference signal (or data amount per referred pixel) is suppressed to be lower than the component signal in the case of 4:1:1 of the method (C), the image quality naturally deteriorates. (case of 4:0.5:0.5=8:1:1, for instance)
However, there are advantages as follows. Since data amount of the image is wholly lessened, memory amount, circuit scale and consumption power, used in digital image apparatus, can be lessened. As the image data amount is lessened, specific data, such as data of one frame, can be transmitted in a short time when the same transmission rate is used. It is also possible to transmit more redundant data, character data and encoded data in a specific time.
Conventionally, taking the above advantages and the disadvantage of the image quality deterioration into consideration, 4:1:1 is actually used as a practical level coding method.
FIGS. 155, 156 and 157 show models of pixel configuration in each of the stated methods 4:4:4, 4:2:2 and 4:1:1.
There is a conventional signal processing apparatus in which the component image signals, made of the luminance signal Y and the color difference signals U and V of 4:4:4, are input and the luminance signal Y and the color difference signals U and V of 4:2:2 or 4:1:1 are output. In the signal processing apparatus, a specific filtering process, with respect to sampling frequency ratio 4:2:2 or 4:1:1, is performed for the luminance signal Y and the color difference signals U and V. It is well known that method conversion from the coding method of 4:4:4 to the coding method of 4:2:2 or 4:1:1 is performed in a signal processing block shown in FIG. 158.
Related Art 12.
Conventionally, an image source in image data is formed in proportion to a ratio of length to breadth of a final image. The liquid crystal panel used in image generation means is also formed in proportion to the ratio of the length to breadth.
The above will now be explained with reference to FIG. 159 showing one example of the conventional image generation apparatus.
A liquid crystal panel 910 has the same ratio of length to breadth as that of a desired image for generating, in the conventional image generation apparatus. Supposing that the ratio of length to breadth of the desired image for generating is 3:4, the ratio of length to breadth of physical form of the liquid crystal panel 910 is 3:4. An image signal 900 input into the liquid crystal panel 910 has the same ratio of length to breadth as that of the liquid crystal panel 910.
An image generation means 920 inputs the image signal 900 into the liquid crystal panel 910. The image generation means 920 modulates light 905 optically based on the input image signal 900 and generates an optically modulated image 930.
Since the conventional image generation apparatus is constructed as the above, it is restricted to use the liquid crystal panel corresponding to the ratio of length to breadth of an input picture source. It is also restricted to use the liquid crystal panel corresponding to an aspect ratio (ratio of length to breadth) of the picture finally seen by the user.
Because of the above restriction, since a section of the optical path of the light source is generally circular, utilization efficiency of the light source is extremely decreased in the case of a high definition television whose ratio of length to breadth is large to be 9:16, for instance. Although the utilization efficiency of a square panel is 64%, that of the case of 9:16 is 54%.
A shape, with which the panel can be taken as many as possible from one quartz panel, is required in the case of smaller sized panel, such as the TFT liquid crystal panel of the poly. Si, being used in order to reduce the cost.
Accordingly, it is ideal to decide the ratio of length to breadth of the panel by considering the above respects, not based on the image signal.
One of the objects of the present invention is to solve a problem caused by a large number of optical components and complicated optical structure. For instance, in the related art described with reference to FIG. 123, there are too many optical components and the structure is very complicated, since the reflecting mirror and the dichro, which compose the structure, have not become module.
Another object of the present invention is to solve a problem caused by the cross dichro, which is applied in order to reduce the number of components. The joints of the cross dichro are projected on the screen when the distance between at least one of the cross dichros and the panel becomes short. Accordingly, the prism cross dichro, having no joints and being expensive, is needed to be used instead of the cross dichro.
Another object of the present invention is to solve a structural problem caused by a difference between size of the lighting equipment used for the liquid crystal projector and the projection lens and size of the liquid crystal panel and the dichroic mirror. Compared with the size of the lighting equipment and the projection lens, the size of the liquid crystal panel and the dichroic mirror has been reduced. According as the size of the liquid crystal panel has been reduced from 3 inches to less than 1 inch, the size of the reflecting mirror and the dichroic mirror has also been reduced, which totally makes the apparatus compact.
On the other hand, for the purpose of irradiating enough light, it is necessary for the size of the lighting equipment to be more than some specific size. In other words, it is impossible for the lighting equipment to be less than some specific size. Then, it is also necessary to use the projection lens whose size is relatively large.
When there is a big size difference between the size of the lighting equipment and the size of the liquid crystal panel, the light is not effectively utilized. It is needed to narrow diameter of the light being projected and to keep a specific length of the optical path between the lighting equipment and the liquid crystal panel so as to effectively utilize the light.
It is very important to efficiently allocate the components having different size, to keep a necessary length of the optical path and to apply the most appropriate structure for the apparatus in order to totally make the apparatus compact.
Another object of the present invention is to solve a problem caused by a difference between length of the optical path from the lamp 10 to image generation of red or blue and length of the optical path from the lamp 10 to image generation of green. Especially, it can be the problem that illuminance distribution on the liquid crystal panels 30 and 32 for red and blue differs from the illuminance distribution on the liquid crystal panel 31 for green. The reason is that even when the main reflecting mirror 11 having a perfect rotating parabolic surface is used, the light can not be perfectly parallel because of the arc length of the lamp. Generally, the longer the distance from the light source becomes, the brighter the center becomes and the darker the outlying part becomes in the liquid crystal panel. Accordingly, the illuminance distribution difference that bright at the center and dark at the outlying part easily occurs in the liquid crystal panels for red and blue having the longest optical paths. In order to correct the above phenomenon, it is necessary to give a white balance correction signal to a picture signal circuit from the wellknown picture signal correction circuit (not shown).
However, if much correction amount is given, a gradation display faculty originally obtained will be restricted. Namely, a problem of white balance break caused by the optical length difference and a problem of gradation faculty decline of the projected image caused by correcting the white balance break, will easily occur.
Another object of the present invention is to solve a problem caused by inconsistency in the optical path lengths. In order to prevent color disparity, it is necessary to perform precise mutual convergence for each pixel among liquid crystal panels for red, green and blue in the apparatus wherein the optical path lengths are different. This procedure makes adjustment time in manufacturing process long.
Another object of the present invention is to solve a problem caused by the polarization film, which generates much heat and has low tolerance of light, being applied close to the liquid crystal panel. The tolerance level for the light of a small-sized liquid crystal depends upon the polarization film. Then, it is necessary to apply a method wherein the polarization film is not needed, in the liquid crystal display apparatus whose output luminous flux is large. The method of using material of the polymer dispersed liquid crystal or using the polarization converting element has been suggested instead of the method using the polarization film.
Another object of the present invention is to solve a problem relating to the polarization converting element. After the luminous flux from the light source, whose section is circular, passes through the polarization converting element, the circular section of the luminous flux is transformed to a rectangle whose ratio of length to breadth is 2:1. This indicates that the luminous flux is not effectively utilized because the most appropriate shape for the liquid crystal panel is a square or a rectangle whose ratio of length to breadth is at most 4:3. In order to compensate this disadvantage, the ratio of the length to breadth of the luminous flux can be changed by a cylindrical lens and then the luminous flux can be input into the thin film element. However, there is a possibility of the image quality being deteriorated by the cylindrical lens. The reason is that rates of dispersion of the light inclination in the length and in the breadth, which are important for various thin film elements installed after the cylindrical lens, are different in some luminous fluxes. This may bring about the image quality deterioration.
Another object of the present invention is to solve a problem relating to a large amount of cubic of the optical system.
The element in which the thin film is used, such as the dichroic mirror and the polarization beam splitter, has a low permissible value against the dispersion existed at the incident angle of the incident light.
It is required for gap length of the light source to be some specific length in order to keep long life time. Consequently, it is necessary for the diameter of the main reflecting mirror of the light source system to have some long length for the purpose of suppressing a dispersion value of angle of irradiated light.
According to the above reason, it is needed for the dichroic mirror and the polarization beam splitter to have some large size so as to obtain a desired characteristic. In addition, the element in which the thin film is used, has to be installed with its surface applied thin film, inclining around 45 degrees. Since the area amount relates to a cubic amount, the cubic of the optical system also becomes large.
Another object of the present invention is to solve a problem caused by shading. There is a dichroic mirror system for synthesizing, whose optical path length is long, between the liquid crystal panel and the projection lens for projecting light. Therefore, the length of the optical path between the liquid crystal panel and the projection lens is long. Then, the shading is generated in the simple telecentric system, which often makes the projected image dim.
FIG. 160 illustrates the shading. In the case of parallel light, the light can be projected on the liquid crystal panel and utilized, regardless of the optical path length. In the cases of light A and light B, it depends upon the length of optical path whether or not the light is utilized. Both lights A and B can be utilized in a short optical path A. However, only the light B is utilized and the light A is not utilized in a long optical path B. Accordingly, it is necessary to make the optical path as short as possible in order to effectively utilize the light.
Another object of the present invention is to solve a cost problem caused by using the dichroic mirror. In the front type projecting apparatus wherein distance between the projector and the screen is not fixed, it is necessary to put the dichroic mirror, being expensive, for synthesizing between the liquid crystal panel and the projection lens. On the other hand, it is not necessary to put the dichroic mirror in the rear type projecting apparatus wherein the distance between the projector and the screen is fixed.
Another object of the present invention is to solve a problem relating to size of the dichroic mirror and the liquid crystal panel. In the conventional apparatus, the size of the dichroic mirror is often a base for deciding the size of the liquid crystal panel whose cost per specific area is expensive. A permissible value for the incident light dispersion of liquid crystal element is primarily higher than that of the element, such as the dichroic mirror, in which the thin film is used. However, since the simple telecentric system has been conventionally used, the size of the liquid crystal panel corresponding to the size of the dichroic mirror is used.
Another object of the present invention is to solve a problem relating to the liquid crystal panel of poly. Si. Since the quartz glass, which is expensive, is used in the liquid crystal panel of poly. Si, it is necessary to make the area of the panel small. Compared with the panel of a --Si, the panel of poly Si can have much higher aperture efficiency per a specific area. Namely, the size of the liquid crystal panel of poly. Si can be small. It is earnestly desired for the liquid crystal panel of poly. Si to develop a method of effectively projecting the light on the small area.
Another object of the present invention is to solve a problem of the crosstalk described with reference to FIG. 147. Since the glass plate for enclosing the liquid crystal has some thickness, the crosstalk is generated when the light of high luminance, whose incident angle is more than a specific value, is input.
Another object of the present invention is to solve a problem relating to the aspect ratio. When the aspect ratio of the main reflecting mirror used in the light source system is small, dispersion of inclination of irradiated light from the main reflecting mirror has become large even in the same diameter and the same light source length. The reason of the small aspect ratio being used is that the smaller the aspect ratio is, the more the light collecting efficiency of the main reflecting mirror becomes.
Another object of the present invention is to solve a problem that the human visual characteristic has not been thoroughly utilized in the signal process, optical modulation process and image synthesizing process at the conventional liquid crystal projecting apparatus. Therefore, it has been needed to make the pixel size minute and raise the power of the white light source up in order to progress the image quality, in the conventional method. Signal assignment and operational processing adapted to the human visual characteristic, having a high resolution for luminance and a low resolution for color, have been desired. In addition, suiting the pixel size of the liquid crystal panel to the signal and simple processing of image synthesis have also been desired.
Another object of the present invention is to solve a problem relating to the ratio of length to breadth of the panel. As stated above, since the section of the optical path of the light source is generally circular, utilization efficiency of the light source is extremely decreased in the case of the high definition television whose ratio of length to breadth is large to be 9:16, for instance. Although the utilization efficiency of a square panel is 64%, that of the case of 9:16 is 54%.
In addition, the shape, with which panel can be taken as many as possible from one quartz panel, is required in the case of smaller sized panel, such as the TFT liquid crystal panel of the poly. Si, being used in order to reduce the cost.
Accordingly, it is ideal to decide the ratio of length to breadth of the panel by considering the above respects, not based on the image signal.
To sum up, the object of the present invention is to provide the projecting apparatus, which is composed of a small number of components and is compact, whose structure is adapted to the human visual characteristic, wherein a high precise projected image can be realized.

Method and apparatus for image generation


According to at least one embodiment, a system comprises an interface means for supporting a plurality of functional accesses; and means for selectively limiting on a per user basis access to certain ones of said plurality of functional accesses via the interface means.

Multi-function computer ports that support a plurality of different types of functions are available. For instance, USB (Universal Serial Bus) ports are commonly implemented in computer systems. Such USB ports support interfacing input devices (e.g., keyboards, mice, joysticks, scanners, cameras, etc.), output devices (e.g., printers, etc.), storage devices (e.g., USB mass storage drives, etc.), audio devices (e.g., MP3 players, etc.), communication devices (e.g., modems, etc.), and networking devices (e.g., Network Interface Card (NIC), etc.), as examples. For example, at one instance a user may couple an input device to a USB port for use in inputting information to the computer system, and at another instance the user may couple a storage device to the USB port for storing information from the computer system to the coupled storage device. Thus, various types of functions may be performed through such a multi-function port, such as inputting information to the computer system, outputting information from the computer system, storing information from the computer system (e.g., to a mass storage device), etc.

System and method for managing access to functions supported by a multi-function port

Hewlett-Packard Development Company, L.P.


A pulse width modulation (PWM) Regulator System with automatically switching pulse skipping mode (PSM) is disclosed. The PWM regulator system comprises a PWM regulator, a PSM switching module and a pulse generator. The PWM regulator converts the input voltage by PWM. The PSM switching module determines to enter or exit the PSM. The pulse generator adaptively produces pulse signal for the switching regulator to operate in PSM.

1. Field of the Invention
The present invention relates to a regulator system with automatically switching pulse skipping mode, and more particularly relates to a pulse width modulation regulator system with automatically switching pulse skipping mode.
2. Description of the Prior Art
In the switching mode regulator, the pulse width modulation (PWM) is extensively used in the method of voltage transformation. When the circuit is needed to provide voltage during the heavy load, the inductor current of the circuit is maintained in the Continuous Conduction Mode (CCM). And the switching speed of PWM is high enough to provide higher power density for heavy load. However, when the circuit is only needed to provide voltage during light load, the inductor current of the circuit is maintained in the Discontinuous Conduction Mode (DCM). And the switching speed of PWM is low enough to provide lower power density for light load.
In the prior art, the PWM regulator is only needed to provide low voltage for light load. In order to reduce the power switching loss, the solution of the prior art is to use pulse skipping mode (PSM) to reduce the number of times to turn on or off the switch in the circuit and achieve the power saving. The PWM regulator with PSM in the prior art, when the circuit is operated in the Continuous Conduction Mode, the output voltage will come with bigger pulse because of the PSM and the phase is not stable.
The traditional asynchronous step-down converter with PSM is shown in FIG. 1. The circuit comprises an error amplifier 100, a comparator 103, a ramp generator 106, a pulse generator with minimum-on-time 109, an OR gate 112, an activator 115, a switch SW1, a diode D1, an inductor L1 and a capacitance C1. In the traditional circuit of FIG. 1, the circuit is added a pulse generator 109 with minimum-on-time to have the pulse skipping mode and save the power. And the inductor current of the circuit can be in the Continuous Conduction Mode (CCM) or the Discontinuous Conduction Mode (DCM). Because the Continuous Conduction Mode (CCM) is not exited in the traditional circuit and the circuit will be adaptively turned off the pulse skipping mode, the switching ratio of the input voltage and the output voltage is very high in the circuit. And the output voltage will be big because of the PSM and the phase will be not stable.
FIG. 2 is a traditional Asynchronous step-up converter with pulse skipping mode (PSM). The circuit comprises an error amplifier 200, a comparator 203, a ramp generator 206, a pulse generator 209 with minimum-on-time, an OR gate 212, an activator 215, a switch SW2, a diode D2, an inductor L2, and a capacitance C2. In the traditional circuit of FIG. 2, the circuit is also added a pulse generator 209 with minimum-on-time in order to have the PSM and save the power. And the inductor current of the circuit can be Continuous Conduction Mode (CCM) or Discontinuous Conduction Mode (DCM). Because the Continuous Conduction Mode (CCM) is not exited in the traditional circuit and the circuit will be adaptively turned off the pulse skipping mode, the switching ratio of the input voltage and the output voltage is very high in the circuit. And the pulse of the output voltage will be big because of the PSM and the phase will be not stable.
Because of the reason described above, the circuit can be operated in Discontinuous Conduction Mode (DCM) to save the power because of the pulse skipping mode (PSM). However, when the circuit is operated in the Continuous Conduction Mode (CCM), the pulse of the output voltage is too big and the phase is not stable. It is necessary to provide a method or system with an operative mode to enable or disable the pulse skipping mode.

Pulse width modulation regulator system with automatically switching pulse skipping mode


A router apparatus used in an IPv6 multihome network and capable of exchanging a network prefix as route information with an adjacent router is disclosed. The router apparatus comprises a pair information generator (52) configured to associate a prefix assigned from the network with a default route to generate a pair information item, and a routing protocol processor (51) configured to distribute the pair information item of the prefix and the associated default route to the adjacent router using a routing protocol.

The present invention relates to a router apparatus used in an IPv6 multihome network, a route information distributing method implemented by the router apparatus, and a communications system including the router apparatus.
Multihome environments are structured in IP networks. In an IP multihome network, an end site, such as a local area network of a company or a university, is connected to multiple Internet service providers (ISPs). By establishing connection to multiple upstream resources, load distribution is guaranteed and the reliability is improved. An example of such a multihome technique is disclosed in RFC (Request for Comments) 2260 and RFC 3178 documented by the Internet Engineering Task Force (IETF), which is a private organization for promoting standardization of Internet techniques.
The Internet Protocol version 6 (IPv6) address has a bit length four times as large as the conventional IPv4 address, and a global IP address can be assigned to a number of machines and pieces of equipment. In IPv6, Internet service providers (ISPs) assign 48-bit prefixes to an end site, which is a site of an end user or a subscriber contracted with the Internet service providers, unlike the IPv4. The site assigns 64-bit prefixes to host apparatuses arranged in that site.
FIG. 1 is an example of the conventional IPv6 multihome network. Site 100 is connected to two Internet service providers (ISP_a and ISP_b) 110 and 120. A host apparatus 50 located in the site 100 is connected to the Internet service providers (ISP_a and ISP_b) 110 and 120 via a router (not shown).
The host 50 has the prefixes assigned by the Internet service providers (ISP_a and ISP_b) 110 and 120. The prefixes are, for example, ISP_a 2002:1000:1000::/48, and ISP_b 2002:2000:2000::/48, respectively, where "/48" represents the IPv6 prefix length. The prefix is divided every 16 bits, and expressed as a hexadecimal value using colons.
FIG. 2 shows the IPv6 address structure in the site. The IPv6 address consists of a 48-bit prefix 151 assigned from an Internet service provider (ISP), a 16-bit subnet ID 152 assigned to a LAN in the site, and a 64-bit host address 153 for identifying each host apparatus used in the LAN.
The host 50 in the site 100 has multiple IP addresses with different prefixes, as illustrated in FIG. 1. In this example, the host 50 has the following IP addresses:                FECO:1000:1000:0001:0011:2233:4455:6677,        2002:1000:1000:0001:0011:2233:4455:6677, and        2002:2000:2000:0001:0011:2233:4455:6677.The IP address starting from "FECO::" is a site local address given by a site manager, which is unique within a specific area defined as "site".
In the IPv6 multihome environment, the number of bits of the prefix assigned by an Internet service provider (ISP) to a site is fixed at 48 bits, such that address modification becomes the minimum when the site changes the connected ISP.
In many sites, each router creates a routing table using a routing protocol, such as RIPng (Routing Information Protocol next generation) defined in RFC 2080 or OSPF (Open Shortest Path First) defined in RFC 2740. All the packets addressed outside the site are transmitted collectively using a route called a "default route" having a length of 0 bits (0.0.0.0/0) because these packets are simply supplied to a gateway (GW) router, which is a connection node between the multihome site and an Internet service provider. The default route is a general-purpose route used when no route information is found.
An ordinary in-site route is generated by the adjacent router of this link, and delivered in the site. In contrast, the default route is created by a gateway provided at the connection node between the site and the ISP, and delivered to each of the routers located in the site using a routing protocol. Upon receiving a packet, each router checks the destination address. If there is a corresponding route, that is, if the destination address is in the site, then the packet is transmitted in that direction. On the other hand, if there is no corresponding route, that is, if the destination address is outside the site, then the packet is output to the default route.
In this manner, each gateway (GW) router connected to an associated ISP creates a default route for in-site routing. With the current IP routing protocol, an appropriate route is determined based on distance when multiple default routes exist. Accordingly, a packet addressed to outside of the site is supplied to the nearest gateway router located closest to the sender.
Meanwhile, RFC 2267 provides an ingress filter, which checks the source address of a packet transmitted from a site and rejects the packet if the packet does not use the address prefix given by the ISP. According to this policy, if a packet is supplied through the in-site default route to a gateway router that does not correspond to the prefix, this packet is discarded by the ingress filter at the ISP.
FIG. 3 illustrates the packet filtering. In the site 100, a gateway router a (GWa) 51, which is a connection node to ISP_a 110, and a gateway router b (GWb) 52, which is a connection node to ISP_b 120 are provided. A router Ra 53 is connected to the gateway router a (GWa) 51. A router Rb 54 is connected to the router Ra 53 and the gateway router b (GWb) 52. A host 50 is connected to the router Rb 54.
The host 50 can generate a source address using a prefix 2002:1000:1000::/48 assigned the ISP_a 110, which is referred to as address A, and another source address using a frefix 2002:2000:2000::/48 assigned by the ISP_b 120, which is referred to as address B. The host 50 can set one of the addresses A and B as the source address when transmitting a packet.
When a packet is transmitted from the host 50 to outside of the site 100, the packet is first received at the router Rb 54, and supplied to the gateway router b (GWb) 52 through the shortest default route (with distance 1 in this example). If, in this case, the prefix 2002:1000:1000::/48 assigned by ISP_a 110 is used in the source address, this packet is discarded by the ingress filter of ISP_b 120.
To avoid this problem, RFC 3178 provides a technique for preventing undesirable packet from being discarded at the ingress filter, as illustrated in FIG. 4. With this technique, negotiation is manually made between a multihome site and an ISP, and static IP tunnels are provided between the gateway router a (GWa) 51 and ISP_b 120, and between the gateway router b (GWb) 52 and ISP_a 110. The gateway routers 51 and 52 check the source address of a packet to send this packet to the correct ISP.
JP 2002-359638A discloses a router system capable of automatically setting a temporary default route upon linking up of the port when activating a router. This technique can improve the reliability of switching operations of a routing table.
The technique using the static IP tunnel defined in RFC 3178 has a problem in that a redundant route may be generated. FIG. 5 illustrates an example of such a redundant route. When a packet is transmitted from the host "a" (50) to host "b" (60) located outside the site, the packet may be delivered through the IP tunnel connecting the gateway router b (GBb) 52, ISP_b 120, and ISP_a 110.
To be more precise, if the host a (50) transmits a packet to host b (60) using the prefix assigned from ISP_a 110 as the source address, the packet is first sent to the gateway router b (GWb) 52 located nearest to the destination address. This packet is further supplied to ISP_a 110 via ISP_b 120 through the static tunnel.
This means that the packet is transmitted through a redundant route, instead of using the optimum (shortest) route (extending via ISP_b 120, ISP_c 130, and the Internet 200 to the host b (60) in this example).
Using a static tunnel is also disadvantageous in a dynamically changing environment, such as in multihome mobile networks. Whenever the connection point to the ISP changes, the gateway address and the address prefix of the multihome site change, which makes it difficult to maintain the IP tunnel provided between the ISP and the gateway. No technique has been proposed so far to automatically supply such a dynamic change in address prefix to the router in the site.

Router apparatus, route information distributing method, and communications system


A method includes: if it is detected that at least one first menu on a first application interface receives a touch event entered by a user, starting a second application interface corresponding to any one of the at least one first menu. In the method of the embodiments, transparency processing can be performed on a partial region of the upper-layer second application interface. In addition, the first application interface and the second application interface simultaneously displayed on a terminal device are simultaneously in an active state, and the first application interface and the second application interface can implement operation interaction.

To improve efficiency of operating a terminal device by a user, the prior art provides a multi-screen operation method. As shown in FIG. 1, a display screen of a terminal device is split into a split screen region A and a split screen region B. The terminal device obtains two applications selected by a user, displays an application interface of one application in the split screen region A, and displays an application interface of the other application in the split screen region B.
Although a user can simultaneously view application interfaces of a plurality of different applications by using the split screen operation method provided in the prior art, the plurality of different applications simultaneously displayed on a display screen of a terminal device are independent from each other, and the user can activate an application interface of an application only in one split screen region at a same time. As shown in FIG. 1, if the user operates the application running in the split screen region A, the application in the split screen region B is in an inactive state and runs in the background of the terminal device. That is, the user cannot simultaneously operate different applications displayed on the display screen of the terminal device, reducing efficiency of operating an application by the user.

Application interface display method and terminal device


An ultraviolet type white color light emitting device (Q) including a 340 nm–400 nm ultraviolet InGaN-LED, a first fluorescence plate of ZnS doped with more than 1×1017cm−3 Al, In, Ga, Cl, Br or I for absorbing ultraviolet rays and producing blue light (fluorescence), a second fluorescence plate of ZnSSe or ZnSe doped with more than 1×1017 cm−3 Al, In, Ga, Cl, Br or I for absorbing the blue light, producing yellow light (fluorescence) and synthesizing white color light by mixing the yellow light with the blue light.

1. Field of the Invention
This invention relates to a simple, light, small, long-life, high color rendering property white color light emitting device which is suitable for lighting, displaying and liquid crystal backlighting.
2. Background
Plenty of light emitting diodes (LEDs) and laser diodes (LDs) have been widely produced and sold on the market as small, long-life, inexpensive light emitting devices. High luminescence light emitting diodes (LEDs) have been already obtained for red, yellow, green and blue. Red light emitting diodes (LEDs) are LEDs having AlGaAs active layers or GaAsP active layers. The active layer means a thin layer which produces and emits light. Energy of emitted light is equal to the bandgap of an active layer. Color of emitted light depends upon the bandgap of the active layer. Yellow and green light can be produced by GaP-LEDs having GaP active layers. Orange/yellow light can be yielded by LEDs having AlGaInP active layers.
Production of blue light which requires a wide bandgap material had been one of the difficult problems. SiC (silicon carbide) type LEDs, ZnSe (zinc selenide) type LEDs and GaN (gallium nitride) type LEDs which had wide bandgap active layers had competed with each other for accomplishing practical, high luminescent, long lifetime blue light LEDs for a while. High-luminescence and long lifetime had allowed the GaN type LEDs to win a victory in the blue light LED race. Gallium nitride type light emitting diodes (GaN-LEDs) have an indium gallium nitride (InGaN) active layer. InGaN is a mixture crystal of InN (indium nitride) and GaN (gallium nitride). A mixture rate x, which means the ratio of components, is omitted here. The GaN type LEDs are denoted by InGaN-LEDs or GaN-LEDs hereafter. The InGaN-LEDs are made upon sapphire substrates (Al2O3). All the light emitting diodes (LEDs) or laser diodes (LDs) which produce light by electron bandgap transitions emit light of a single color whose energy is equal to the bandgap of the active layer. Monochromatic emission is one of the excellent features of semiconductor light emitting devices (LEDs & LDs) which make use of the bandgap transitions. Semiconductor light emitting devices (LEDs & LDs) are inherently monochromatic light sources. Monochromacity, however, forbids semiconductor devices from generating light including a plurality of colors. No single semiconductor light emitting device can yield complex color light.

Abstract: A blue light type white color light emitting device (R) including a 410 nm–470 nm blue light InGaN-LED, a fluorescence plate of ZnSxSe1-x (untreated 0.2≦x≦0.6; heat-treated 0.3≦x≦0.67) doped with more than 1×1017 cm−3 Al, In, Ga, Cl, Br or I for absorbing the blue light, producing 568 nm–580 nm yellow light (fluorescence) and synthesizing white color light by mixing the yellow light with the blue LED light.


Embodiments of the invention efficiently improve the stray field robustness without changing the size or weight of the drive on a large scale. Because a stray field concentrates on a magnetic field element, a symptom in which already recorded data is erased is prevented. According to one embodiment, in a magnetic recording system that uses a perpendicular recording medium having a soft magnetic under layer, a soft magnetic shield is provided on an upper part and a side of a slider on which a magnetic head is mounted, and a distance C between the shield and the medium is made smaller than a distance A or B between the shield and the slider.

The present invention relates to a magnetic recording system, and, particularly to a magnetic recording system provided with double-layered perpendicular media having a soft magnetic under layer and a single pole type write head.
To advance the high recording density of a magnetic disk drive, a double-layered perpendicular recording system is considered advantageous in which double-layered perpendicular media having a recording layer and a soft magnetic under layer, and a magnetic head which mounts a single pole type write head are combined. The single pole type write head is provided with a main-pole, a return-pole, and a coil for generating a write field applied from the main-pole to the double-layered perpendicular media. Moreover, the single pole type write head is usually fabricated in one body together with a read head which arranges a magneto-resistive effect element between a pair of regenerated shields. The double-layered perpendicular media have the soft magnetic under layer, the recording layer, and a protective layer formed on a substrate. Also in some cases, the double-layered perpendicular media may insert a non-magnetic layer between the recording layer and the soft magnetic under layer, or between the soft magnetic under layer and the substrate for the purpose of improving the crystalline of the upper layer and film adhesion. As an applicable composition, the single pole type write head provides a soft-magnetic material in the vicinity of the main-pole to change recording field distribution. In this description, a magnetic pole for generating a magnetic field in which magnetization is finally formed on the recording medium is defined as the main-pole.
In a conventional longitudinal magnetic recording system, data is written to the medium using the magnetic field leaked from a write head gap. On one hand, in the case of using the double-layered perpendicular media, because a magnetic flux generated from a recording element flows into the soft magnetic under layer, the recording field applied to the recording layer increases, and this characteristic is an advantage for high density. On the other hand, when a magnetic field is applied from the outside of the drive, as shown in FIG. 14, the magnetic flux concentrates on the magnetic head/soft magnetic under layer, and there is the possibility of an extremely large magnetic field being generated in comparison with an externally applied magnetic field, directly under the main-pole of the magnetic head. When a stray field concentrates on the magnetic head in this manner, the erasure of recorded data or the deterioration of signal quality occurs. There is the possibility of this problem being reduced due to the structure/magnetic property of the magnetic head and the structure/magnetic property of the soft magnetic under layer. However, because a double-layered perpendicular recording system is more sensitive to an external field than the conventional longitudinal magnetic recording system, this problem cannot be prevented theoretically in the double-layered perpendicular recording system.
To prevent this problem, it is proposed heretofore that a soft magnetic shield is added to the entirety or a part of the drive. Even in either case, the object is to bypass the magnetic flux by the external field via the shield by adding the shield, and reduce the magnetic flux flowing into the head and recording medium inside the drive. The shield in this case uses the outside of the drive or, at least, the outside of the head and the entirety of the medium as a circumventing path of the magnetic flux.

Magnetic recording system with improved magnetic shield

Hitachi Global Storage Technologies Netherlands B.V.


A negative photosensitive composition comprising (a-1) a compound having at least two organic groups of the following formula (I): EQU --OCH.sub.2 OR.sup.1 (I). wherein R.sup.1 is an alkyl group, in its molecule and an alkali-soluble resin, or (a-2) an alkali-soluble resin having at least two organic groups of the above formula (I) in its molecule, and (b) a photo-acid-generating material.

1. Field of the Invention
The present invention generally relates to a negative photosensitive composition sensitive to radiation and a method for forming a resist pattern by means thereof. More particularly, it relates to a negative photosensitive composition useful as a negative photoresist suitable especially for production of semiconductor integrated circuits.
2. Discussion of Background
A negative photoresist in which cyclized rubber and a bisazide compound as a crosslinking agent are used, is well known as a resist useful for photolithography. However, with such a negative photoresist, since swelling takes place during development by the action of an organic solvent used as a developer, the resolution of the resist results in 3 .mu.m at best. Therefore, such a negative photoresist is not suitable for the production of a highly integrated device. Further, the organic solvent used for the development is problematic from the aspects of environmental pollution and health or in view of inflammability.
Whereas, a positive photoresist comprising an alkali-soluble novolak resin and a compound containing a naphthoquinone diazide moiety as a photo-acid-generating material, is well known as a photoresist to which an alkali developer can apply. However, such a positive photoresist has a substantial absorption at 300 nm or less and thus has a drawback that when exposure is conducted at a short wavelength, the pattern profile will be very poor. Therefore, exposure has to be conducted at a wavelength of at least about 350 nm, whereby the resolution is limited, and it is not useful enough for lithography of less than a half micron level.
Although a conventional lithography employing exposure at 350 nm or longer has not sufficient resolutoin, a high level of resolution can be obtained even by employing a light with a wavelength of at least 350 mm such as i-line (365 nm) of a mercury lamp, if exposure is conducted by a phase shifting method using an improved reticle. However, it is well known that for the phase shifting method, a negative resist is more advantageous than the positive resist.
For example, Japanese Unexamined Patent Publication No. 164045/1987 discloses a negative photoresist composed of a combination of an acid curable resin and a photo-acid-generating material. Specifically, such a photoresist contains methoxymethylated melamine (Cymel.RTM. 303, trade name, manufactured by Mitsui Cyanamid) as a crosslinking agent. Japanese Unexamined Patent Publication No. 170165/1990 discloses a radiation sensitive composition which comprises a compound capable of generating an acid when irradiated with a radiation and an alkali-soluble phenol resin containing methoxymethyl groups, methylol groups or both of them, particularly an alkali-soluble polymer having at least structural units of the following formula: ##STR1## wherein each of n and m is an integer of 1 or 2, and R is hydrogen or methyl.
However, it is still desired to develop a negative photoresist having various properties including resolution further improved.

Negative photosensitive composition and method for forming a resist pattern by means thereof


The invention relates to a power factor correction (PFC) circuit (20), comprising an inductor (21) which is configured to provide a discharge current, a capacitor (23) which is connected to the inductor (21) via a switch (24) and which can be charged with said discharge current, a control unit (14) which is configured to alternately switch the switch (24) on and off based on a feedback control, wherein the control unit (14) has an input interface (42) for receiving a feedback signal (ZXCS) which represents a discharge voltage of the inductor (21), wherein the control unit (14), in a DCM mode, is further configured to calculate a switch on time (Ton) of the switch (24) which is after a first local minimum of the discharge voltage, and wherein, after switching off the switch (24), the control unit is configured to: either switch on the switch (24) at a next or closest local minimum of the inductor voltage after Ton, in case Ton is less than a directly or indirectly set reference time (Tref), or close the switch (24) at Ton, in case Ton is equal to or exceeds Tref.

Power factor correction (PFC) circuits are known to convert a supplied DC or AC voltage to a higher level. At the same time, such circuits can be designed to guarantee a power factor of almost 1, in which, for example, the current drawn by this circuit has a sinusoidal time curve similar to the one of the mains voltage.
Boost PFC circuits are a special type of PFC circuits. In a boost PFC circuit an inductor, such as a charging coil, is connected in series with a boost diode, whereby the connection point between the boost diode and the charging coil can selectively be connected to ground via a switch. An AC or DC voltage can be applied to the charging coil, which charges the charging coil when the switch is closed. When the switch is open, a charging capacitor is charged via the boost diode. Typically, the switch is closed again as soon as the charging coil has completely discharged, i.e. as soon as the current through the charging coil has dropped to zero. This type of operation of the PFC circuit is called borderline mode.
At low loads, it is already known to operate the PFC circuit alternatively in the so-called discontinuous conduction mode (DCM). Thereby, the switch is not switched on again immediately at the first local minimum of the voltage at the switch, respectively the first zero crossing of the corresponding current, but at a later point in time. To be more precise, the switch is not switched on again until the second or further local minimum, whereby these local minimums are caused by oscillation processes of the charging coil.
A problem now is that this particular clocking of the switch requires the detection of several zero crossings within a switch-off phase, i.e. while the switch is in an open position. However, the ringing, i.e. the oscillation, of the voltage decays over time such that after a view ringing circles it can become very difficult to properly determine the valleys in view of the decaying amplitude of the voltage.
In addition, the voltage oscillation at the charging coil is a damped oscillation, therefore the amplitude of the feedback signal detected via a secondary winding will also decrease towards a control unit following the damping. Thus, there is a risk that the maximum amplitude of the oscillation will drop below the discrimination threshold for zero crossing detection. This means that further zero crossings can no longer be detected. In extreme cases, the switch will not be switched on, i.e. closed, again.
In case no valley can be detected anymore, the PFC switch can be switched on again after a safety march in time period. However, as this safety march in time period is spaced substantially after the calculated switching on time, this leads to a fluctuation of the produced output voltage (Bus-voltage) of the PFC, which internally can lead to a visible relatively low frequency flicker in the light output of the lighting means.
Thus, it is an objective to provide an improved PFC circuit and an improved method for performing a power factor correction, which avoid the above-mentioned disadvantages.