BACKGROUND OF INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a scanner, and more particularly, to a method and related apparatus for compensating light inhomogeneity of a light-distributing device of a scanner.  
           [0003]    2. Description of the Prior Art  
           [0004]    Owing to the rapid development of digital image processing systems, digital information can be displayed and transmitted in a high-speed and low-cost way. Thus, the demand for scanners used for transforming image information to digital information has increased recently. For example, a transmissive scanner, a type of optical scanner, is used to scan a transparent document such as a projection transparency. The transmissive scanner transforms the image information of the transparent document into digital information by projecting light onto the document.  
           [0005]    Please refer to FIG. 1A and FIG. 1B. A housing  12  covers the principal parts of a scanner  10 . A transparent platform  14  is positioned on the housing  12  for a document to be placed on. The housing  12  comprises a track  18  positioned inside the housing  12  parallel with a scanning direction A 1  of the scanner  10 . The housing also comprises a scanning module  20  installed on and able to slide on the track  18  for sensing the light generated from a light-distributing device  16  passing through a transparent document  17 , and for generating a corresponding scan signal. Users can use the light-distributing device  16  as a light source to scan the document  17  with the help of an auxiliary frame  15 . The conventional frame  15  has a scanning opening  13 A and a light-calibrating opening  13 B. Both openings of the frame  15  allow light to pass through, whereas the remaining part of the frame  15  is lightproof. The opening  13 A corresponds to the position of the document  17 .  
           [0006]    The operation of the scanner  10  when scanning a transparent document is as follows. Please refer to FIG. 1B. The light-distributing device  16  is placed on top of the frame  15 . The document  17  is placed accordingly into the opening  13 A. A light  19 , generated by the light-distributing device  16 , passes through the document  17 , opening  13 A, platform  14 , and finally projects onto the module  20 . The module  20  transforms the received light into digital information.  
           [0007]    According to the previous description, the module  20  cannot sense any image information of the document  17  until the light  19  generated from the light-distributing device  16  has projected onto the module  20 . If the light-distributing device  16  cannot generate light evenly, the module  20  cannot accurately sense the image information of the document  17  and scanning errors occur.  
           [0008]    Please refer to FIG. 2. The scanning module  20  comprises a plurality of sensors  22  (4 sensors numbered from  22   a  to  22   d  are depicted as an example) for sensing the light projecting onto the module  20  and then generating corresponding pixel-scan-signals. A scan signal of the module  20  is formed by a combination of pixel-scan-signals of the sensors  22   a  to  22   d . A combination of different scan signals is generated by the module  20  moving along the track  18  and forms the complete image information of the document  17 . When the module  20  is moving along the corresponding opening  13 A, a light  24  generated from the light-distributing device  16  penetrates the document  17  and projects onto each sensor of the module  20 . For example, when the module  20  moves to a position P 4 , a pixel-scan-signal D 41  is generated by the sensor  22   a  and represents the intensity of light sensed by the sensor  22   a .Similarly, the sensor  22   d  senses the light generated by the light-distributing device  16 , passing through an area R 44  and projecting onto the module  20 , and generates a pixel-scan-signal D 44 . When the module  20  moves to the position P 4 , a combination of the four pixel-scan-signals, D 41  to D 44 , forms a scan signal  204 . The signal  204  represents the image information of one row (A 2  direction) of the document  17  placed on the scanning opening  13 A. When the module  20  moves to a position P 3 , the sensor  22   a  senses the light passing through an area R 31  and then generates a pixel-scan-signal D 31 . Likewise, the sensor  22   d  senses the light passing through an area R 34  and generates a pixel-scan-signal D 34 . Combining all the pixel-scan-signals of each sensor while the module  20  is positioned at P 3  forms a corresponding scan signal  203 . As a result, when the module  20  moves along the track  18  across positions P 1  to P 4 , the respective scan signals  201  to  204  are generated. A combination of the above scan signals forms scanned image information of the document  17 .  
           [0009]    However, as mentioned previously, the light generated from the light-distributing device  16  is not evenly distributed. In other words, the intensity of the light projecting onto different areas of the platform  14  varies. Please refer to FIG. 2 again. For example, the intensity of light projecting onto an area R 41  is stronger than that projecting onto an area R 44 . Thus, even if the actual images of the document  17  in the areas R 41  and R 44  are identical, the pixel-scan-signals D 41  and D 44  are different because of the different intensities of light projecting onto these two areas. Therefore, scanning errors occur. That is, an uneven distribution of light generated from the light-distributing device  16  causes an inconsistency in the image information between the original document and the scanned one.  
           [0010]    The opening  13 B, included in the frame  15 , is used to correct the abovementioned scanning errors(refer to FIG. 1A and FIG. 1B). The operation of the scanner  10  while calibrating the light generated from the light-distributing device  16  is illustrated as follows. It is important that no document is placed on the opening  13 B. The module  20  moves to a position P 0  under the opening  13 B. Each sensor of the module  20  senses the projected light and generates a corresponding pixel-calibration-signal. Each calibration signal is converted into a corresponding correction factor after comparison to a standard value.  
           [0011]    As shown in FIG. 2, a correction factor g 1  corresponds to the sensor  22   a ,factor g 4  corresponds to the sensor  22   d , and so on. Because no document is placed in the opening  13 B when the scanner  10  is calibrating light, the light sensed by each sensor of the module  20  positioned at P 0  is that which is directly generated from the light-distributing device  16 . If the amplitude of the pixel-calibration-signal generated by each sensor is different, the light generated from the light-distributing device  16  is unevenly distributed. If the pixel-calibration-signal generated by a sensor is stronger than the standard value, the intensity of the light projecting onto the corresponding area is too strong and the corresponding correction factor will be smaller than 1. On the contrary, if the pixel-calibration-signal generated by a sensor is weaker than the standard value, the intensity of light projecting onto the corresponding area is too weak and the corresponding correction factor will be larger than 1. The pixel-calibration-signal will approach the standard value after being multiplied by the correction factor. The pixel-calibration-signal generated by each sensor corresponds to a particular correction factor. For example, the correction factors g 1  to g 4  correspond to the sensors  22   a  to  22   d , respectively.  
           [0012]    The prior art method for calibrating a scan signal employs multiplying a pixel-calibration-signal by the corresponding correction factor to form a calibrated scan signal. As shown in FIG. 2, the respective pixel-scan-signals D 11  to D 14  of the scan signal  201 , corresponding to the sensors  22   a  to  22   d , multiplied by the corresponding correction factors g 1  to g 4  form a calibrated scan signal  301 . Similarly, the pixel-scan-signals D 21  to D 24  of the scan signal  202  multiplied by the corresponding factors g 1  to g 4  form a calibrated scan signal  302 . Likewise, the pixel-scan-signals generated by each sensor of the scan signals  203 ,  204  are multiplied by a corresponding correction factor to generate calibrated scan signals  303 ,  304 .  
           [0013]    The conventional calibration principle assumes that the unevenly distributed light generated by the light-distributing device  16  and projecting through the opening  13 B has the same distribution as the unevenly distributed light projecting through the opening  13 A. For example, the prior art assumes that the distribution of light generated by the light-distributing device  16  and projecting onto the area R 04  of the calibrating opening  13 B is the same as that light projecting onto the areas R 34  and R 44 . This results in the pixel-calibration-signal and the corresponding correction factor g 4 , generated by the sensor  22   d  when positioned at the area R 04  of the opening  13 B, being used to calibrate the pixel-scan-signal generated by a sensor when positioned at the areas R 34  and R 44 . If a pixel-calibration-signal generated by the sensor  22   d  is weak (weaker than the standard value), the pixel-scan-signal generated by the sensor  22   d  when positioned at the areas R 34  and R 44  will be amplified by the same correction factor g 4 . However, the intensity of the light generated by the light-distributing device  16  varies not only along the A 2  direction, but also along the A 1  direction. In other words, the intensity of the light generated by the light-distributing device  16  and projecting onto the areas R 34  and R 44  is different. The prior art does not calibrate for this variation of light intensity. Moreover, even if an uneven distribution of light only occurs along the A 2  direction, the pattern of the unevenly distributed light projected at the opening  13   b  is not guaranteed to be identical to that at the opening  13 A. The prior art cannot completely solve the problem of uneven light distribution while the scanner  10  is scanning a document along the A 1  direction.  
         SUMMARY OF INVENTION  
         [0014]    It is therefore an objective of the claimed invention to provide a method for calibrating two-dimensional light inhomogeneity of a light-distributing device of a scanner to solve the problems of the conventional method of only calibrating one-dimensional light inhomogeneity.  
           [0015]    According to the claimed invention, a scanner includes a housing having a transparent platform positioned on the housing for placing a document, a light-distributing device positioned above the transparent platform for projecting light on the document placed on the transparent platform, a track positioned inside the housing parallel with a scanning direction of the scanner, and a scanning module movably positioned on the track for sensing the light passing through the document and generating a corresponding scan signal. The method includes amplifying or decaying the scan signal generated from the scanning module according to a position of the scanning module located on the track when the scanning module slides along the track to scan the document.  
           [0016]    It is an advantage of the claimed invention that the method can calibrate the scan signal of the document in two dimensions. The claimed invention can be used to detect when a light-emitting cell of the light-distributing device has malfunctioned. The claimed invention can use a different method to generate a common correction factor so as to save memory space in a computer. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0017]    [0017]FIG. 1A is a pictorial view of a conventional scanner.  
         [0018]    [0018]FIG. 1B is a cross-sectional view of the scanner  10  in FIG. 1A scanning a transparent document.  
         [0019]    [0019]FIG. 2 is a schematic diagram illustrating a prior art method of calibrating an uneven light distribution.  
         [0020]    [0020]FIG. 3 is an exploded perspective view of a scanner according to the invention.  
         [0021]    [0021]FIG. 4 is a schematic diagram illustrating a method of calibrating for an uneven light distribution according to the invention.  
         [0022]    [0022]FIG. 5 is a schematic diagram of a second embodiment illustrating a method for calibrating for an uneven light distribution.  
         [0023]    [0023]FIG. 6 is a schematic diagram of a third embodiment illustrating a method for calibrating for an uneven light distribution. 
     
    
     DETAILED DESCRIPTION  
       [0024]    Please refer to FIG. 3. A housing  32  covers the principal parts of a scanner  30 . The scanner  30  comprises a transparent platform  34  positioned on the housing  32  for a document to be placed on. A scanning module  40  is installed on and able to slide on a track  38  and can move along an A 1  direction for scanning a document. The scanner  30  also comprises a processor  46  for controlling the operation of the scanner  30  and a storing circuit  48  for storing the information necessary for the scanner  30  to function. The scanner  30  includes a light-distributing device  36  and an auxiliary frame  35  to scan a transparent document  37 , such as a projection transparency. The light-distributing device  36  is used to project light when the scanner  30  is scanning the document  37 . The document  37  is placed on a light-penetrable scanning opening  33  positioned in the central part of the frame  35 . When the light-distributing device  36  is placed upon the frame  35 , the light generated from the light-distributing device  36  projects through the document  37 , penetrates through the opening  33  and the platform  34 , and is finally sensed by the module  40 .  
         [0025]    Please refer to FIG. 4. Similar to the prior art scanner, the module  40  has a plurality of sensors (only four sensors  42   a  to  42   d , as shown in FIG. 4, are used as an example). The light penetrating through areas positioned along an A 2  direction of the opening  33  projects onto different sensors and then these sensors generate corresponding pixel-scan-signals. A combination of the pixel-scan-signals generated by each sensor forms a scan signal. The scan signal represents a row (A 2  direction) image of a document  47 . When the module  40  moves from one end of the scanner  30  to the other end, along the track  38  in the direction A 1 , a combination of the scan signals formed at different positions forms a complete image signal of the document  47 . Scan signals  401  through  404 , shown in FIG. 4, are respectively representative of when the module  40  is positioned at positions P 1  through P 4 . The pixel-scan-signals D 41  to D 44  of the scan signal  404 , for example, represent the corresponding signals generated by the sensor  42   a  to  42   d  when the module  40  is positioned at the position P 4 .  
         [0026]    Unevenly distributed light generated by the light-distributing device  36  causes an image to be scanned inaccurately. To calibrate for this inaccuracy, the scanner  30 , according to the invention, scans the opening  33  completely without a document being placed on the opening  33  and generates corresponding pixel-scan-signals and scan signals. Each of these pixel-scan-signals and scan signals are in effect pixel-calibration-signals and calibration signals. Because no document is placed on the opening  33 , the light sensed by the module  40  is the light directly generated by the light-distributing device  36  and projected onto the module  40 . A correction factor can thus be generated, according to the invention, by determining the corresponding pixel-calibration-signal generated by each sensor when the module  40  is positioned at a different position. For example, the module  40  is positioned at the position P 3 . If a pixel-calibration-signal, generated by the sensor  42   a ,is stronger than a standard value, the intensity of the light projected onto an area Z 31  is too strong and a correction factor G 31  with a value smaller than 1 is generated. Similarly, if the module  40  is positioned at the position P 4  and a pixel-calibration-signal, generated by a sensor  42   d , is weaker than the standard value, the intensity of the light projected onto an area Z 44  is too weak and another correction factor G 44  with a value greater than 1 is generated. The result of multiplying the correction factor by the corresponding pixel-calibration-signal will approach the standard value. In other words, when the scanning module  40  is positioned at the position P 1 , the correction factors G 11  to G 14  can be generated by determining the corresponding pixel-scan-signal generated by each sensor  42   a  to  42   d . Generally, different correction factors can be generated by determining different pixel-calibration-signals generated by each sensor no matter what position the module  40  is at.  
         [0027]    The embodiment method to calibrate a scan signal is described as follows. Pixel-scan-signals D 11  through D 14 , generated by the corresponding sensors when the module  40  is positioned at the position P 1  are multiplied by the corresponding correction factors G 11  through G 14  (the corresponding correction factor of each pixel-calibration-signal when the module  40  is positioned on the position P 1 ) to generate a calibrated scan signal  601 . Similarly, pixel-scan-signals D 41  through D 44  of the signal  404  are multiplied by the corresponding correction factors G 41  through G 44  to generate calibrated scan signal  604 . The signalsand  403  are also thus modified by the corresponding correction factors tocalibrated scan signals  602  and  603 . In this way, the scanner  30  is calibrated for the inaccuracy caused by unevenly distributed light generated by the light-distributing device  36 .  
         [0028]    In contrast to the prior art, the correction factors are generated by sensors sensing the light passing through the opening  33  rather than through the opening  13 B (shown in FIG. 1A). The invention calibration method uses different correction factors to calibrate the pixel-scan-signals generated at different positions so that the scanned image is corrected not only in the A 2  direction but also in the A 1  direction. This is superior to the prior art method of only calibrating the scanner in the A 2  direction.  
         [0029]    In practical application, the invention method determines the correction factors by only scanning the opening  33  once while no document is present on the platform  34 , and then stores these correction factors in the storing circuit  48 . The scanner  30  will calibrate a scanned document according to the correction factors stored in the circuit  48 . Another user can use these correction factors to calibrate another scanned document. Usually, a scanner is connected to a computer and an application stored on the computer controls the scanner. In such a circumstance, a hard disk or other memory device of the computer stores the application and the correction factors to correct the original scan signals. Additionally, because the light distribution of a scanner may change over time, the application can remind a user to update the correction factors by repeating the described calibration procedure.  
         [0030]    The correction factors can also be generated by the following method. Please refer to FIG. 5. The difference between this embodiment and the former one is that correction factors L 11 , L 12 , L 21 , and L 22  are generated according to pixel-calibration-signals generated by corresponding sensors when the module  40  moves to different positions. For example, the factor L 11  is generated according to four pixel-calibration-signals generated by the sensors  42   a ,  42   b  when the module  40  moves to the positions P 1 , P 2 . Averaging these four pixel-calibration-signals and dividing the average by a standard value can generate the corresponding correction factor L 11 . The correction factor L 12  can be generated in a similar way using the sensors  42   c ,  42   d . Multiplying the pixel-scan-signals generated by the sensors  42   a  to  42   d  while the module  40  moves across the positions P 1 , P 2  by the corresponding correction factors L 11 , L 12  generates calibrated scan signals  701 ,  702 . In a similar manner, the factors L 21 , L 22  can be used to generate calibrated scan signals  703 ,  704 , as shown in FIG. 5. The advantage of this embodiment is that the memory required for storing the correction factors is smaller because the number of correction factors is reduced. In addition, some light-distributing devices project evenly distributed light within a small area, so using an averaged correction factor will not impact scanning quality. Of course, in practical application, users can use as many pixel-scan-signals as desired multiplied by a common correction factor to generate a calibrated scan signal. Users can also select the pixel-calibration-signals generated by different sensors while the module  40  moves to different positions to generate another common correction factor.  
         [0031]    The number of pixel-scan-signals used for generating a common correction factor can be changed according to different sensors and different scanning positions. Please refer to FIG. 5 again. For example, nine pixel-calibration-signals, generated by the three sensors  42   a  to  42   c  while the module  40  moves across the three positions P 1  to P 3 , could be used to generate the common correction factor L 11  for calibrating pixel-scan-signals D 11 -D 13 , D 21 -D 23 , and D 31 -D 33 . Only three pixel-calibration-signals, generated by the sensors  42   a  to  42   c  after the scanning module  40  moves to the position P 4 , would then be used to generate the common correction factor L 21 . Alternately, six pixel-calibration-signals, generated by the sensor  42   a  to  42   c  while the module  40  moves across the positions P 3  and P 4 , could also be used to generate the common correction factor L 21 . Because some light-distributing devices generate evenly distributed light from a central portion but unevenly distributed light from a peripheral portion, the method of using different numbers of pixel-calibration-signals to generate common correction factors can save memory space in a computer.  
         [0032]    Please refer to FIG. 6. Some light-distributing devices are formed by a plurality of light-emitting cells disposed on the light-distributing device. Occasionally a light-emitting cell of the light-distributing device malfunctions, and thus the light generated by the device is unevenly distributed. A user can determine which light-emitting cell has malfunctioned by utilizing the present invention. For example, if a pixel-scan-signal generated by a sensor  42   a  while the scanning module moves to a position P 3  is weaker than a threshold value, a light-emitting cell disposed on a light-distributing device corresponding to an area Z 31  is judged to have malfunctioned. In such a circumstance, the result of calibrating the scan signal by a correction factor is not adequate. Instead, an interpolation method using the pixel-scan-signals generated by the neighboring sensors while the scanning module moves to the neighboring positions is used to calibrate the pixel-scan-signal. In this example, a pixel-scan-signal  131 of a calibrated scan signal  803  at the corresponding area Z 31  can be generated by interpolating pixel-scan-signals D 21 , D 22 , D 32 , D 42 , and D 41 . The remaining pixel-scan-signals of the scan signal can be calibrated by the method mentioned in the first embodiment of the invention to generate corresponding pixel-scan-signals of the calibrated scan signals  801  to  804 .  
         [0033]    Additionally, the light-distributing device of some scanners generates light of different colors. Each color of light generates corresponding scanned image information, which is then combined to form a full color scanned image. The method of the invention can also generate different correction factors according to different colored light of a color image. These correction factors can be used to calibrate the corresponding color of light of the color image.  
         [0034]    In summary, the method for calibrating light of the invention first uses a scanning module of a scanner to scan an opening while no document is placed on a transparent platform, and then utilizes a corresponding method of calibration, such as determining correction factors. When the scanner scans a document, it can use the correction factors to calibrate the document so the image quality is improved. In contrast to the prior art, the invention can calibrate a document image in two dimensions, not just in one dimension. Furthermore, the auxiliary frame  35  of the invention does not comprise a calibrating opening (refer to FIG. 1A and FIG. 1B according the prior art, and FIG. 3 according to the invention), so the area covered by the light-distributing device is decreased resulting in the cost of manufacturing the scanner being decreased.  
         [0035]    Following the detailed description of the invention above, those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.
A method for calibrating an image generated from a scanner when scanning a document is provided. The scanner includes a housing having a transparent platform positioned on the housing for placing the document, a light-distributing device positioned above the transparent platform for projecting light on the document, a track positioned inside the housing parallel with a scanning direction of the scanner, and a scanning module movably positioned on the track for sensing the light passing through the document and generating a corresponding scan signal. The method includes amplifying or decaying the scan signal generated from the scanning module according to a position of the scanning module located on the track when the scanning module slides along the track to scan the document.