FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to a method and device for driving an image display apparatus, such as for instance in a television, a monitor, etc. 
         [0002]    In general, the invention can be applied to several types of image display apparatus having a backlight. In the following, the invention will be described for an image display device of the LCD type, but it is to be noted that it is not intended to restrict the invention to LCD image display devices. 
       BACKGROUND OF THE INVENTION 
       [0003]    In an image display device, an image consists of a large number of image points, each having a specific grey value or color and a specific brightness. In a specific class of image display devices, a viewer is watching a display screen behind which a light source is arranged, the so-called backlight. The display screen comprises a plurality of pixels which can be controlled to pass light or to block light. In a specific embodiment, a pixel is implemented as a liquid crystal cell. A controller receives a video signal with image data, and on the basis of these image data it generates control signals for the liquid crystal cells. In the following, a control signal for pixel cells will be indicated as S CP , and it will be assumed to have a minimum value 0 and a maximum value 1. 
         [0004]    The image data can range from perfect black to perfect white. The image data are translated by the controller to a certain value for the control signal S CP . In the case of perfect black, the brightness data in de video signal will be assumed to have a minimum value 0. It is noted that, in response to receiving a control signal S CP =0, the pixel cell should block all light from the backlight. In practice, however, a pixel cell will always “leak” to some extent. In the case of perfect white, the brightness data in de video signal will be assumed to have a maximum value 1. It is noted that, in response to receiving a control signal S CP =1, the pixel cell should pass all light from the backlight. In practice, however, a pixel cell will always reflect and/or absorb to some extent. So, generally speaking, the transmission rate of a pixel cell, indicated as H, will range from a minimum value α to a maximum value β, wherein 0&lt;α&lt;β&lt;1. 
         [0005]    In an actual image, the darkest portions may be lighter-than-black and the brightest portions may be darker-than-white. Thus, the transmission rate for all pixels of the image will be in a range from α* to β*, with α&lt;α*&lt;β*&lt;β. The values α* en β* determine the contrast of the image: a high contrast ratio means that the distance between α* and β* is as large as possible. 
         [0006]    Apart from the actual value of the transfer rate H, the amount of light I P  emanating from a pixel, as viewed by the viewer, depends on the brightness of the backlight, in other words the intensity I BL  of the light generated by the backlight. This might be expressed in a formula as follows: 
         [0000]        I   P   =H·I   BL   (1) 
         [0007]    Thus, with a certain setting of the intensity I BL  of the backlight, the brightness I P  of a pixel can range from α·I BL  to β·I BL . 
         [0008]    Under certain circumstances, it may be desirable to increase the light output. This may for instance be the case if the level of the ambient light is relatively high. Increasing the light output may be done by shifting the range [α*,β*] to higher values, or at least by shifting the upper limit β* of this range to higher values. 
         [0009]    On the other hand, under certain other certain circumstances, it may be desirable to decrease the light output. This may for instance be the case if the level of the ambient light is relatively low. Decreasing the light output may be done by shifting the range [α*,β*] to lower values, or at least by shifting the lower limit a* of this range to lower values. 
         [0010]    However, increasing or decreasing the light output can also be achieved by increasing or decreasing the intensity I BL  of the backlight. 
         [0011]    From formula 1, it follows that the same pixel brightness I P  can be achieved for different settings of the brightness I BL  of the backlight. If the brightness I BL  of the backlight is multiplied by a certain factor X, and simultaneously the transfer rate H of a pixel cell is divided by the same factor X, the resulting product (X·I BL )·(H/X)=I P . This fact is utilized in backlight boosting and backlight dimming. 
         [0012]    In the case of backlight boosting, the intensity I BL  of the backlight is increased. This can be used to enhance white parts of an image. By increasing the backlight intensity I BL , those parts appear to be “better white” for the viewer. In grey parts of the image, the grey value can be maintained by simultaneously reducing the control signal S CP  for the pixel cells, so that the pixels cells pass less light. 
         [0013]    In the case of backlight dimming, the brightness I BL  of the backlight is decreased. This can be used to enhance black parts of an image. By decreasing the backlight intensity I BL , those parts appear to be “better black” for the viewer. In grey parts of the image, the grey value can be maintained by simultaneously increasing the control signal S CP  for the pixel cells, so that the pixels cells pass more light. 
         [0014]    By alternating backlight boosting and backlight dimming, the overall contrast ratio of the display device can be enhanced, and energy can be saved. 
         [0015]    An image display device is designed for a certain nominal setting of the backlight light source. In this nominal setting, the backlight light source consumes a certain amount of power, and consequently it generates a certain amount of heat; the image display device is designed to handle this amount of heat. It should be clear that changing the contrast range [α*,β*] of the transmission rate of the screen pixels does not change the power consumption of the backlight. When using backlight dimming, energy is saved, but when using backlight boosting, the backlight light source produces more heat than the image display device is designed to handle. If this situation continues for a prolonged amount of time, the apparatus may become too hot. This problem might be mitigated by using additional cooling means, but this would add to the hardware costs and the energy bill of the apparatus. 
       SUMMARY OF THE INVENTION 
       [0016]    The present invention proposes a method for driving an image display device using backlight dimming and backlight boosting such that, on average, the power consumed by the backlight does not exceed a predetermined power rating. In darker scenes, the backlight is dimmed and the display control signals are increased. In very bright scenes, the backlight can temporarily be boosted. 
         [0017]    The predetermined power setting may be equal to the nominal power setting; in that case, brighter images are achieved. However, the predetermined power setting may also be lower than the nominal power setting; in that case, an overall power saving for the display apparatus is achieved. 
         [0018]    Backlight dimming saves energy, but backlight boosting spends more energy. In order not to exceed the predetermined average, it is only possible to perform backlight boosting if it is preceded by a period of backlight dimming. It might be said that backlight dimming provides an energy reserve that can be consumed to perform backlight boosting. However, such reserve is limited. The present invention provides a method for backlight boosting which uses the energy reserve in an efficient manner and, when the energy reserve gets exhausted, reduces the excess energy consumption in an effective manner. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    These and other aspects, features and advantages of the present invention will be further explained by the following description with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: 
           [0020]      FIG. 1  schematically illustrates a transmission characteristic of a pixel; 
           [0021]      FIG. 2  schematically illustrates backlight dimming; 
           [0022]      FIG. 3  is a block diagram schematically illustrating a display apparatus; 
           [0023]      FIG. 4  is a graph comparable to  FIG. 2 , also including an apparatus characteristic; 
           [0024]      FIGS. 5A-E  are graphs illustrating a backlight dimming characteristic of a controller of the apparatus of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]      FIG. 1  is a graph schematically illustrating a transmission characteristic of a pixel, for instance an LCD cell. The horizontal axis represents a control signal S CP , ranging from 0 for minimum transmission to 1 for maximum transmission. The vertical axis represents a transmission ratio H of the pixel, ranging from 0 for perfectly blocking to 1 for 100% transmission. Line  1  shows that for control signal S CP =0, the transmission ratio H( 0 )=α&gt;0, indicating that the minimum transmission of a pixel is always somewhat larger than zero. Further, line  1  shows that for control signal S CP =1, the transmission ratio H( 1 )=β&lt;1, indicating that the maximum transmission of a pixel is always less than 100%. The characteristic line  1  is shown as a straight line, but this is not essential. 
         [0026]      FIG. 2  is a graph schematically illustrating backlight dimming. The vertical axis downwards represents the control signal S CP , and the horizontal axis represents the transmission ratio H of the pixel, so that quadrant IV of this graph corresponds to the graph of  FIG. 1 . The vertical axis upwards represents the amount of light I P  emanating from the pixel, also indicated as pixel intensity (normalized on the nominal backlight intensity). It is assumed that the pixel intensity I P  obeys the above formula (I). Thus, at a certain nominal backlight intensity I BL , represented by line  2 , if the pixel control signal S CP  has a certain value S 1 , the transmission ratio H of the pixel has a certain value H 1  and the pixel intensity I P  has a certain value I P (x). The same value I P (x) is achieved if the backlight intensity is reduced (indicated by line  3 ) and the pixel control signal S CP  is suitably increased to an increased value S 2 , in which case the transmission ratio H of the pixel has an increased value H 2 . 
         [0027]    In  FIG. 2 , it can also be seen that, if the pixel control signal S CP  is maintained, reducing the backlight intensity causes a reduction of the pixel intensity I P , which particularly can be used to enhance “black” performance. Assume a dark scene, associated with a certain low value S 4  of the pixel control signal S CP . With the nominal backlight intensity I BL  (line  2 ), the pixel intensity I P  has a relatively high value I P ( 4 ). With reduced backlight intensity I BL  (line  5 ), the pixel intensity I P  has a substantially lower value I P ( 5 ). 
         [0028]    Conversely, backlight boosting results in higher pixel intensity I P  when the pixel control signal S CP  is maintained, which can be used to enhance “white” performance. Assume a bright scene, associated with a certain high value S 6  of the pixel control signal S CP . With the nominal backlight intensity I BL  (line  2 ), the pixel intensity I P  has a relatively low value I P ( 6 ). With increased backlight intensity I BL  (line  7 ), the pixel intensity I P  has a substantially higher value I P ( 7 ). 
         [0029]    It is noted that backlight boosting and backlight dimming are known per se. Backlight dimming can for instance be performed by driving a backlight lamp with a duty cycle less than 1. Backlight boosting can for instance simply be implemented if the nominal power setting of a backlight lamp corresponds to a duty cycle less than 1: in that case, the duty cycle can be increased. If, in order to improve the display performance in the case of moving images, a backlight lamp is normally driven at a duty cycle of 30%, a boost factor of over 300% is available. 
         [0030]      FIG. 3  is a block diagram schematically illustrating a display apparatus  100 , comprising a display device  110  and a controller  120 . The display device  110  comprises at least one backlight lamp  111  and a display screen  112 . It is noted that display devices with backlight are known per se. The backlight lamp may for instance be implemented as an array of fluorescent tube, or as an array of LEDs. The display screen may for instance be implemented as an array of LCD cells, or any other type of light valve. 
         [0031]    The controller  120  has a light control output  121  coupled to the backlight  111 , for communicating lamp control signals S CL  to the backlight  111 , and has a pixel control output  122  coupled to the display screen  112 , for communicating pixel control signals S CP  to the display screen  112 . The controller  120  has an image input  123  for receiving image data D (video signals), and has a user control input  124  for receiving user control signals U. With the lamp control signals S CL , the controller  120  controls the power setting of to the backlight  111 ; it is noted that the intensity or brightness of the backlight  111  is proportional to the lamp power in a good approximation. 
         [0032]      FIG. 4  is a graph comparable to  FIG. 2 , but having added a left-hand horizontal axis representing image data D, wherein D=0 represents perfect black and D=1 represents perfect white. A line  8  represents a setting of brightness and contrast. If a scene is relatively dark, its pixel data will have values relatively close to zero (range A), resulting, with the nominal backlight intensity I BL  (line  2 ), in relatively low level pixel intensity (range B). In such case, the controller  120  may reduce its lamp control signals S CL  (line  2 ′) and simultaneously increase its pixel control signals S CP  (line  8 ′) to achieve a reduction in power consumption while maintaining the image brightness. It should be clear that backlight dimming in this way is only possible in the case of relatively dark scenes, so it depends on the image contents. 
         [0033]    On the other hand, if a scene is relatively bright, the controller  120  will try to increase the brightness of the backlight by increasing its lamp control signals S CL  (line  2 ″), resulting in a wider area of higher pixel intensity (range C). As mentioned above, a problem may then be that the average power of the backlight becomes too high. 
         [0034]    The solution to this problem proposed by the present invention is described below. 
         [0035]    According to a first aspect of the present invention, the controller  120  sets a maximum to the backlight brightness, i.e. a maximum of the backlight power. This maximum, that will be indicated as I BL (max), corresponds to a maximum S CL (max) of the lamp control signals S CL  to be outputted at the light control output  121 . This is illustrated in  FIG. 5 , which is a graph illustrating a characteristic of the controller  120 . The horizontal axis represents the calculated lamp control signals S CL (D) as calculated by the controller  120  on the basis of the content of the data signals D and the user setting U alone. It is noted that the user setting U may be considered to be constant, but the content of the data signals D changes dynamically with time. The vertical axis represents the actually outputted lamp control signals S CL (A). The graph of  FIG. 5  shows a straight line  51  representing the relationship S CL (A)=ξ·S CL (D), wherein ξ is a factor which, by way of preferred example, in the following will be taken to be equal to 1. The graph of  FIG. 5  further shows a horizontal line  52  representing the limit value S CL (max). Curve  53  illustrates the behavior of the controller  120 . For relatively low values of the calculated control signals S CL (D), the controller  120  sets its output lamp control signals S CL (A) to be equal to the lamp control signals S CL (D) as calculated on the basis of the data content alone: in this region I, curve  53  follows line  51 . 
         [0036]    If the calculated control signals S CL (D) exceed the maximum value S CL (max), the controller  120  sets its output lamp control signals S CL (A) to be equal to the maximum value S CL (max); in this region III, curve  53  follows line  52 . 
         [0037]    It is possible that curve  53  follows lines  51  and  52  up till the intersection of these lines, to achieve a “hard” limitation. However, it is preferred that the limitation is softer, illustrated by a transition portion of curve  53  in the transition region II. Curve  53  follows line  51  between S CL (D)=0 and S CL (D)=S CL ( 1 ), indicated by a point P, wherein S CL ( 1 ) is a first transition value lower than the maximum value S CL (max). Curve  53  follows line  52  for S CL (D)≧S CL ( 2 ), indicated by a point Q, wherein S CL ( 2 ) is a second transition value higher than the maximum value S CL (max). Between S CL ( 1 ) and S CL ( 2 ), curve  53  follows a path connecting points P and Q. Thus, the function that describes the relationship between S CL (A) and S CL (D) is a continuous function. Such path may be a straight line itself. Preferably, and as illustrated, such path is a curved path of which, in points P and Q, the end portions have the same direction as lines  51  and  52 , respectively. The exact shape of this curved path is not essential, but it is preferred that it is a smooth shape. Preferably, the function that describes the relationship between S CL (A) and S CL (D) between S CL ( 1 ) and S CL ( 2 ) has a second derivative that is always negative. 
         [0038]    The transition points P and Q may be calculated from the maximum value S CL (max) in several ways. It is possible that the transition values are calculated according to 
         [0000]        S   CL (1)= S   CL (max)−Δ1 and  S   CL (2)= S   CL (max)+Δ2 
         [0039]    Δ1 may be equal to Δ2. 
         [0040]    It is also possible that the transition values are calculated according to 
         [0000]        S   CL (1)= S   CL (max)/α1 and  S   CL (2)= S   CL (max)−α2 
         [0041]    α1 may be equal to α2. 
         [0042]    According to a second aspect of the present invention, the controller  120  is provided with a feedback loop  130  comprising a power calculator  131  and an average calculator  132 . The power calculator  131  has an input receiving the actual lamp control signals S CL (A) outputted by the controller  120 , and is designed to calculate a value that is proportional to the power consumed by the backlight  111 . Alternatively, it could be possible to actually measure the power consumption by the backlight  111 , but that is more complicated. The average calculator  132  calculates a time-average of the power-representing value as calculated by the power calculator  131 , and provides the result as an average signal S AV  to the controller  120  at its power average input  126 . The time constant of the average calculator  132  may be set in relationship with the warming-up and cooling-down properties of the display device  110 ; in general, the average calculator  132  may calculate the average over a time period in the order of several minutes. 
         [0043]    In a relatively simple embodiment, the power consumed by the backlight  111  is proportional to the lamp control signals S CL (A); in that case, a separate power calculator may be omitted, and the average calculator  132  may simply calculate the time-average of the lamp control signals S CL (A). It is noted that circuitry or software for calculating a time-average are known per se. 
         [0044]    According to a third aspect of the present invention, the controller  120  compares the average signal S AV  with a predetermined reference value S REF , received at a reference input  125 . The reference value S REF  may be stored in a memory (not shown) associated with the controller. The controller  120  sets the maximum value S CL (max) proportional to the difference (S REF −S AV ): if the average signal S AV  becomes smaller, the maximum value S CL (max) increases. Ultimately, the maximum value S CL (max) may be higher than the practical range of backlight settings. If the average signal S AV  rises, the controller  120  decreases the maximum value S CL (max). 
         [0045]    This is illustrated in an exaggerated manner in  FIGS. 5A-E . 
         [0046]      FIG. 5A  illustrates a situation at a certain time t 1 . An assumed value for the reference value S REF  is indicated. Assume that in this situation the average signal S AV (t 1 ) is substantially lower than the reference value S REF , meaning that in the recent history the power consumption has been relatively low, i.e. a recent history of backlight dimming. Assume further that the calculated lamp control signal S CL (D) has a certain relatively low value S CL (D,t 1 ), and that the actually outputted control signal S CL (A) has a certain value S CL (A,t 1 ) close to the average value S AV (t 1 ). Because the average value S AV (t 1 ) is currently substantially lower than the reference value S REF , the maximum value S CL (max) is high. 
         [0047]      FIG. 5B  illustrates the situation at a later time t 2 . Assume a bright scene, so that the calculated lamp control signal S CL (D) has a certain relatively high value S CL (D,t 2 ), although (in the example) lower than the first transition value S CL ( 1 ). The corresponding actually outputted control signal S CL (A,t 2 ) is higher than S CL (A,t 1 ), and is even higher than S REF : the backlight is boosted. 
         [0048]    Because the actually outputted control signal S CL (A,t 2 ) is higher than S AV (t 2 ), the average S AV  is increasing (arrow X 1 ), and consequently the maximum value S CL (max) is decreasing (arrow X 2 ).  FIG. 5C  illustrates the situation at a later time t 3 . It is assumed that the calculated lamp control signal S CL (D,t 3 ) on time t 3  is equal to S CL (D,t 2 ).  FIG. 5C  illustrates that the average value S AV (t 3 ) on time t 3  has increased with respect to S AV (t 2 ), but it is still lower than the reference value S REF . The decreased maximum value S CL (max,t 3 ) is indicated by a horizontal line  56  lower than line  52  (shown dotted in  FIG. 6 ), and the resulting controller characteristic is shown by a curve  57 . It is noted that, with the decreasing maximum value S CL (max), also the first and second transition values S CL ( 1 ) and S CL ( 2 ) have decreased. In  FIG. 5C , the calculated lamp control signal S CL (D,t 3 ) is still lower than the first transition value S CL ( 1 ), so S CL (A,t 3 ) is equal to S CL (A,t 2 ). 
         [0049]    Because the actually outputted control signal S CL (A) is still higher than S AV , the average S AV  is still increasing (arrow X 3 ), and consequently the maximum value S CL (max) is still decreasing (arrow X 4 ).  FIG. 5D  illustrates the situation at a still later time t 4 . The average value S AV (t 4 ) on time t 4  has increased with respect to S AV (t 3 ), but it is still lower than the reference value S REF . The decreased maximum value S CL (max,t 4 ) is indicated by a horizontal line  58 , and the resulting controller characteristic is shown by a curve  59 . It is assumed that the calculated lamp control signal S CL (D,t 4 ) on time t 4  is still equal to S CL (D,t 2 ). The first transition value S CL ( 1 ) is now lower than the calculated lamp control signal S CL (D,t 4 ), and consequently the actually outputted control signal S CL (A,t 4 ) is lower than S CL (A,t 3 ). 
         [0050]    Although the actually outputted control signal S CL (A,t 4 ) is reduced with respect to S CL (A,t 3 ), it is still higher than S AV , so the average S AV  is still increasing (arrow X 5 ), and consequently the maximum value S CL (max) is still decreasing (arrow X 6 ). It should be clear that, with the decreasing maximum value S CL (max), also the actually outputted control signal S CL (A,t 4 ) is decreasing, so that the rate of increase of the average S AV  is decreasing. 
         [0051]      FIG. 5E  illustrates that a steady state is reached when the actually outputted control signal S CL (A) is equal to the average S AV . When that happens, the average S AV  will be close to but lower than S REF . Thus, on average, the power consumed by the backlight does not exceed a predetermined power rating corresponding to S REF . 
         [0052]    The predetermined reference value S REF  can be a design parameter, or a parameter that can be set by the user. In one embodiment, the predetermined reference value S REF  can be equal to the original nominal design power of the backlight, indicated as 100%. However, in another embodiment the predetermined reference value S REF  can be set to a lower value, for instance 70%. In that case, occasional backlight boosting to values of 100% or more can be combined with the guarantee that the overall power consumption is reduced. Of course, the amount of backlight boost, in terms of percentage or duration, depends on the history of dark scenes as well as on the setting of the reference value S REF , as should be clear to a person skilled in the art. 
         [0053]    When the above-mentioned steady state is reached, i.e. when the actually outputted control signal S CL (A) is equal to the average S AV , backlight boosting is no longer possible. It can be said that the energy reserve is exhausted. Only when further dark scenes happen, the backlight is dimmed, as explained earlier, so that the average power consumption decreases. Simultaneously, the maximum value S CL (max) is increased, and backlight boosting becomes possible again. The lower the average power consumed over the recent time period is at the moment when backlight boosting is requested, the further the backlight intensity can be increased, or an the longer the increased intensity can be maintained. 
         [0054]    By decreasing the maximum value S CL (max), instead of simply reducing the actual backlight intensity, the result is that the boosting of the brightest scenes is limited first, whereas the less bright scenes can be boosted longer. 
         [0055]    It should be clear to a person skilled in the art that the present invention is not limited to the exemplary embodiments discussed above, but that several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. For instance, in  FIG. 3  the feedback loop  130  is shown as being external to the controller  120 , but the feedback loop  130  may alternatively be integral part of the controller  120 . 
         [0056]    It is noted that amending the maximum value S CL (max) can be done at predetermined time intervals, for instance 60 times per second, or continuously. 
         [0057]    In the above, it was mentioned that the controller  120  sets the maximum value S CL (max) proportional to the difference (S REF −S AV ). The function that describes the relationship between S CL (max) and the difference (S REF −S AV ) may be a linear, first order function. However, this function may also comprise second order or higher order terms. The function may also have a zero-th order term unequal to zero. 
         [0058]    In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.
A method for driving an image display device ( 110) comprising at least one backlight lamp ( 111) and a display screen ( 112) is described. The method comprises the steps of: receiving an image data signal (D); on the basis of the image data signal (D), calculating a content-related backlight control signal (S CL (D)) for the backlight lamp ( 111) for setting the intensity of the backlight; generating an average signal (S AV) that represents a time-average of the power consumed by the backlight lamp ( 111); comparing the average signal (S AV) with a reference signal (S REF); on the basis of the calculated content-related backlight control signal (S CL (D)), taking into account the result of the said comparison, generating an actual backlight control signal (S CL (A)) for the backlight lamp ( 111).