Abstract:
An image display device which has a display element that optically modulates an incident light for displaying an image. A lamp is a light source of the incident light to the display element. A lamp driver drives the lamp. A lamp power level calculator calculates a driving power level of the lamp corresponding to a level of an input video signal. A lamp temperature controller controls a lamp temperature depending on changes in the input signal from the lamp power level calculator so as to change the lamp temperature within a predetermined range in response to fluctuations of the input video signal. This image display device can further include a projection lens for projecting and displaying images formed in the display element on a screen.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to image display devices for displaying high-contrast video images.  
       BACKGROUND ART  
       [0002]     Projection-type display devices and direct-viewing liquid crystal display devices are popular image display devices employing a display element that spatially modulates light emitted from a light source (lamp) for displaying video images by means of either transmissive or reflective optical modulation.  
         [0003]      FIG. 20  shows a configuration of an example of a projection-type display device employing a transmissive liquid crystal panel.  
         [0004]     Each component in  FIG. 20  works as described below.  
         [0005]     Lamp  105  illuminates liquid crystal panel  115  as a light source. Projection lens  116  magnifies and projects a video image displayed on liquid crystal panel  116  onto screen  117 . Cooling fan  108  for controlling the lamp temperature cools down lamp  105  when it gets too hot.  
         [0006]     Lamp driver  104  drives lamp  105 . Fan control signal generator  107  drives cooling fan  108  for controlling the lamp temperature.  
         [0007]     Liquid crystal panel driver  114  converts and processes video signals into signals required for driving liquid crystal panel  115 , and displays images on liquid crystal panel  115 .  
         [0008]     The above components except for screen  117  are included in projector  118  (projection-type display device).  
         [0009]     In a projection television (PTV), screen  117  is placed inside the display device, and images are projected from the back of the screen. In the case of a direct viewing liquid crystal display device, projection lens  116  and screen  117  are excluded from the above configuration, and the user looks at images directly on the liquid crystal panel.  
         [0010]     Compared to an image display device employing a self-luminescent display element such as a CRT, black rise tends to occur more in dark portions in image display devices employing a display element for optical modulation such as liquid crystal panels.  
         [0011]     One proposed method for suppressing this black rise and improving the contrast of displayed images is to dynamically change the lighting luminance of the lamp to match the input video images. For example, the Japanese Laid-open Application Nos. H5-127608 and H6-160811 disclose this method.  
         [0012]     The Japanese Laid-open Application No. H5-127608 proposes the following method.  
         [0013]     The characteristic of an input video signal is detected from the maximum and minimum values of the input video signal. When the mean of the maximum and minimum values is higher than a predetermined threshold, the luminance of the lamp is reduced to maintain the luminance of the displayed image within a certain range.  
         [0014]     The Japanese Laid-open Application No. H6-160811 proposes the next method.  
         [0015]     The maximum value of the input video signal is detected. If the maximum value is higher than a predetermined threshold, the luminance of the lamp is increased. If the maximum value is lower than the predetermined threshold, the lamp luminance is decreased. In addition, the amplitude of the video luminance signal when the maximum value is low is reduced compared to the amplitude of the video luminance signal when the maximum value is high. This enhances the relative contrast of images between the high and low maximum values.  
         [0016]     As described above, when the luminance of the lamp, the light source, is dynamically controlled in accordance with the video signal, scenes with low or high lamp driving level are considered to continue for a long period in some input video sources.  
         [0017]     If this state occurs with a discharge type light source such as high-pressure mercury lamp or xenon lamp, the lamp bulb temperature increases or decreases out of the reliability-assured range.  
         [0018]     This causes so-called blackening or whitening by shading of the lamp bulb due to attachment of foreign particles or degradation of the lamp, resulting in reduced reliability of the lamp.  
       DISCLOSURE OF INVENTION  
       [0019]     An image display device of the present invention employs a display element that optically modulates the incident light for displaying images. A lamp is a light source of the incident light to the display element. A lamp driver drives the lamp. A lamp power level calculator calculates the driving power level of the lamp corresponding to the level of the input video signal. A lamp temperature controller controls the lamp temperature depending on changes in input signal from the lamp power level calculator so as to change the lamp temperature within a predetermined range in response to fluctuations of the input video signals.  
         [0020]     The image display device of the present invention can further include a projection lens for projecting and displaying images formed in the display element on a screen. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is a block diagram of an image display device in accordance with a first exemplary embodiment.  
         [0022]      FIG. 2  is a block diagram of the image display device in accordance with the first exemplary embodiment.  
         [0023]      FIG. 3  is a block diagram of an image display device in accordance with a second exemplary embodiment.  
         [0024]      FIG. 4  is a block diagram of the image display device in accordance with the second exemplary embodiment.  
         [0025]      FIG. 5  is a block diagram of an image display device in accordance with a third exemplary embodiment.  
         [0026]      FIG. 6  is a block diagram of the image display device in accordance with the third exemplary embodiment.  
         [0027]      FIG. 7  is a block diagram of the image display device in accordance with the third exemplary embodiment.  
         [0028]      FIG. 8  is a block diagram of an image display device in accordance with a fourth exemplary embodiment.  
         [0029]      FIG. 9  is a block diagram of an image display device in accordance with a fifth exemplary embodiment.  
         [0030]      FIG. 10  illustrates correlation between an input video signal and lamp driving power.  
         [0031]      FIG. 11  illustrates the operation of a lamp driving level signal integrator.  
         [0032]      FIG. 12  illustrates an operation of a lamp driving level corrector.  
         [0033]      FIG. 13  is another figure illustrating an operation of the lamp driving level corrector.  
         [0034]      FIG. 14  illustrates fan revolution control.  
         [0035]      FIG. 15  illustrates fan revolution control.  
         [0036]      FIG. 16  illustrates lamp driving level correction and fan revolutions number control.  
         [0037]      FIG. 17  illustrates lamp driving level correction and fan revolutions number control.  
         [0038]      FIG. 18  illustrates accumulated lamp ON time correction.  
         [0039]      FIG. 19  illustrates gamma correction in accordance with a lamp driving level.  
         [0040]      FIG. 20  illustrates configuration of an example of a conventional projection-type image display device employing a transmissive liquid crystal panel. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0000]     First Exemplary Embodiment  
         [0041]      FIGS. 1 and 2  are block diagrams illustrating configurations of an image display device in an exemplary embodiment of the present invention.  
         [0042]     FIGS.  10  to  13  illustrate the concept of the present invention.  
         [0043]     In  FIG. 1 , lamp  5  is a light source for illuminating liquid crystal panel  15 . Projection lens  16  magnifies and projects the image displayed on liquid crystal panel  15 . The image is projected on screen  17 .  
         [0044]     Liquid crystal panel driver  14  converts and processes the video signal into a signal required for driving liquid crystal panel  15 , and displays the image on liquid crystal panel  15 .  
         [0045]     The image display device of the present invention can also be a direct-viewing liquid crystal display device in which the user directly looks at the image displayed on liquid crystal panel  15 .  
         [0046]     In  FIG. 2 , lamp driving level signal integrator  9  and lamp driving level corrector  10  configure lamp driving level correcting unit  3 .  
         [0047]     The input video signal is input from video signal input terminal  1  to lamp power level calculator  2 . The output signal from lamp power level calculator  2  is input to lamp driving level signal integrator  9 . A signal integrated in lamp driving level signal integrator  9  and the output signal of lamp power level calculator  2  are input to lamp driving level corrector  10 . The lamp driving level signal from lamp driving level corrector  10  is input to lamp driver  4 . The output signal of lamp driver  4  drives lamp  5 .  
         [0048]     In this exemplary embodiment, lamp temperature controller  6  is composed of lamp driving level correcting unit  3 .  
         [0049]     The operation is described next in sequence from the input.  
         [0050]     Lamp power level calculator  2  calculates the lamp driving power level corresponding to the input video signal. For example, the maximum value of the luminance signal related to brightness of the input image is calculated.  
         [0051]     The operation of lamp power level calculator  2  is described next with reference to an example of correlation between the input video luminance signal and lamp driving power shown in  FIG. 10 .  
         [0052]     When the maximum value of the luminance signal per field is 100% of the input dynamic range, the maximum (MAX) lamp driving power is assigned. When the maximum value is 0% of the input dynamic range, the minimum (MIN) lamp driving power is assigned.  
         [0053]     When the luminance signal for gradation whose maximum value per field is between 100% and 0% of the input dynamic range is input, the lamp driving power is interpolated to a value between the maximum and minimum.  
         [0054]     Accordingly, the lamp driving power is increased to display a bright image when the level of input video signal is high for a bright scene. When a scene is judged to be dark, based on a low input signal level, the lamp power is decreased to display a dark image. This improves the contrast of the displayed images that the user notices when watching the images.  
         [0055]     Only the maximum value of the luminance level of the video signal is used in this exemplary embodiment for extracting the characteristic of the input video signal. However, the characteristic of the input video signal can also be determined based on the input RGB signal. The color signal information can also be included in the determination. Moreover, minimum value, mean value and distribution information such as the histogram of the video signal such as luminance signal can also be used.  
         [0056]     The lamp driving power level corresponding to the input video signal obtained as above is input to lamp driving level correcting unit  3 .  
         [0057]     Next, the operation of lamp driving level correcting unit  3  configuring lamp temperature controller  6  in this exemplary embodiment is described with reference to  FIG. 2 .  
         [0058]     First, the operation of lamp driving level signal integrator  9  in lamp driving level correcting unit  3  is described.  
         [0059]      FIG. 11  shows the change of the maximum value of the input video luminance signal versus time, which becomes the basis of the lamp driving level, between a predetermined reference time and time T 1 , a branch point of operation.  
         [0060]     Shaded portion  111  in this Figure is the integrated value obtained by time integration of the maximum value of the input video luminance signal between time  0  and TR 1 . This integrated value indicates the total lamp driving power in time T 1 .  
         [0061]     Lamp driving level signal integrator  9  calculates the area of this shaded portion  111 . At every T 1 , lamp driving level signal integrator  9  is reset, and the integrated value is re-calculated. Time T 1  is optimized depending on the lamp used. This integrated value  111  of the lamp driving power level at every fixed time T 1  and the output signal from aforementioned lamp power level calculator  2  are input to lamp driving level corrector  10 . The output signal from lamp driving level corrector  10  is input to lamp driver  4 . The output signal from lamp driver  4  drives lamp  5 .  
         [0062]     Next, the operation of lamp driving level corrector  10  in lamp driving level correcting unit  3  is described with reference to  FIGS. 12 and 13 .  
         [0063]     Changes in the lamp driving power versus time up to time T 1  in  FIG. 12  correspond to changes in the maximum value of the input video luminance signal up to T 1  in  FIG. 11 .  
         [0064]     When lamp driving level corrector  10  determines that the driving power level is higher than a predetermined threshold based on information input on integrated value of the lamp driving power level up to fixed time T 1 , lamp driving level corrector  10  reduces the lamp driving power until lamp  5  reaches the temperature at which reliability of lamp  5  is assurable.  
         [0065]     In the example shown in  FIG. 12 , the lamp driving power is reduced from MAX to MAX 1 . In this case, transition of the driving power from MAX to MAX 1  is preferably slow to the extent of preventing a sense of incongruity on the display screen. The temperature of lamp  5  can thus be reduced to where the reliability of lamp  5  is assurable by decreasing the lamp driving power level itself when the high lamp driving power level continues.  
         [0066]     When the lamp driving power stays low, the temperature of lamp  5  is considered to fall too low.  
         [0067]     In this case, as shown in  FIG. 13 , when the output of lamp driving level signal integrator  9  is smaller than the predetermined threshold, lamp driving level corrector  10  determines that the lamp driving power level up to time T 1  is low. Lamp driving level corrector  10  then increases the lamp driving power until lamp  5  rises to the temperature at which reliability of lamp  5  is assurable.  
         [0068]     In an example shown in  FIG. 13 , lamp driving level corrector  10  increases the lamp driving power from MIN to MIN 1 . The temperature of lamp  5  can thus be increased to where the reliability of lamp  5  is assurable by increasing the lamp driving power level itself when low lamp driving power continues.  
         [0069]     As described above, the lamp temperature is controlled in response to fluctuations of the input video signal and the lamp temperature is adjusted only within a predetermined range.  
         [0070]     The level for driving the lamp is calculated based on the maximum, minimum and mean values and distribution information, such as histogram, of the input video signal.  
         [0071]     The continuity of the low power driving level or high power driving level is detected respectively from the information on integrated value that is obtained by time integration of the lamp driving level over a fixed time. The lamp driving level is then shifted respectively to a higher power driving level or a lower power driving level. This prevents temperature drop or rise of the lamp bulb, and contributes to ensuring the reliability of the lamp, such as the life of lamp and preservation of the lamp luminance.  
         [0072]     In this exemplary embodiment, the output of lamp power level calculator  2  is used for obtaining information on the lamp driving power level over a fixed period. It is apparent that the video signal input from video signal input terminal  1  can also be used for this purpose.  
         [0000]     Second Exemplary Embodiment  
         [0073]      FIGS. 3 and 4  are block diagrams illustrating the configuration of an image display device in the second exemplary embodiment of the present invention.  
         [0074]      FIGS. 10, 11 ,  14  and  15  describe the concept of the present invention.  
         [0075]     The same reference numerals are given to the same components as those in  FIGS. 1 and 2  for the first exemplary embodiment of the present invention, and these components operate in the same way.  
         [0076]     In the second exemplary embodiment, lamp temperature control fan  8  cools down lamp  5 . Fan control signal generating unit  7  drives lamp temperature control fan  8 .  
         [0077]     Liquid crystal panel  15 , projection lens  16 , screen  17  and liquid crystal panel driver  14  shown in  FIG. 1  are omitted here.  
         [0078]     An input video signal from video signal input terminal  1  is input to lamp power level calculator  2 . The output signal of lamp power level calculator  2  is input to lamp driver  4  and fan control signal generating unit  7 .  
         [0079]     Information obtained by integral calculation using lamp driving level signal integrator  9  and the output signal of lamp power level calculator  2  are input to fan control signal generator  11 . The output signal of fan control signal generator  11  is input to lamp temperature control fan  8  for controlling the operation of control fan  8 . In this way, the lamp temperature is controlled.  
         [0080]     The output signal of lamp power level calculator  2  described above is input to lamp driver  4 . Lamp driver  4  drives lamp  5 .  
         [0081]     In the second exemplary embodiment, the lamp temperature controller  6  includes fan control signal generating unit  7  and lamp temperature control fan  8 .  
         [0082]     The operation is described next in sequence from the input.  
         [0083]     The input video signal from video signal input terminal  1  is input to lamp power level calculator  2 . Lamp power level calculator  2  calculates lamp driving power level corresponding to the input video signal.  
         [0084]     Lamp power level calculator  2  operates in a way as described previously using correlation between the input video luminance signal and lamp driving power shown in  FIG. 10 .  
         [0085]     In this exemplary embodiment, only the maximum value of the luminance level of the video signal is used for extracting the characteristic of the input video signal. However, the characteristic of the input video signal can be determined based on the input RGB signal. The color signal information can also be included in the determination. Moreover, the minimum value, mean value and distribution information of the luminance signal can be used.  
         [0086]     The lamp driving power level corresponding to the input video signal obtained as above is input to fan control signal generating unit  7 .  
         [0087]     Next, the operation of fan control signal generating unit  7  is described using an example in  FIG. 4 .  
         [0088]     Fan control signal generating unit  7  includes lamp driving level signal integrator  9  and fan control signal generator  11 .  
         [0089]     The operation of lamp driving level signal integrator  9  in fan control signal generating unit  7  is the same as that in the first exemplary embodiment.  
         [0090]      FIG. 11  shows the change of the maximum value of the input video luminance signal versus time, which becomes the basis of the lamp driving level, between a predetermined reference time and time T 1 , a branch point of operation. Accordingly, shaded portion  111  is the integrated value obtained by time integration of the maximum value of the input video luminance signal between time  0  and T 1 . This integrated value indicates the total lamp driving power in time T 1 .  
         [0091]     The integrated value of the lamp driving power level at every fixed time T 1 , the output from lamp driving level signal integrator  9 ; and the output signal from lamp power level calculator  2  are input to fan control signal generator  11 . The output signal from fan control signal generator  11  is input to lamp temperature control fan  8 , and the operation of lamp temperature control fan  8  is controlled. In this way, the lamp temperature is controlled.  
         [0092]     On the other hand, the output signal of aforementioned lamp power level calculator  2  is input to lamp driver  4 . Lamp driver  4  drives lamp  5 .  
         [0093]     The operation of fan control signal generator  11  is described with reference to  FIGS. 14 and 15 .  
         [0094]     In  FIG. 14 , line  142  shows the time transition of the lamp driving power. Changes in the lamp driving power versus time up to time T 1  in  FIG. 14  correspond to changes in the maximum value of the input video luminance signal up to T 1  in  FIG. 11 .  
         [0095]     Line  141  indicates changes in the number of revolutions of the temperature control fan versus time.  
         [0096]     When fan control signal generator  11  determines that the driving power level indicated by line  142  is higher than a predetermined threshold based on information input on integrated value of the lamp driving power level up to fixed time T 1 , fan control signal generator  11  increases the number of revolutions of the fan as shown by line  141  until lamp  5  reaches the temperature at which reliability of lamp  5  is assurable.  
         [0097]     In the example shown in  FIG. 14 , fan control signal generator  11  increases the number of revolutions of the fan from R to R 1 . In this case, it is preferable that transition of fan revolutions from R to R 1  is made in slow speed to the extent that the change cannot be noticed as noise. The temperature of lamp  5  can thus be reduced to where the reliability of lamp  5  is assurable by increasing the number of revolutions of the cooling fan when the high lamp driving power level continues.  
         [0098]     In  FIG. 15 , line  152  indicates the change in the lamp driving power versus time. Changes in the lamp driving power versus time up to time T 1  in  FIG. 15  correspond to changes in the maximum value of the input video luminance signal up to T 1  in  FIG. 11 .  
         [0099]     Line  151  indicates changes in the number of revolutions of the temperature control fan versus time.  
         [0100]     When the lamp driving power stays continuously low, the temperature of lamp  5  is considered to drop excessively. As shown in  FIG. 15 , when fan control signal generator  11  determines that the lamp driving power level up to fixed time T 1  is low, based on the fact that the output of lamp driving level signal integrator  9  indicated by line  152  is below a predetermined threshold, fan control signal generator  11  reduces the number of revolutions of the cooling fan until the temperature of lamp  5  reaches the temperature at which reliability of lamp  5  is assurable. In the example in  FIG. 15 , the number of revolutions of the fan indicated by line  151  is reduced from R to R 2 .  
         [0101]     As described above, fan control signal generator  11  reduces the number of revolutions of the cooling fan when low lamp driving power continues. The temperature of lamp  5  can thus be increased to where the reliability of lamp  5  is assurable.  
         [0102]     In this exemplary embodiment, the output of lamp power level calculator  2  is used for obtaining information on the lamp driving power level over a fixed period. It is apparent that video signal input from video signal input terminal  1  can also be used for this purpose.  
         [0000]     Third Exemplary Embodiment  
         [0103]     FIGS.  5  to  7  show block diagrams illustrating the configuration of an image display device in the third exemplary embodiment.  
         [0104]     The same reference numerals are given to the same components as those in FIGS.  1  to  4  for the first and second exemplary embodiments of the present invention, and these components operate in the same way.  
         [0105]     In  FIG. 5 , liquid crystal panel  15 , projection lens  16 , screen  17  and liquid crystal panel driver  14  shown in  FIG. 1  are omitted.  
         [0106]     The input video signal from video signal input terminal  1  is input to lamp power level calculator  2 . Lamp power level calculator  2  calculates the lamp driving level in accordance with the input video signal. Details of the operation of lamp power level calculator  2  are the same as those in the first and second exemplary embodiments. Lamp temperature controller  6  includes lamp power level correcting unit  3 , fan control signal generating unit  7  and lamp temperature control fan  8 .  
         [0107]     As shown in  FIG. 6 , lamp driving level correcting unit  3  includes lamp driving level signal integrator  9  and lamp driving level corrector  10 .  
         [0108]     Lamp power level calculator  2  operates in a way as described previously using correlation between the input video luminance signal and lamp driving power shown in  FIG. 10 .  
         [0109]     Only the maximum value of the luminance level of the video signal is used in this exemplary embodiment for extracting the characteristic of the input video signal. However, the characteristic of the input video signal can also be determined based on the input RGB signal. The color signal information can also be included in the determination. Moreover, minimum value, mean value and distribution information of the luminance signal can also be used.  
         [0110]     The lamp driving power level corresponding to the input video signal obtained in the way above is input to lamp driving level correcting unit  3 .  
         [0111]     The operation of lamp driving level correcting unit  3  and fan control signal generating unit  7  is described with reference to the examples in  FIGS. 6 and 7 .  
         [0112]     The operation of lamp driving level correcting unit  3  and lamp driving level signal integrator  9  in fan control signal generating unit  7  is the same as those in the first and second exemplary embodiments.  
         [0113]     As described previously,  FIG. 11  shows the change of the maximum value of the input video luminance signal versus time, which becomes the basis of the lamp driving level, between a predetermined reference time and time T 1 , a branch point of operation. Accordingly, shaded portion  111  is the integrated value obtained by time integration of the maximum value of the input video luminance signal between time  0  and T 1 . This integrated value indicates the total lamp driving power in time T 1 .  
         [0114]     The integrated value of the lamp driving power level at every fixed time T 1  which is the output from lamp driving level signal integrator  9  is output to lamp driving level corrector  10  and fan control signal generator  11 .  
         [0115]      FIG. 16  shows changes in the lamp driving power versus time. Changes in the lamp driving power versus time up to time T 1  in  FIG. 16  correspond to changes in the maximum value of input video luminance signal up to T 1  in  FIG. 11 .  
         [0116]     Line  161  indicates the change in the number of revolutions of the temperature control fan versus time.  
         [0117]     When it is determined that the driving power level is higher than a predetermined threshold based on information input on integrated value of the lamp driving power level up to fixed time T 1 , lamp driving level correcting unit  3  reduces the lamp driving power in a way shown by line  162  until lamp  5  reaches the temperature at which reliability of lamp  5  is assurable. At the same time, fan control signal generating unit  7  increases the number of revolutions of the temperature control fan in a way shown by line  161 .  
         [0118]     In the example shown in  FIG. 16 , the lamp driving power is reduced from MAX to MAX 1 , and the number of revolutions of the temperature control fan is increased from R to R 1 . The temperature of lamp  5  can thus be reduced to where the reliability of lamp  5  is assurable by decreasing the lamp driving power level itself and increasing the number of revolutions of the fan when the high lamp driving power level continues.  
         [0119]     When the lamp driving power stays low, the temperature of lamp  5  is considered to fall too low.  
         [0120]     In this case, as shown in  FIG. 17 , when the output of lamp driving level signal integrator  9  shown by line  172  is smaller than the predetermined threshold and the lamp driving power level up to fixed time T 1  is small, lamp driving level correcting unit  3  increases the lamp driving power in a way shown by line  172  until lamp  5  rises to the temperature at which its reliability is assurable. At the same time, the number of revolutions of the temperature control fan is reduced in a way shown by line  171 .  
         [0121]     In  FIG. 17 , the lamp driving power is increased from MIN to MIN 1 , and fan control signal generating unit  7  decreases the number of revolutions of the temperature control fan from R to R 2 . The temperature of lamp  5  can thus be increased to where the reliability of lamp  5  is assurable by increasing the lamp driving power level itself and decreasing the number of revolutions of the fan when the low lamp driving power level continues.  
         [0122]     In this exemplary embodiment, the output of lamp power level calculator  2  is used for obtaining information on the lamp driving power level over a fixed period. It is apparent that the video signal input from video signal input terminal  1  can also be used for this purpose.  
         [0000]     Fourth Exemplary Embodiment  
         [0123]      FIG. 8  is a block diagram illustrating the configuration of an image display device in the fourth exemplary embodiment.  
         [0124]     The same reference numerals are given to the same components as those in FIGS.  1  to  7  for the first to third exemplary embodiments, and these components operate in the same way.  
         [0125]     In  FIG. 8 , liquid crystal panel  15 , projection lens  16 , screen  17  and liquid crystal panel driver  14  in  FIG. 1  and lamp temperature control fan  8  and fan control signal generating unit  7  in FIGS.  3  to  7  are omitted.  
         [0126]     Lamp  5  is driven in accordance with the input video signal level. In addition, the output signal whose lamp driving level is corrected is output from lamp driving level correcting unit  3 , which is lamp temperature controller  6 , so as to maintain the temperature of lamp  5  in a range where its reliability is assurable when a low power driving state or high power driving state continues. This output signal is input to lamp driver  4 , and lamp driver  4  drives lamp  5 . At the same time, the output of lamp driving level corrector  10  is input to lamp ON time totalizer  12 . A signal for notifying whether the lamp is turned on or off is also input from lamp driver  4  to lamp ON time totalizer  12 . Lamp ON time totalizer  12  measures and totals the time that the lamp ON signal from lamp driver  4  is effective, and uses it as reference data for the lamp use time.  
         [0127]     In the exemplary embodiment, as shown in  FIG. 8 , the correction coefficient is determined in accordance with the lamp driving power level, and practical high-precision lamp use time is calculated by multiplying the use time by the correction coefficient. As a general rule, the correction coefficient is 1 or below during low power driving periods, and the coefficient is 1 or above during high power driving periods. This coefficient is optimized depending on the type of lamp used.  
         [0128]     In  FIG. 18 , ON time T′ after correction and accumulated ON time T before correction are represented by the following equation:
 
 T′=Ka·Ta+Kb·Tb+Kc·Tc ; and
 
 T=Ta+Tb+Tc, 
 
         [0129]     whereas  
         [0130]     time at around maximum driving power “A”, around average driving power “B” and around minimum driving power “C” are respectively Ta, Tb and Tc; and correction coefficient Ka, Kb and Kc are Ka&gt;1, Kb=1 and Kc&lt;1.  
         [0131]     If low power driving time occupies most of the time, as in  FIG. 18 , T′&lt;T. In this case, accumulated ON time after correction is sufficiently small compared to the accumulated ON time before correction. Accordingly, correction allows more practical calculation of a high-precision ON time for the lamp.  
         [0132]     Accurate measurement of lamp accumulated ON time allows estimation of the change in luminance versus time in line with the lamp accumulated ON time, and thus accurate lamp power supply corresponding to a change in luminance is enabled. This in turn extends the service life of the lamp.  
         [0133]     In addition, in recent years, the lamp is prevented from turning on when the lamp reaches its specified service life time, determined based on the lamp accumulated ON time measured, in equipment using a lamp as a light source, such as projectors. This ensures that the user replaces the lamp. The present invention is effective for improving the accuracy of this kind of measurement.  
         [0000]     Fifth Exemplary Embodiment  
         [0134]      FIG. 9  is a block diagram illustrating the configuration of an image display device in the fifth exemplary embodiment.  
         [0135]     The same reference numerals are given to the same components as those in FIGS.  1  to  8  for the first to fourth exemplary embodiments of the present invention, and these components operate in the same way.  
         [0136]     Liquid crystal panel  15  and liquid crystal panel driver  14  are given the same reference numerals as the liquid crystal panel and liquid crystal panel driver described in  FIG. 1 . Projection lens  16  and screen  17  in  FIG. 1 , and lamp temperature control fan  8  and fan control signal generating unit  7  shown in FIGS.  3  to  7  are omitted.  
         [0137]     The input video signal from video signal input terminal  1  is input to lamp power level calculator  2 . Lamp power level calculator  2  calculates the lamp driving power level in accordance with the input video signal.  
         [0138]     As described previously, the lamp power level calculator calculates the lamp driving power level corresponding to the input video signal from an example of the correlation between the input video luminance signal and lamp driving power shown in  FIG. 10 .  
         [0139]     The lamp driving power level corresponding to the input video signal obtained in this way is input to lamp driver  4 . Lamp driver  4  drives the lamp in the same way as that in the first exemplary embodiment. Lamp driver  4  drives lamp  5  by its output. The light emitted using lamp  5  as a light source is spatially modulated on liquid crystal panel  15 , which is a display element having transmissive or reflective optical modulation, and an image is displayed.  
         [0140]     At this point, the driving power level of lamp  5  changes in accordance with the input video signal. The characteristic of liquid crystal panel  15  changes according to its energy, and thus the gamma curve of liquid crystal panel  15  changes. This is considered to make the displayed image different from the original image.  
         [0141]     To prevent this from happening, the output from lamp power level calculator  2  and video signal input terminal  1  is input to gamma corrector  13 . Gamma corrector  13  changes the gamma curve in line with the lamp driving level.  
         [0142]     Accordingly, gamma corrector  13  corrects the gamma of the input video signal in accordance with the lamp driving level, and outputs the video signal whose gamma is corrected. Liquid crystal panel driver  14  displays an image based on the video signal after the gamma is corrected on liquid crystal panel  15 .  
         [0143]     Accordingly, variations in the gamma characteristic due to changes in lamp luminance are corrected. This makes grayscale expression on the liquid crystal display stable independent of lamp luminance.  
         [0144]     Variations in gamma characteristic due to lamp luminance are thus corrected, and an image based on the input video signal is displayed.  
         [0145]      FIG. 19  shows gamma curve  192  when the lamp is driven at a low power level and gamma curve  191  when the lamp is driven at a high power level. In this way, the gamma curve changes in line with the driving power level of the lamp.  
         [0146]     Here, gamma corrector  13  can switch the gamma curve, based on a look-up table, to the degree required in accordance with the lamp driving power level. The look-up table stores the gamma curve for each lamp driving power level.  
         [0147]     Alternatively, gamma corrector  13  can create a gamma curve by storing typical gamma curves and compensating only for required steps. Here, the gamma curve is optimized in line with the characteristic of the liquid crystal panel used.  
         [0148]     The present invention describes an example of lamp luminance modulation for changing lamp luminance immediately corresponding to the video signal. The present invention is also applicable to fixed high power driving mode and fixed low power driving mode for the lamp.  
         [0149]     The above description centers on projection display devices. However, it is apparent that the present invention is applicable to other types of display devices employing screens, such as projection TVs (PTVs), in which the image is projected from the back of the screen; direct-viewing display devices in which the user directly sees the liquid crystal panel; display devices using the lamp as a light source; and lighting units using the lamp as a light source.  
       Industrial Applicability  
       [0150]     In display devices employing a transmissive or reflective display element in which the lamp luminance is modulated in response to the input video signal, the present invention maintains the temperature of the lamp bulb, which fluctuates in accordance with the video signal, within a range over which the reliability is assurable. This improves the contrast of the image display devices, prevents shortening of the service life of the lamp and prevents reduction of lamp luminance. Moreover, the image display device of the present invention displays high-contrast images by controlling lamp luminance and video luminance.