Abstract:
Aspects of the invention provide a projector capable of comparatively easily achieving a contrast ratio higher than the original contrast ratio of the liquid-crystal light valve by a simple mechanism. The illumination light modulated by liquid-crystal light valves, i.e., image light, can be combined together in a cross dichroic prism, and then light intensity of suitable pixels is reduced by a proper amount by a liquid-crystal light valve, and then sent to a projection lens. The image light entering the projection lens can be projected to a projection surface. Because the light intensity of the image light formed by the liquid-crystal light valves is appropriately attenuated at suitable pixel areas by the liquid-crystal light valve, image light projection with a contrast ratio exceeding by far the contrast ratio achievable by the liquid-crystal light valves alone is possible due to cumulative light-intensity adjustment.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field of Invention  
         [0002]     Aspects of the invention relate to a projector for projecting an image by use of a liquid-crystal light valve and other light modulating devices.  
         [0003]     2. Description of Related Art  
         [0004]     Some related art projectors form a color image by combining, through a special prism, images of respective colors separately formed by sending three colors, RGB, of illumination light to three liquid-crystal light valves, for example. See, for example, JP-A-2002-365720. The contrast ratio achievable on such related art projector, at present, has the upper limit of approximately 1000:1.  
         [0005]     Meanwhile, on the CRT or a film, a contrast ratio of 3000:1 or higher is obtained, depending on the viewing environment. Incidentally, according to movie directors and movie critics, where contrast ratio is concerned the ultimate target images desired for movies, require a 5000:1 ratio. Taking account of film overshoot (peaks), nearly 6000:1 is considered required in the future.  
         [0006]     In the related art projector as above, the black portion brightness can be reduced by regulating the voltage applied to the lamp which is the light source. However, usually, the amount of light can be reduced only to 80% of maximum because of lamp characteristics.  
         [0007]     Meanwhile, there is a proposal that an illumination light amount adjusting device be arranged between the light source and the liquid-crystal light valve, as another method to improve the dynamic range of projection images. See, for example, JP-A-2001-100699. Here, the illumination light amount adjusting device can have a polarization plate for achieving a desired illumination light amount by rotation to a certain direction and a ultrasonic motor for regulating the rotation.  
       SUMMARY OF INVENTION  
       [0008]     However, with a related art projector in which the illumination light amount adjusting device is arranged between the light source and the liquid-crystal light valve, the illumination light amount adjusting device tends to be large sized because of the mechanical mechanism etc. used, and operation is not easy to control. Particularly, illumination light amount cannot be adjusted at high speed because of response time limitation of the mechanical mechanism, and the like. Furthermore, because illumination light amount cannot be adjusted minutely on a pixel unit basis, it is impossible to increase the contrast ratio, the light-intensity difference within one projection surface.  
         [0009]     Therefore, an aspect of the invention can provide a projector capable of easily achieving a contrast ratio higher than the contrast ratio resulting from the nature of the liquid-crystal light valve alone, by use of a simple mechanism.  
         [0010]     An exemplary projector according to the invention can include an illuminating device having a light source, forming illumination light by utilization of the light source, a main light modulating device to be illuminated by the illumination light from the illuminating device and for modulating the illumination light to form image light, an auxiliary light modulating device for attenuating, by modulation, light intensity of the image light formed by the main light modulating device, a projection optical system for projecting the image light formed by the main light modulating device and auxiliary light modulating device to a projection surface, and a signal processing circuit for generating, from an inputted image signal, a drive signal to the main light modulating device and auxiliary light modulating device which increases the contrast ratio of the image light projected, by harmonizing operation of the main light modulating device and the auxiliary light modulating device. Although contrast ratio, in principle means the maximum light-intensity ratio among the pixels in a display face that can be expressed, here it means the maximum light-intensity ratio between the pixels within one projection surface or the maximum light-intensity ratio between a plurality of continuing projection surfaces.  
         [0011]     In the above projector, because the auxiliary light modulating device attenuates the light intensity of image light formed by the main light modulating device by modulation, light intensity can be adjusted cumulatively by the main and auxiliary light modulating devices. In this case, the main and auxiliary light modulating devices are operated harmoniously by the signal processing circuit, making it possible to project image light at a contrast ratio exceeding by far the contrast ratio that can be realized by the main light modulating device singly.  
         [0012]     In an example of the invention, in the projector, the illuminating device generates different three colors of illumination light, the main light modulating device having three color-light modulating devices illuminated respectively by the three colors of illumination light and separately modulating the respective colors of illumination light, and a light-combining member for combining together image light of the respective colors from the three color-light modulating devices. In this case, the three colors of illumination light can be modulated separately and combined together, thus making possible to project a color image high in brightness.  
         [0013]     In an example form of the invention, in the projector, the auxiliary light modulating device can be arranged between the light combining member and the projection optical system. In this case, the auxiliary light modulating device for attenuation is not provided for the color-light modulating device for each color but the light intensity of image light that has passed through the main light modulating device is attenuated by a single auxiliary light modulating device, such as a single liquid-crystal light valve, thus making the auxiliary light modulating device or the projector cheap in price.  
         [0014]     In another example of the invention, the three color-light modulating devices and the auxiliary light modulating device are all liquid-crystal light valves having the same structure. In this case, the auxiliary light modulating device can be made with a simple structure matched to the color-light modulating device.  
         [0015]     In another example of the invention, the signal processing circuit determines drive signals to be sent to the main and auxiliary light modulating devices by making a predetermined operation on the inputted signals of respective colors. In this case, a desired contrast can be achieved by this processing of the image signals of respective colors.  
         [0016]     In another example of the invention, the signal processing circuit can determine the drive signal to be sent to the auxiliary light modulating device by referring to the maximum value of the signals of respective colors in the inputted image signal. In this case, the main light modulating device can express the intensity level of each color accurately while the auxiliary light modulating device can achieve a desired light reduction, making possible to project an image high in contrast and image quality.  
         [0017]     In another example of the invention, the signal processing circuit can use signals of respective colors in the inputted image signal as they are as the drive signals to the main light modulating device, and the drive signal to the auxiliary light modulating device is determined by making a predetermined operation on the signals of respective colors in the inputted image signal. In this case, a desired contrast can be achieved by processing the image signals of respective colors. Besides, operation processing is satisfactorily performed on the auxiliary light modulating device only. Thus, the burden of image processing is lessened.  
         [0018]     In another example of the invention, the auxiliary light modulating device can modulate the light intensity of image light one unit at a time, the unit being at least one pixel in the main light modulating device. In this case, the intensity modulation by the auxiliary light modulating device can be made on the basis of pixel units in the main light modulating device. Otherwise, the intensity modulation by the auxiliary light modulating device can be made collectively on the unit basis of a plurality of pixels of the main light modulating device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The invention will be described with reference to the accompanying drawings, wherein like numerals reference like elements, and wherein:  
         [0020]      FIG. 1  shows an exemplary block diagram explaining a projector according to one embodiment of the invention;  
         [0021]      FIG. 2  shows an exemplary diagram explaining a modification of a projector shown in  FIG. 1 ; and  
         [0022]      FIG. 3  shows a graph explaining contrast ratio increase by the projector shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0023]      FIG. 1  is an exemplary view explaining the construction of a projector according to an embodiment of the invention. The projector  10  can include a light source  21  for generating source light, a light-splitting system  23  for splitting the source light from the light source  21  into three colors of RGB, a main light modulator  25  illuminated by the respective colors of light exiting from the light-splitting system  23 , a light-combining optical system  27  for combining the respective colors of image light from the main light modulator  25 , an auxiliary light modulator  28  which is arranged on the light-exit side of the light-combining optical system  27  and which carries out two-dimensional light intensity modulation on the image light, and a projection lens  29  for projecting the image light passed the auxiliary light modulator  28  to a projection surface. Furthermore, the projector  10  can include a signal-processing circuit  40  for operating the light modulators for respective colors, etc. built in the main light modulator  25  on the basis of image signals inputted from outside, and a control unit  50  for suitably operating the light source  21 , the light modulators  25 ,  28 , the signal-processing circuit  40 , etc. to thereby totally control the overall operations of the projector  1 .  
         [0024]     The light source  21  has a light-source lamp  21   a,  a pair of fly-eye optical systems  21   b,    21   c,  a polarization conversion member  21   d  and a superimposing lens  21   e.  Here, the light-source lamp  21   a  can include a high-pressure mercury lamp, for example, and has a concave mirror for collimating the source light. Meanwhile, the pair of fly-eye optical systems  21   b,    21   c  comprises a plurality of element lenses in a matrix arrangement. By these element lenses, the source light from the light-source lamp  21   a  is split and separately focused/dispersed. The polarization conversion member  21   d  converts the source light exiting from the fly-eye optical system  21   c  into a particular polarization component and supplies it to the next-stage optical system. The superposed lens  21   e  suitably focuses the entire illumination light that has passed the polarization conversion member  21   d  and enables superimposed illumination onto the spatial light modulators provided for the respective colors. Namely, the illumination light that has passed both the fly-eye optical systems  21   b ,  21   c  and the superposed lens  21   e  focused so as to be superimposed on the liquid-crystal light valves  25   a - 25   c  for respective colors provided in the main light modulator  25  through the light splitting system, detailed below. [ 00231  The light splitting system  23  has first and second dichroic mirrors  23   a ,  23   b , three field lenses  23   f ,  23   g ,  23   h  and reflection mirrors  23   i ,  23   j ,  23   k , and constitute a illuminating device together with the light source  21 . The first dichroic mirror  23   a  reflects R-light of the three colors RGB and allows G-light and B-light to pass through. Meanwhile, the second dichroic mirror  23   b  reflects G-light of the two remaining colors GB and allows B-light to pass through. In this light splitting system  23 , R-light reflected by the first dichroic mirror  23   a  is reflected by the reflection mirror and enters the field lens  23   f  for incident angle adjustment. Meanwhile, G-light that passed the first dichroic mirror  23   a  and was reflected by the dichroic mirror  23   b  enters the field lens  23 g. Furthermore, B-light that passed through the second dichroic mirror  23   b  passes through relay lenses LL 1 , LL 2  for optical-path difference compensation, is reflected by reflection mirrors  23   j ,  23   k , and enters the field lens  23   h  for incident angle adjustment.  
         [0025]     The main light modulator  25  can include the following devices: three liquid-crystal light valves  25   a - 25   c  which are color-light modulating devices, and three sets of polarization filters  25   e - 25   g  arranged to sandwich the respective light valves  25   a - 25   c . R-light, reflected by the first dichroic mirror  23   a , passes through the field lens  23   f  to the liquid-crystal light valve  25   a , illuminating it uniformly. G-light, that passes through the first dichroic mirror  23   a  and was reflected by the second dichroic mirror  23   b , passes through the field lens  23   g  to the liquid-crystal light valve  25   b , illuminating it uniformly. B-light, that passes through both the first and second dichroic mirrors  23   a ,  23   b , passes through the field lens  23   h  to the liquid-crystal light valve  25   c , illuminating it uniformly. The liquid-crystal light valves  25   a - 25   c  are non light-emission type display devices for modulating the spatial or two-dimensional light intensity distribution of incident illumination light. The three colors of light respectively incident on the liquid-crystal light valves  25   a - 25   c  are modulated according to a drive signal inputted as an electric signal to the liquid-crystal light valves  25   a - 25   c.  On this occasion, the polarization filter  25   e - 25   g  regulates the polarization direction of the illumination light entering the liquid crystal light valve  25   a - 25   c  and extracts modulated light of a predetermined polarization direction out of the modulated light exiting the liquid-crystal light valve  25   a - 25   c.    
         [0026]     The cross dichroic prism  27  is a light-combining member incorporating a dielectric multi-layer film  27   a  for R-light reflection and a dielectric multi-layer film  27   b  for B-light reflection orthogonal to each other. The dielectric multi-layer film  27   a  in the cross dichroic prism  27  reflects R-light from the liquid-crystal light valve  25   a  so that the light turns toward the right and exits, allows G-light from the liquid-crystal light valve  25   b  to travel straight through the dielectric multi-layer films  27   a  and  27   b  and exit, and the dielectric multi-layer film  27   b  reflects B-light from the liquid-crystal light valve  25   c  so that it turns toward the left and exits.  
         [0027]     The auxiliary light modulator  28  has, as an auxiliary light modulating device, a liquid-crystal light valve  28   a  for light modulation and a pair of polarization filters  28   h  arranged in a manner sandwiching the liquid-crystal light valve  28   a . The liquid-crystal light valve  28   a  has a similar structure to the liquid-crystal light valve  25   a - 25   c  provided in the main light modulator  25 . The pair of polarization filters  28   h  has a similar structure to the pairs of polarization filters  25   e - 25   g  sandwiching the liquid-crystal light valve  25   a - 25   c . By the auxiliary light modulator  28 , the image light of combined light combined by the cross dichroic prism  27  can be attenuated on a pixel unit basis, particularly in the lower intensities, making it possible to supplementally increase the contrast in the projection image. For this reason, a special drive signal which is generated based upon drive signals, etc. inputted to the liquid-crystal light valves  25   a - 25   c  for respective colors is inputted in the liquid-crystal light valve  28   a.    
         [0028]     The projection lens  29  of the projection optical system projects the images from the liquid-crystal light valves  25   a - 25   c  for respective colors to the projection surface together with the image from the liquid-crystal light valve  28   a  a contrast-adjusting two-dimensional filter. Consequently, the focal point to the rear of the projection lens  29  is deepened in focal depth. Incidentally, as shown in  FIG. 2 , it is possible to arrange a refocus lens  160  between the light-combining cross dichroic prism  27  and the contrast-adjusting light valve  28   a , for focusing the images from liquid-crystal light valves  25   a - 25   c  at the liquid-crystal light valve  28   a . In the above, by somewhat defocusing the image on the liquid-crystal light valve  28   a  side, there arises an advantage that the black matrix on the liquid-crystal light valve  28   a  side becomes less visible.  
         [0029]     The signal processing circuit  40  has an image processing circuit  41  for carrying out a suitable signal processing on an image signal inputted from the outside and obtaining an image data signal in a digital format for example, and a liquid-crystal drive circuit  43  for generating a drive signal for operating the liquid-crystal light valves  25   a - 25   c , etc. provided in the main light modulator  25  on the basis of the image data signal outputted from the image processing circuit  41 . To the image processing circuit  41 , the image signal inputted is a video image signal, etc. from a video reproducing apparatus connected externally. In the image processing circuit  41 , on the basis of an input signal such as a video image signal, generated is an image data signal serving as a basis to operate the liquid-crystal light valves  25   a - 25   c  in the main light modulator  25 , and also generated is an auxiliary signal for operating the liquid-crystal light valve  28   a  in the auxiliary light modulator  28 . The liquid-crystal drive circuit  43  generates a drive signal for operating the liquid-crystal light valves  25   a - 25   c ,  28   a  on the basis of an image data signal, etc. generated by the image processing circuit  41 , and outputs it to these liquid-crystal light valves  25   a - 25   c ,  28   a.    
         [0030]     The control unit  50  outputs a control signal to the signal processing circuit  40  and controls indirectly the operation state of the liquid-crystal light valves  25   a - 25   c ,  28   a . The signal processing circuit  40  generates the above image data signal and auxiliary signal, on the basis of an image signal inputted from the outside and an instruction signal from the control unit  50 . Specifically, prepared are a main image data signal for color display directed for the liquid-crystal light valves  25   a - 25   c  for respective colors, and an auxiliary image data signal for contrast increase directed for the liquid-crystal light valve  28   a.    
         [0031]     In the below, explanation is made on a calculation processing method of an image data signal, etc. in the control unit  50  and signal processing circuit  40 .  
         [0032]     In the first method, in the signal processing circuit  40  are prepared main image data signals for driving the liquid-crystal light valves  25   a - 25   c  for respective colors, which are corrected so as to enhance the light intensity to the upper limit. Furthermore, in the signal processing circuit  40 , prepared is an auxiliary image data signal for driving the liquid-crystal light valve  28   a  for compensating for the correction in the main image data signal and returning the entire light intensity to the former light intensity.  
         [0033]     Here, a particular pixel of an image frame will be considered in order to make a concrete explanation. The digitized versions of the inputted RGB image signals are named SR, SG and SB, and the maximum value among them is named PMAX. Here, in the case that the maximum value PMAX is that of the image data signal SR, SR=PMAX. Correction is made after setting the most intense R-light image data signal to be the upper limit value. Namely, of the main image data signals, the R-light signal corresponding to R-light is made SR′, and the R-light signal SR′ is changed to the upper limit value. 
 
SR′=100(%)   (1) 
 
         [0034]     By adjusting the auxiliary image signal correspondingly to this, transmissivity is reduced at the corresponding pixel of the liquid-crystal light valve  28   a . Namely, taking the light-reducing signal corresponding to the auxiliary image data signal as SY, the following is set. 
 
SY=SR   (2) 
 
         [0035]     Namely, the light-reducing signal SY corresponds to the initial image signal of R-light. Meanwhile, the G-light signal of the main image data signals corresponding to G-light is made SG′, and the following is set in order to compensate for transmissivity decrease at the corresponding pixel of the liquid-crystal light valve  28   a  and keep color balance. 
 
SG′=SG/SR   (3) 
 
         [0036]     Likewise, the B-light signal of the main image data signals corresponding to B-light is made SB′, and the following is set. 
 
SB′=SB/SR   (4) 
 
         [0037]     An example of the data conversion as above with concrete numerals is summarized in Table 1 below.  
                                                                                                                                                   TABLE 1                                           Light intensity           Light intensity               before light           after light           Actual input   reduction   Input to panel   Output (total)   reduction                SR   SG   SB   LR0   SR′   SG′   SB′   SY   R   G   B   LR1                        White   100   100   100   100.00   100   100   100   100   100   100   100   100.00           100   50   50   100.00   100   50   50   100   100   50   50   100.00           100   20   10   100.00   100   20   10   100   100   20   10   100.00           100   0   0   100.00   100   0   0   100   100   0   0   100.00           50   30   20   50.08   100   60   40   50   50   30   20   50.08           40   10   20   40.10   100   25   50   40   40   10   20   40.10           20   0   5   20.13   100   0   25   20   20   0   5   20.13       Black   0   0   0   0.17   0   0   0   0   0   0   0   0.00                  
 
         [0038]     In the above Table 1, the R-light intensities LR 0 , LR 1  before and after light reduction are expressed in a standardized arbitrary unit. As apparent from the Table, the signals in respective colors SR′, SG′, SB′ maintain their relative light intensity ratios, and a total light-intensity increase caused by the respective color signals SR′, SG′, SB′ is offset by a light reducing signal SY. Namely, the image light passing the liquid-crystal light valves  25   a - 25   c  and liquid-crystal light valve  28   a  as a whole has the former light intensity and tone, but also is an integration of the maximum light-reduction degree executed by the liquid-crystal light valves  25   a - 25   c  and the maximum light-reduction degree executed by the liquid-crystal light valve  28   a , at the darkest black signal level. Explaining it with a concrete example, in the case that the maximum contrast ratios (light-reduction degrees) according to the specifications when operating the liquid-crystal light valves  25   a - 25   c  and  28   a  are all 600: 1, the maximum contrast ratio obtainable is 360000:1. However, the characteristic of the liquid-crystal light valve  28   a  may be set to obtain a contrast ratio of approximately 10:1 instead of the maximum value according to the specifications, to thereby make the maximum obtainable contrast ratio approximately 6000:1. In this case, the drive voltage range of the liquid-crystal light valve  28   a  is adjusted, or the characteristic or angular direction is adjusted of the polarization filter  28   h  arranged on the light exit side of the liquid-crystal light valve  28   a.    
         [0039]     In the second method, the signal processing circuit  40  utilizes the input image signal as it is as a main image data signal for driving the liquid-crystal light valves  25   a - 25   c  for respective colors, but it also prepares a signal in which the light intensity of the main image data signals of low-light intensity pixels is further reduced commensurate with their original intensity, as an auxiliary image data signal for driving the liquid-crystal light valve  28   a.    
         [0040]     For explanation purposes, all the pixels in an image frame of a certain frame will be considered. Here, the upper limit value (corresponding to transmissivity 100%) of the light intensity signals SY 0  at the pixels is named TMAX, and if there exists a pixel corresponding to the upper limit value TMAX, then light reduction is not performed on this image data signal. Instead, the maximum value TMAX is made a standard in a filtering process for light intensity reduction that is performed on the other pixels through the liquid-crystal light valve  28   a , or else, a filtering process for light intensity reduction can be made through the liquid-crystal light valve  28   a , with the maximum value of the light intensity signals SY 0  being made a standard. Namely, by applying such a filtering as to further increasing the darkness of a dark pixel, the light intensity lower limit is extended and the contrast ratio is increased. Incidentally, the light intensity signal SY 0  can use a value determined by operation from the light tone values SR, SG, SB of R, G and B color signals (specifically, the general relationship between a color signal and a light-intensity signal SY 0 =0.3SR+0.6SG+0.1SB).  
         [0041]     Here, an exemplary operation processing method is explained in the case of using the upper limit value mentioned above. It is assumed that the maximum light intensity adjustment range of each of the first-stage liquid-crystal light valves  25   a - 25   c  is a 0 , the light intensity at the lower limit value is b 0 , the maximum light intensity adjustment range over both the first-stage liquid-crystal light valves  25   a - 25   c  and the next-stage liquid-crystal valve  28   a  is a 1 , and the light intensity at the lower limit value is b 1 . Meanwhile, the light intensity data of a particular pixel is made SY 0  (expressed as a percentage). Furthermore, the contrast ratio among all the liquid-crystal light valves  25   a - 25   c  is made C 0 , and the contrast ratio of the liquid-crystal light valve  28   a  is C 1 . In this case, the following relationship is fulfilled: 
 
 a   0 + b   0 = a   1 + b   1    (5) 
 
 C   0 =( a   0 + b   0 )/ b   0    (6) 
 
 C   1 = b   0 / b   1    (7). 
 
         [0042]     For reference, in the graph of  FIG. 3  are visually shown the significances of the references a 0 , b 0 , a 1 , b 1  used in the above expressions. In the graph, the axis of ordinate represents light intensity of illumination light or image light incident on the liquid-crystal light valve  25   a - 25   c  or the like. From the above equations (5)-(7), the extension ratio of light intensity range RE=a 1 /a 0  is given as: 
 
 RE =( C   0 −1 /C   1 )÷( C   0 −1)   (8). 
 
         [0043]     Here, the light intensity after passage of the first-stage liquid-crystal light valves  25   a - 25   c  (light intensity before light reduction) L 0  depends upon the light intensity data provided to the liquid-crystal light valve  25   a - 25   c , as in the formula: 
 
 L   0 =( a   0  /100) SY   0 + b   0 =( a   0 /100) SY   0 + a   1 − a   0 + b   1    (9). 
 
         [0044]     At the liquid-crystal light valve  28   a , light-reduction process is made on this L 0  to thereby obtain the final light intensity L 1  (light intensity after light reduction) by which the relationship between tones is kept within the light intensity adjustment range al. This L 1  is changed to: 
 
 L 1=( a   1 /100) SY   0 + b   1    (10). 
 
         [0045]     Finding the conversion ratio F=L 1 /L 0  of the liquid-crystal light valve  28   a  (note that computation is made expediently removing the lowest baseline light intensity b 1  which is left even if light is made to pass completely through the liquid-crystal light valve  28   a , on the assumption that, in the liquid-crystal light valve  28   a , transmissivity changes linearly in accordance with the input signal), the following results: 
 
 F=RE×SY   0 ÷{ SY   0 +100×( RE −1)}  (11) 
 
         [0046]     By converting this into percentage, the transmissivity TR (percentage) through the liquid-crystal light valve  28   a  is calculated as:  
                   TR   =     100   ⁢   F                 =     100   ⁢   RE   ×   SY   ⁢           ⁢     0   ÷       {       SY   ⁢           ⁢   0     +     100   ×     (     RE   -   1     )         }     .                       (   12   )             
 
         [0047]     Thus, the light intensity data SY 1  of the liquid-crystal light valve  28   a  can be easily calculated from the light intensity data SY 0  at a particular pixel, expressed as transmissivity TR (percentage). Incidentally, the light intensity of the light actually passing through the liquid-crystal light valve  28   a  is calculated by multiplying transmissivity TR with and adding light intensity b 1  to the light intensity of illumination of the liquid-crystal light valve  28   a.    
         [0048]     The data conversion as above was exemplified with concrete numerals, which is summarized in the below Table 2.  
                                                                                                                               TABLE 2                                           Light intensity       Light intensity               before light       after light           Actual input   reduction   Input to panel   reduction                SR   SG   SB   SY0   L0   SR   SG   SB   ST1   L1                        White   —   —   —   100   100.00   —   —   —   100.00   100.00           —   —   —   80   80.03   —   —   —   99.96   80.00           —   —   —   60   60.07   —   —   —   99.89   60.01           —   —   —   40   40.10   —   —   —   99.75   40.01           —   —   —   20   20.13   —   —   —   99.34   20.01           —   —   —   5   5.16   —   —   —   96.94   5.02           —   —   —   1   1.17   —   —   —   85.86   1.02       Black   —   —   —   0   0.17   —   —   —   0.00   0.02                  
 
         [0049]     In the above Table 2, the light intensity L 0 , L 1  before and after light reduction is expressed in a standardized arbitrary unit. As apparent from the table, the light intensity data SY 1  for the liquid-crystal light valve  28   a  for light reduction can be calculated from the light intensity data of image data signals of the liquid-crystal light valves  25   a - 25   c  for respective colors. Namely, the image light passing the liquid-crystal light valves  25   a - 25   c  and liquid-crystal light valve  28   a , although as whole being an image maintaining the former tone, is an image having a high contrast ratio in which the darker the pixel, the more the blackness is increased. In other words, it is possible to achieve a contrast ratio which the cumulated value of the maximum light-reduction degree due to the liquid-crystal light valves  25   a - 25   c  and the maximum light-reduction degree due to the liquid-crystal light valve  28   a . Concretely, the liquid-crystal light valves  25   a - 25   c  in operation has a contrast ratio (light-reduction degree) of 600:1, the maximum value according to the specifications. In the case that the liquid-crystal light valves  25   a - 25   c  in operation has a contrast ratio (light-reduction degree) of 10:1, the maximum contrast ratio obtainable is given 6000:1.  
         [0050]     Incidentally, in the case that the liquid-crystal light valve  28   a  has specification of a contrast ratio of 600:1 similarly to the others, the drive voltage range on the liquid-crystal light valve  28   a  can be adjusted so as to obtain a contrast ratio of 10:1. Alternatively, a similar effect is obtainable by such measures as adjusting the characteristic or angular direction of the polarization filter  28   h  arranged on the light-exit side of the liquid-crystal light valve  28   a.    
         [0051]     The image data signal calculation processing method in the signal processing circuit  40 , etc. is not limited to the above. For example, the light-intensity data signals SR′, SG′, SB′ and light-intensity data signals SR, SG, SB serving as signals for operating the liquid-crystal light valves  25   a - 25   c  for respective colors may be subjected to a suitable nonlinear correction for enhancing the difference of lightness/darkness, etc. Likewise, the light-intensity data signals SY 0 , SY 1  for operating the liquid-crystal light valve  28   a  may be subjected to a suitable nonlinear correction for enhancing the difference of lightness/darkness, etc.  
         [0052]     Further, in the above calculation process, the light-intensity data signal SY 1  or the like does not need to be calculated in real time. By preparing ahead of time a conversion table for assigning a suitable light-intensity levels to intensity levels in a set range, the light-intensity data signal SY 1  can be determined by looking up the value read from the present light intensity signal SY 0  in the conversion table.  
         [0053]     Here explanation is made of the overall operation of the projector  10  according to the exemplary embodiment. The source light from the light source  21  is color-split by the first and second dichroic mirrors  23   a ,  23   b  provided in the light splitting system  23 , and sent as illumination light respectively to the corresponding liquid-crystal light valves  25   a - 25   c . The liquid-crystal light valves  25   a - 25   c  are modulated by the image signal so as to have a two-dimensional refraction-index distribution to in turn spatially modulate the illumination light on a pixel unit basis. In this manner, the illumination light modulated by the liquid-crystal light valves  25   a - 25   c , i.e., image light, is combined at the dichroic prism  27 , reduced in intensity by a suitable amount at appropriate pixels by the liquid-crystal light valve  28   a , and then sent to the projection lens  29 . The image light entering the projection light  29  is projected onto a (not-shown) projection surface. On this occasion, the light intensity of the image light formed by the liquid-crystal light valve  25   a - 25   c  at appropriate pixel areas is appropriately attenuated by the above-mentioned liquid-crystal light valve  28   a . Thus, it is possible to project image light having a contrast ratio exceeding by far the contrast ratio that could be realized by the liquid-crystal light valves  25   a - 25   c  alone.  
         [0054]     In the above, although the invention was explained along with the present embodiment, the invention is not limited to the above embodiment. For example, although, in the above embodiment, three liquid-crystal light valves  25   a - 25   c  were separately illuminated by the RGB respective colors, local light-intensity adjustment of the image can be made by the filter of the liquid-crystal light valve  28   a  shown in  FIG. 1  even in a projector of a type where illumination is made by a white-light source to a color display panel, using a single liquid-crystal light valve and arranging RGB filters on the respective pixels, making it possible to project image light at a high contrast ratio.  
         [0055]     Meanwhile, although, in the above explanation, brightness adjustment is made on the pixels of the liquid-crystal light valves  25   a - 25   c  by placing the pixels of the liquid-crystal light valve  25   a - 25   c  and the pixels of the liquid-crystal light valve  28   a  in one-to-one correspondence, brightness adjustment can be made on the basis of domain units comprising a plurality of adjacent pixels of the liquid-crystal light valves  25   a - 25   c  (e.g. four pixels). In this case, the pixels of the liquid-crystal light valve  28   a  can be comprised of domain units of the liquid-crystal light valve  25   a - 25   c.  Incidentally, in the case of carrying out light adjustment on a domain unit basis, it is possible to determine the light-reduction amount based on the maximum intensity pixel within the domain, or determine the light-reduction amount based on the average values of pixels within the domain.  
         [0056]     Meanwhile, in the above explanation, although the transmissivity through the liquid-crystal light valve  28   a  was locally adjusted on the pixel unit basis for example, the overall light reduction can be achieved by adjusting transmissivity over the entirety of liquid-crystal light valve  28   a . In this case, in one embodiment the light-reduction amount for the whole frame is determined based on the maximum intensity-leveled pixel of within the projection surface.  
         [0057]     While this invention has been described in conjunction with the specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, preferred embodiments of the invention as set forth herein are intended to be illustrative, not limiting. There are changes that may be made without departing from the spirit and scope of the invention.