Abstract:
A display system capable of raising limits on the response speed of the spatial light modulator and the rate of pulse-width modulation signal transfer is provided at a low cost. The display system comprises: a color switch filter unit used in the sequential color separation of white light from the light source; a spatial light modulator which is illuminated by lights of plural color elements from the color switch filter unit and generates image lights of the respective color elements; and an intensity switch filter unit for switching three or more intensity levels of the respective lights of plural color elements, being separate from the color switch filter unit.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a display system for producing color images by light-modulating (spatially light-modulating) the lights of different color elements obtained through sequential color separation and, more particularly, to a display system which adopts a pulse-width modulation for light modulation.  
         BACKGROUND OF THE INVENTION  
         [0002]    A pulse-width modulating display system is generally provided with a spatial light modulator that generates image lights by partially reflecting or transmitting light from a light source, and sequentially projects the image lights produced by the modulator on a screen to display images. The spatial light modulator generates the image lights on the basis of input image signals from external devices such as PCs and video equipment. The contrast of images displayed on the screen is normally defined by modulating the pulse-width of signals that execute ON/OFF control of the modulator.  
           [0003]    There is described an example of the pulse-width modulating display system in Japanese Patent Application laid open No. HEI10-78550. The conventional display system obtains color images by light-modulating the lights of different color elements which have undergone sequential color separation.  
           [0004]    [0004]FIG. 1 is a block diagram showing the configuration of the conventional display system. With reference to FIG. 1, the display system comprises a light source  1 , a color switch filter unit  31  used in the sequential color separation of white light from the light source, a spatial light modulator  2  for sequentially receiving the lights of different color elements obtained by the sequential color separation and generating the image lights of the color elements by partially reflecting the input lights in a prescribed direction, a projection lens  3  for projecting the image lights of the color elements sequentially generated by the spatial light modulator  2  on a screen  4 , and a drive circuit  51  for driving the spatial light modulator  2  and color switch filter unit  31  in synchronism based on an image signal  101  and a frame synchronous signal  102  sent from the outside (e.g. PC).  
           [0005]    Examples of the spatial light modulator  2  include a digital micromirror device (DMD) comprised of hundreds of thousand of micro-mirrors whose angles of gradient are adjustable. The micro-mirrors of DMD correspond to, one each, the picture elements (pixels) of images displayed on the screen  4 . Any images can be presented by controlling the angle of each micro-mirror. To be specific, each of the micro-mirrors is selectively adjusted at the first angle for reflecting the light in a direction to avert it from the projection lens  3  or the second angle for reflecting the light to the projection lens  3 , thus creating desired images on the screen  4 . The angle control of the micro-mirrors is executed based on a modulation signal  103  fed from the drive circuit  51 .  
           [0006]    The color switch filter unit  31  includes a color wheel  41  provided with plural color filters (Red, Green, Blue) which have different spectral transmittance characteristics and are arranged one by one in the circumferential direction, a motor  11  which supports the center of the color wheel  41  for rotating the wheel  41  in a prescribed direction, a couple of elements (light emitting element  12  and light acceptance element  13 ) disposed opposite to each other with the color wheel  41  between them, and a color wheel control circuit  81  for controlling the rotation of the motor  11 .  
           [0007]    The color wheel control circuit  81  receives a color wheel phase signal  112  from the light acceptance element  13  as well as a color switch control signal  104  from the drive circuit  51 , and sends a motor control signal  111  to the motor  11 . The color wheel phase signal  112  includes information on the rotation cycle of the color wheel  41 . The information is derived from the timing of the reception of light that the light emitting element  12  emits through a hole made in a prescribed position on the color wheel  41  to the light acceptance element  13 .  
           [0008]    The image signal (video signal)  101  fed from the outside consists of image signals relative to respective colors Red, Green and Blue (including intensity information), which are sequentially output with respect to each frame. Besides, frame synchronization is performed based on the frame synchronous signal  102 . The modulation signal  103  is a signal for controlling the angles of the micro-mirrors of the spatial light modulator  2  according to the image signal  101  (image signals for colors R, G and B). That is, the respective micro-mirrors are set at the first angle or the second angle based on the modulation signal  103 . The color switch control signal  104  controls color filter (R, G, B) switching to execute the color separation by the color wheel  41 . The color filters can be switched in timing with the changeover of the image signal  101  based on the frame synchronous signal  102 . The color filter switching is carried out by rotating the color wheel  41  in a light path.  
           [0009]    In the following, the image display operation of the above-mentioned conventional display system will be described by taking the case where the image signal (R, G, B)  101  is fed into the drive circuit  51  for example.  
           [0010]    The light radiated from the light source  1  enters in the color switch filter unit  31 . When the image signal (Red)  101  is input on this occasion, the drive circuit  51  controls the angles of the micro-mirrors corresponding to the respective pixels in response to the signal (R)  101 . Concretely, the drive circuit  51  feeds the modulation signal  103  with the spatial light modulator  2  to switch the angles of the respective micro-mirrors to the first angle (at which the micro-mirror reflects light to divert it from the projection lens  3 ) or the second angle (at which the micro-mirror reflects light toward the projection lens  3 ).  
           [0011]    In addition, the drive circuit  51  switches the color filters of the color wheel  41  in synchronism with the angle control of the micro-mirrors (spatial light modulation in the spatial light modulator  2 ) according to the frame synchronous signal  102 . More specifically, the drive circuit  51  feeds the color wheel control circuit  81  with the color switch control signal  104  to switch the filter set in the light path to the color filter (R) when the image signal (R)  101  is input. The color wheel control circuit  81  sends the motor control signal  111  to the motor  11  based on the color switch control signal  104  and the color wheel phase signal  112  received from the light acceptance element  13 . In response to the motor control signal  111 , the motor  11  rotates the color wheel  41  so that the light radiated from the light source  1  enters the color filter (R).  
           [0012]    The light having entered the color filter (R) transmits therethrough and becomes light (R). The light (R) then enters into the spatial light modulator  2 . The light modulator  2  spatially light-modulates the light (R) to generate image light (R). The image light (R) is projected on the screen  4  by the projection lens  3 .  
           [0013]    Subsequently, the image signal (Green)  101  is input. With this the drive circuit  51  feeds the spatial light modulator  2  with the modulation signal  103  to execute the angle control of the micro-mirrors according to the image signal (G)  101 . At the same time, the drive circuit  51  feeds the color wheel control circuit  81  with the color switch control signal  104  to switch the filter of the color wheel  41  to the color filter (G). The color wheel control circuit  81  sends the motor control signal  111  to the motor  11  based on the color switch control signal  104  and the color wheel phase signal  112  received from the light acceptance element  13 . In response to the motor control signal  111 , the motor  11  rotates the color wheel  41  so that the light radiated from the light source  1  enters the color filter (G).  
           [0014]    The light having entered the color filter (G) transmits therethrough to become light (G). The light (G) then enters into the spatial light modulator  2 . The spatial light modulator  2  spatially light-modulates the light (G) to generate image light (G). The image light (G) is projected on the screen  4  by the projection lens  3 .  
           [0015]    After that, the image signal (Blue)  101  is input. Accordingly, the drive circuit  51  feeds the spatial light modulator  2  with the modulation signal  103  to execute the angle control of the micro-mirrors in conformity with the image signal (B)  101 . At the same time, the drive circuit  51  feeds the color wheel control circuit  81  with the color switch control signal  104  to switch the filter of the color wheel  41  to the color filter (B). The color wheel control circuit  81  sends the motor control signal  111  to the motor  11  based on the color switch control signal  104  and the color wheel phase signal  112  received from the light acceptance element  13 . In response to the motor control signal  111 , the motor  11  rotates the color wheel  41  so that the light radiated from the light source  1  enters the color filter (B).  
           [0016]    The light having entered the color filter (B) transmits therethrough to become light (B). The light (B) then enters into the spatial light modulator  2 . The spatial light modulator  2  spatially light-modulates the light (B) to generate image light (B). The image light (B) is projected on the screen  4  by the projection lens  3 .  
           [0017]    As a result of these operations, the image lights R, G and B are sequentially projected to an enlarged scale on the screen  4 . The switch in the image lights (R, G, B) is inappreciable to the human eye. Consequently, images in the respective colors shown by the image lights (R, G, B) are temporally superimposed, and thus recognized as a color image in human perception. The contrast of color images displayed on the screen  4  can be arbitrarily adjusted by modulating the pulse-width of the modulation signal  103  that controls the operation of the spatial light modulator  2 .  
           [0018]    In the following, a description will be given of the concrete configuration of the drive circuit  51  and color wheel control circuit  81  of the conventional display system.  
           [0019]    [0019]FIG. 2 is a block diagram showing an example of the configuration of the color wheel control circuit  81 . The color wheel control circuit  81  receives the color switch control signal  104  from the drive circuit  51  and the color wheel phase signal  112  from the light acceptance element  13  as its input signals. The color wheel control circuit  81  includes a frequency phase comparator  83  for comparing the frequency phases of the input signals to output an error signal, and an amplifier  84  for amplifying the error signal output from the frequency phase comparator  83  to output the amplified signal as the motor control signal  111 .  
           [0020]    [0020]FIG. 3 is a block diagram showing an example of the configuration of the drive circuit  51 . The drive circuit  51  has a pulse-width modulating circuit and a color switch control circuit  68 . The pulse-width modulating circuit is composed of a memory circuit  61 , a write control circuit  63 , read control circuit  64  and a color switch timing information table  65 .  
           [0021]    The color switch timing information table  65  includes the information indicating the timing for making a switch in color filters of the color wheel  41 . The color switch control circuit  68  outputs the color switch control signal  104  in synchronism with the frame synchronous signal  102  for switching colors in the color switch filter unit  31  shown in FIG. 1.  
           [0022]    The image signal  101  input from the outside is once written into the memory circuit  61 . Necessary data is read out of the memory circuit  61 , and sent to the spatial light modulator  2  as the modulation signal  103 . The write control circuit  63  controls the operation of writing the image signal  101  into the memory circuit  61 . The timing of the writing operation is determined based on the frame synchronous signal  102 . The read control circuit  64  controls the operation of reading necessary data out of the memory circuit  61 . The timing of the reading operation is determined based on the color switch timing in the color wheel  41  derived from the frame synchronous signal  102  and the color switch timing information table  65 .  
           [0023]    In the above-described conventional display system, the spatial light modulator  2  controls the switch between ON/OFF states on a pixel-by-pixel basis according to the modulation signal  103 . The light is conducted to the projection lens  3  in the ON state, whereas the light is not conducted thereto in the OFF state. The produced image becomes brighter as the ON state continues longer. In the pulse-width modulation, the image signal indicates the contrast by taking advantage of this behavior. Examples of the spatial light modulator  2  capable of such pulse-width modulation include surface stabilized ferroelectric liquid crystal displays, etc. in addition to DMD.  
           [0024]    The pulse-width modulation will be more fully described below.  
           [0025]    It is assumed by way of example that 8 bits are used to define the contrast of respective R, G and B colors in the input image signal  101 . FIG. 4 is a schematic diagram showing the per frame data structure of the image signal  101  for explaining the pulse-width modulation in the conventional display system.  
           [0026]    One frame of the image signal  101  is time-divided into sections for data R, G and B. The respective data R, G and B is composed of time slots  1  to  255 . Time slot  255  is allocated to bit  0 , and correspondingly, other time slots are allocated to the respective bits as follows: time slots  254  and  253  for bit  1 , time slots  252  to  249  for bit  2 , time slots  248  to  241  for bit  3 , time slots  240  to  225  for bit  4 , time slots  224  to  193  for bit  5 , time slots  192  to  129  for bit  6 , and time slots  128  to  1  for bit  7 .  
           [0027]    Time (time slots) is allocated to the respective bit  7  (most significant bit: MSB), bit  6 , . . . , bit  1 , bit  0  (least significant bit: LSB) in a ratio of 2 7 :2 6 : . . . :2 1 :2 0 . Provided that time T is allocated to bit  0 , times 128T, 64T, . . . , 4T, 2T are allocated for bit  7 , bit  6 , . . . , bit  2 , bit  1 , respectively. Besides, when the input image signal  101  indicates the contrast levels of 255, 254, . . . , 2, 1, and 0, the periods of ON state corresponding to the respective contrast levels are 255T, 254T, . . . , 2T, T, and 0.  
           [0028]    Bit assignment information indicates the above-mentioned time allocation for each bit, and is normally prepared beforehand in the form of a bit assignment information table. The ON/OFF control of the spatial light modulator  2  is carried out according to the bit assignment information table. For example, in the case of representing a contrast level of 130 for a pixel, the spatial light modulator  2  controls the light in regard to the pixel so that the light reaches onto the screen during the time-domains of the bit  7  (128 unit time) and bit  1  (2 unit time), but does not reach thereto in the time-domains of other bits  6  to  2 .  
           [0029]    However, the conventional display system has the following problems.  
           [0030]    Let it be assumed that, in cases, as for example, where the sequential color separation of white light is performed with the use of a color wheel having filters in three colors R, G and B, the color wheel rotates at a speed of 60 Hz and 8 bits are used to define the contrast of the respective colors. In this case, the minimum switching time in the pulse-width modulation is below 22 microseconds. It is necessary to use a spatial light modulator having response speed faster than the minimum switching time. In addition, the minimum switching time becomes even shorter when increasing the rotational speed or number of colors of the color wheel, or raising the level of contrast. As a result, higher response speed is required of the spatial light modulator. Nevertheless, there is a limit to the response speed of the modulator. Consequently, the minimum switching time in the pulse-width modulation has been limited due to the response speed of the modulator, and it has been difficult to increase the number of bits beyond a certain level. Incidentally, when the setting of the minimum switching time exceeds the response speed of the modulator, the brightness of images displayed by LSB is lowered, resulting in a deterioration of image quality in areas of low brightness.  
           [0031]    Besides, provided that the spatial light modulator supports 1024×768 pixels, the maximum rate of pulse-width modulation signal transfer to the modulator is very high, up to 564 MHz per 64-bit width (=60 Hz*3*255*1024*768/64). Since higher signal frequency produces a higher level of noise, peripheral circuits become susceptible to malfunction, and further, power consumption increases.  
           [0032]    Consequently, there have been made studies on a display system with low rate of pulse-width modulation signal transfer, in which a spatial light modulator having low response speed is usable while maintaining system&#39;s performance. In the following, a description will be given of the display system described in Japanese Patent Application laid open No. HEI9-149350 as an example.  
           [0033]    [0033]FIG. 5 is a schematic diagram of a color wheel used in the above-mentioned system. The color wheel is provided with a plurality of color filters (R, G, B) having different spectral transmittance characteristics, which are arranged one by one in the circumferential direction at a prescribed rate (spacing angle: 120°). The filter B is provided with a low-density segment  34  or a density (intensity) filter B+NDF in a certain angular range from the interface with the filter G. White light transmittance is decreased in the low-density segment  34  as compared to the other areas  32  of the filter B. The filters R and G are likewise provided with low-density segments R+NDF and G+NDF, respectively.  
           [0034]    In the system, white light from the light source is sequentially separated into color lights R, G and B by rotating the color wheel in a light path. After that, the color lights illuminate DMD being the spatial light modulator, and image lights R, G and B from the DMD are sequentially superimposed on the screen. Thus, color images are produced in the same manner as the system depicted in FIG. 1. Since the respective color filters R, G and B of the color wheel are provided with the low-density segments, the intensity of the color lights R, G and B drops in the segments. Consequently, when the ON/OFF switch of light in the time-domains of low bits of an image signal, namely, bit  1  (2 unit time) is carried out for the lights having passed through the low-density segments, it becomes possible to extend the time for the low bits. The response speed of the DMD is regulated by the length of time of the low bits. Therefore, it also becomes possible to reduce the response speed according to the extension of the time for the low bits  
           [0035]    However, there are following problems in the above-described conventional display systems.  
           [0036]    In the display system depicted in FIG. 1, there is a limit to the rate of pulse-width modulation signal transfer as well as to the response speed of the available spatial light modulator, which necessarily causes a disadvantage in design.  
           [0037]    On the other hand, in the display system having the color wheel of FIG. 5, the problem of the limit is resolved. However, there are produced new problems as follows.  
           [0038]    It is preferable to use the intensity filter also for intermediate bits between high bits and low bits in order to smoothly present the contrast. In other words, it is necessary to provide the respective color filters with plural intensity filters of different degrees of intensity to achieve a good contrast presentation. The color wheel of the system has only one intensity filter with respect to each color filter, and it is difficult to achieve a good contrast presentation.  
           [0039]    It is possible, but costly, to provide the respective color filters with plural intensity filters of different levels of intensity because the processes for manufacturing the color wheel are increased. This problem will be explained below.  
           [0040]    The color wheel is generally produced by affixing color filters onto a transparent disk, or by depositing filter material on the surface of a transparent disk in a vacuum chamber. In the color wheel shown in FIG. 5, the respective three color filters are provided with two segments of different density levels, and 3×2=6 filters are needed. Consequently, it is required to repeat the affixing process or depositing process six times. When forming three segments of different density levels in the respective color filters, 3×3=9 filters are needed, thereby requiring nine times of the affixing process or depositing process. As is described above, when producing the color wheel with a disk, the manufacturing process is necessarily repeated {(the number of color filters)×(the number of segments formed for each filter)} times. Thus, setting of plural intensity filters of different density levels in the color wheel increases manufacturing processes and costs.  
           [0041]    In addition, when setting plural patterns of intensity to be switched, it is required to produce the color wheel having filters corresponding to the intensity patterns if color separation and intensity switching are performed by using the same color wheel. This enormously raises the cost of production.  
         SUMMARY OF THE INVENTION  
         [0042]    It is therefore an object of the present invention to provide a display system capable of raising limits on the response speed of the spatial light modulator and the rate of pulse-width modulation signal transfer at a low cost.  
           [0043]    In accordance with an aspect of the present invention, to achieve the above objects, there is provided a display system comprising: a color switch means for switching lights of plural color elements to provide the respective lights of color elements one by one in sequence; a spatial light modulation means which is illuminated by the lights of plural color elements from the color switch means and generates image lights of the respective color elements; and an intensity switch means for switching two or more intensity levels of the respective lights of plural color elements or the respective image lights of the color elements, being separate from the color switch means.  
           [0044]    In the display system of the present invention, the spatial light modulator generates the image lights based on an image signal input from the outside. The intensity switch means switches the intensity level of the respective lights. Accordingly, it becomes possible to allocate the high bits of the image signal for a pulse-width modulation signal in timing with brighter intensity filters, and to allocate the low bits thereof for the pulse-width modulation signal in timing with darker intensity filters. As a result, the minimum switching time of pulse-width modulation can be prolonged, thus enabling a reduction in the rate of pulse-width modulation signal transfer as well as the use of a spatial light modulator with low response speed.  
           [0045]    Besides, the color switch means and the intensity switch means are individually provided to the display system, and therefore, color switching performed by the color switch means and intensity switching performed by the intensity switch means are controlled separately. That is, the color switching is carried out by using a color wheel. The intensity switching is carried out by using an intensity wheel being independent of the color wheel. Consequently, according to the present invention, it is possible to use the intensity wheel with plural different intensity levels for one color wheel. Thus, when setting plural patterns or levels of intensity to be switched, it is only required to produce the intensity wheel having filters corresponding to the intensity patterns, thereby dispensing with the need to produce the color wheel for each intensity pattern.  
           [0046]    In addition, the separate color wheel and intensity wheel lead to a reduction in manufacturing process. In the case of, for example, providing the respective three color filters with segments of three different density levels, 3×3=9 filters are needed for the color wheel shown in FIG. 5 as described above. Consequently, it is required to repeat the affixing process or depositing process nine times. On the other hand, in the display system according to the present invention, three affixing processes or depositing processes are required for manufacturing the respective color wheel and intensity wheel. That is, the affixing process or depositing process is repeated six times, lessened by three times as compared to the conventional display system. The gap in the number of manufacturing processes widens as the segments of different density levels increase.  
           [0047]    Incidentally, the intensity switching may be carried out by a liquid crystal panel, by changing the brightness of a light source, or by changing the aperture of a projection lens. Besides, a light source emitting lights in plural different colors may be adopted as the color switch means. In this case, it is possible to dispense with the color wheel and intensity wheel, and there is no need to perform complicated synchronous control. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0048]    The objects and features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings in which:  
         [0049]    [0049]FIG. 1 is a schematic block diagram showing the configuration of the pulse-width modulating display system disclosed in Japanese Patent Application laid open No. HEI10-78550;  
         [0050]    [0050]FIG. 2 is a block diagram showing an example configuration of a color wheel control circuit of the display system depicted in FIG. 1;  
         [0051]    [0051]FIG. 3 is a block diagram showing an example configuration of a drive circuit of the display system depicted in FIG. 1;  
         [0052]    [0052]FIG. 4 is a schematic diagram showing the frame structure of an image signal input to the display system depicted in FIG. 1;  
         [0053]    [0053]FIG. 5 is a schematic diagram of a color wheel used in the display system disclosed in Japanese Patent Application laid open No. HEI9-149350;  
         [0054]    [0054]FIG. 6 is a block diagram showing the configuration of a display system according to an embodiment of the present invention;  
         [0055]    [0055]FIG. 7 is a block diagram showing an example configuration of a color wheel control circuit of the display system depicted in FIG. 6;  
         [0056]    [0056]FIG. 8 is a block diagram showing an example configuration of a drive circuit of the display system depicted in FIG. 6;  
         [0057]    [0057]FIG. 9 is a schematic diagram showing the frame structure of an image signal input to the display system depicted in FIG. 6;  
         [0058]    [0058]FIG. 10 is a schematic diagram of an intensity wheel used in the display system depicted in FIG. 6. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0059]    Referring now to the drawings, a description of a preferred embodiment of the present invention will be given in detail.  
         [0060]    [0060]FIG. 6 is a block diagram showing the configuration of a display system according to an embodiment of the present invention. The display system of this embodiment is similar to that shown in FIG. 1 except for the presence of intensity switch filter unit  32  and drive circuit  52  as a substitute for the drive circuit  51 . Like reference characters indicate like parts in FIGS. 1 and 6, and the detailed description will not be given of these parts.  
         [0061]    The drive circuit  52  receives the image signal  101  and frame synchronous signal  102 , and sends the modulation signal  103  to the spatial light modulator  2 . In addition, the drive circuit  52  feeds the intensity switch filter unit  32  with an intensity switch control signal  105  as well as feeding the color switch filter unit  31  with the color switch control signal  104 .  
         [0062]    The intensity switch filter unit  32  adjusts the brightness of the lights having transmitted through the color wheel  41  in the color switch filter unit  31 . The intensity switch filter unit  32  includes a intensity wheel  42 , a motor  14  which supports the center of the intensity wheel  42  for rotating the wheel  42  in a prescribed direction, a couple of elements (light emitting element  15  and light acceptance element  16 ) disposed opposite to each other with the intensity wheel  42  between them, and a intensity wheel control circuit  82  for controlling the rotation of the motor  14 .  
         [0063]    The intensity wheel  42  is arranged opposite to the color wheel  41 . The intensity wheel  42  has the first, second and third segments corresponding to the color filters R, G and B of the color wheel  41 , respectively. In each segment, a plurality of filters (NDF) F 1  to F 3  of different photo transmittances are arranged one by one in the circumferential direction at a prescribed rate (area ratio). The intensity wheel control circuit  82  receives an intensity wheel phase signal  114  from the light acceptance element  16  as well as an intensity switch control signal  105  from the drive circuit  51 , and sends a motor control signal  113  to the motor  14 .  
         [0064]    The intensity wheel phase signal  114  includes information on the rotation cycle of the intensity wheel  42 . The information is derived from the timing of the reception of light that the light emitting element  15  emits through a hole made in a prescribed position on the intensity wheel  42  to the light acceptance element  16 . The intensity switch control signal  105  controls the switch operation for the filters F 1  to F 3  of the intensity wheel  42 . The filters (NDF) can be switched in accordance with the respective bits for each color (R, G, B) of the image signal  101 . The filter (NDF) switching is carried out by rotating the intensity wheel  42  in a light path.  
         [0065]    In the construction of the display system of this embodiment, the light emitted from the light source  1  travels through the color wheel  41  in the color switch filter unit  31  and the intensity wheel  42  in the intensity switch filter unit  32  to the spatial light modulator  2 . The spatial light modulator  2  modulates the incident light based on the modulation signal  103  received from the drive circuit  52  to generate visualized light (image light). The projection lens  3  extends and projects the image light generated by the spatial light modulator  2  on the screen  4 .  
         [0066]    Images in three colors R, G and B are sequentially displayed on the screen  4  by the extended projection of the image lights (R, G, B). The images are superimposed and perceived as a color image to the human eye due to an after-image effect. The contrast of the color image displayed on the screen  4  can be adjusted by the intensity wheel  42 &#39;s intensity switch as well as by modulating the pulse-width of the modulation signal  103  that controls the operation of the spatial light modulator  2 .  
         [0067]    In the following, a description will be given of the concrete configuration of the drive circuit  52  and intensity wheel control circuit  82  of the display system.  
         [0068]    [0068]FIG. 7 is a block diagram showing an example of the configuration of the intensity wheel control circuit  82 . The intensity wheel control circuit  82  receives the intensity switch control signal  105  from the drive circuit  52  and the intensity wheel phase signal  114  from the light acceptance element  16  as its input signals. The intensity wheel control circuit  82  includes a frequency phase comparator  85  for comparing the frequency phases of the input signals to output an error signal, and an amplifier  86  for amplifying the error signal output from the frequency phase comparator  85  to output the amplified signal as the motor control signal  113 .  
         [0069]    The motor  14  is controlled on the basis of the motor control signal  113  sent from the intensity wheel control circuit  82 . Thus, the intensity wheel  42  rotates in synchronism with the frame synchronous signal  102  to switch intensity levels.  
         [0070]    [0070]FIG. 8 is a block diagram showing an example of the configuration of the drive circuit  52 . The drive circuit  51  has an intensity switch timing information table  66 , a bit allocation information table  67  and a intensity switch control circuit  69  in addition to the configuration of the drive circuit  51  shown in FIG. 3. Like reference characters indicate like parts in FIGS. 3 and 8. In this embodiment, the pulse-width modulating circuit is composed of the memory circuit  61 , write control circuit  63 , read control circuit  64 , color switch timing information table  65 , intensity switch timing information table  66 , and bit allocation information table  67 .  
         [0071]    The color switch timing information table  65  includes the information indicating the timing for making a switch in color filters of the color wheel  41  on the basis of the frame synchronous signal  102 . The intensity switch timing information table  66  contains the information indicating the timing for making a switch in intensity levels of the intensity wheel  42  on the basis of the frame synchronous signal  102 . The bit allocation information table  67  contains the information on when and which of the bits of the image signal  101  for indicating the contrast is read out of the memory circuit  61  on the basis of the frame synchronous signal  102 . The intensity switch control circuit  69  outputs the intensity switch control signal  105  in synchronism with the frame synchronous signal  102  for switching the intensity filters F 1  to F 3  in the intensity switch filter unit  32 .  
         [0072]    In this construction of the drive circuit  52 , the write control circuit  63  is adjusted in timing based on the frame synchronous signal  102 , and the image signal  101  is written into the memory circuit  61  according to the control by the circuit  63 . Besides, the read control circuit  64  is adjusted in timing based on the frame synchronous signal  102 , and necessary data is read out of the memory circuit  61  by the circuit  64 . The data is sent to the spatial light modulator  2  as the modulation signal  103 . The read control circuit  64  controls the operation of reading necessary data out of the memory circuit  61 . The reading operation is performed based on color switch timing, intensity switch timing and contrast indicating bit read timing derived from the color switch timing information table  65 , intensity switch timing information table  66 , and bit allocation information table  67 , respectively.  
         [0073]    Next, a concrete description will be given of the operation of the display system according to this embodiment. In the following description, the contrast is indicated by k bits of the image signal  101 , the number of colors switched by the color switch filter unit  31  is m, and the number of intensity levels switched by the intensity switch filter unit  32  is n (n≧2).  
         [0074]    Additionally, m color filters in the color switch filter unit  31  are represented as E 1 , E 2 , . . . , Em, and n levels of intensity filters in the intensity switch filter unit  32  are represented as F 1 , F 2 , . . . , Fn in ascending order of intensity level of outgoing light therefrom. The intensity ratio of the outgoing lights from the intensity filters F 1 , F 2 , . . . , Fn stands at: 
           F   1 : F   2 : . . . : Fn=S   1 : S   2 : . . . : Sn   
         [0075]    in which S 1 =1. In pulse-width modulation, allocated time per frame time for bit h of the image signal  101 , color filter Ei and intensity filter Fj is T(h, i, j), where: 
         0 ≦h≦k− 1 
         1 ≦i≦m   
         1 ≦j≦n   
           T (0 , i , 1)= T   0 . 
         [0076]    In the display system of this example, the condition of the following expression is satisfied. 
         ( S   1 * T ( h, i , 1)+ S   2 * T ( h, i , 2)+ . . . + Sn*T ( h, i, n ))/ T   0 =pow(2 , h )  (1) 
         [0077]    In the expression, pow(2, h) indicates 2 to the power of h (2 h ), and * indicates multiplication, namely, the same as “×”. Expression (1) presents the condition for defining the contrast of images or the image signal  101  by the pulse-width modulation in the case of making a switch in plural intensity levels.  
         [0078]    Provided that display time per frame time when color filter Ei is paired with intensity filter Fj is Tc(i, j): 
           Tc ( i, j )= T (0 , i, j )+ T (1 , i, j )+ . . . + T ( k− 1 , i, j )  (2) 
         [0079]    Expression (2) presents the condition for deciding the switching time of the intensity filters.  
         [0080]    Besides, in the display system of this example, the condition of the following expression is satisfied. 
         ( Tc ( i , 1)+ Tc ( i , 2)+ . . . + Tc ( i, n ))/ T   0 =(pow(2 , k )−1)  (3) 
         [0081]    Expression (3) presents the condition for extending the minimum unit time in the pulse-width modulation as compared to the conventional display system.  
         [0082]    The intensity switch timing information table  66  depicted in FIG. 8 contains the information of Tc(i, j) conditioned on the above expressions (1) to (3). The read control circuit  64  uses the information for its control operation. The bit allocation information table  67  contains the information of T(h, i, j) conditioned on the above expressions (1) to (3). The information is also used for the control operation of the read control circuit  64 .  
         [0083]    In the following, it will be explained that there are always variable values which satisfies the above expressions (1) to (3).  
         [0084]    Concerning a certain color of colors sequentially separated by the color switch filter unit  31 , expression (1) is established with respect to k pieces of h, and expression (2) is established with respect to n pieces of j. Consequently, (k+n) conditional expressions are established based on expressions (1) and (2). On the other hand, there are n in number of Sj, (k*n) in number of T(h, i, j)/T 0 , and k in number of h. Thus, the total number of variable values is (n+(k*n)+k). When the number of contrast bits of the image signal  101  and the number of the intensity filters are determined, one conditional expression is established according to expression (3). That is, the number of independent variables is (k*n), and at least 2. Therefore, there are always variable values which meet expressions (1) to (3).  
         [0085]    In the aforementioned conditions, the intensity levels of the outgoing lights from the respective intensity filters may have the ratio of 2 to the power of any integer, and also any bit of the image signal  101  may be allocated for only one intensity filter. The conditions are expressed as following expressions (4) and (5) by using integers j 1 , j 2 , Xj 1  and Xj 2  which satisfy: 
           j   1 +1 =j   2   
         1 &lt;=j   1 &lt; j   2  &lt;= n   
         0 &lt;=Xj   1 &lt; Xj   2 &lt;= k −1. 
         [0086]    [0086]                     Sj1   =     pow        (     2   ,   Xj1     )                   Sj2   =     pow        (     2   ,   Xj2     )               }           (   4   )                                                                      When                 h     &lt;   Xj1     ,         T        (     h   ,   i   ,   j1     )       /   T0     =   0.                                                 When                 Xj1     &lt;                =     h   &lt;   Xj2       ,         T        (     h   ,   i   ,   j1     )       /   T0     =       pow        (     2   ,     h   -   Xj1       )       .                           When                 Xj2     &lt;                =   h     ,         T        (     h   ,   i   ,   j1     )       /   T0     =   0.                     When                 Xj2     =   h     ,         T        (     h   ,   i   ,   j2     )       /   T0     =   1.             }           (   5   )                                 
         [0087]    It is easily understood that expression (1) is satisfied on the conditions according to expressions (4) and (5).  
         [0088]    Next, it will be explained that expression (3) is satisfied on these conditions with respect to k (k: an integer larger than 1).  
         [0089]    Assuming that the number of contrast bits of the image signal  101  is k and the number of the intensity filters is n, the left side of inequality (3) comes out by using expression (2) as follows: 
         ( Tc ( i , 1)+ Tc ( i , 2)+ . . . + Tc ( i, n ))/ T   0 =(pow(2 , k   1 )−1)+(pow(2 , k   2 )−1)+ . . . +(pow(2 , kn )−1)  (6) 
         [0090]    Incidentally, k 1 , k 2 , . . . , kn are positive integers which satisfy k 1 +k 2 + . . . kn=k.  
         [0091]    The combination of n, k 1 , k 2 , . . . , kn for obtaining a maximum value in expression (6) is: 
           n =2 , k   1 =1 , k   2 = k −1, 
         [0092]    In this case, the value derived from expression (6), that is, the left side of inequality (3) is pow(2, k−1), thus satisfying inequality (3) in terms of k.  
         [0093]    A concrete example will be given with reference to FIG. 6. It is assumed by way of example that 8 bits are used to define the contrast in the image signal  101 , the number of colors switched by the color switch filter unit  31  is 3, and the number of intensity levels switched by the intensity switch filter unit  32  is 3. That is, k=8, m=3, n=3.  
         [0094]    Provided that the intensity ratio of the outgoing lights from the intensity filters F 1 , F 2 , F 3  stands at: 
           S   1 : S   2 : S   3 =1:4:32; and 
           T (0 , i , 1)=1 *T   0 ,  T (0 , i , 2)=0 *T   0 ,  T (0, i, 3)=0 *T   0   
           T (1 , i , 1)=2 *T   0 ,  T (1 , i , 2)=0 *T   0 ,  T (1 , i , 3)=0 *T   0   
           T (2 , i , 1)=0 *T   0 ,  T (2 , i , 2)=1 *T   0 ,  T (2 , i , 3)=0 *T   0   
           T (3 , i , 1)=0 *T   0 ,  T (3 , i , 2)=2 *T   0 ,  T (3 , i , 3)=0 *T   0   
           T (4 , i , 1)=0 *T   0 ,  T (4 , i , 2)=4 *T   0 ,  T (4 , i , 3)=0 *T   0   
           T (5 , i , 1)=0 *T   0 ,  T (5 , i , 2)=0 *T   0 ,  T (5 , i , 3)=1 *T   0   
           T (6 , i , 1)=0 *T   0 ,  T (6 , i , 2)=0 *T   0 ,  T (6 , i , 3)=2 *T   0   
           T (7 , i , 1)=0 *T   0 ,  T (7 , i , 2)=0 *T   0 ,  T (7 , i , 3)=4 *T   0   
         [0095]    the condition of expression (1) is satisfied. Besides, according to expression (2): 
         [0096]    [0096] Tc ( i , 1): Tc ( i , 2): Tc ( i , 3)=3:7:7. 
         [0097]    This means that the area ratio of the intensity filters F 1 , F 2  and F 3  on the intensity wheel  42  which rotates at a constant speed is supposed to stand at 3:7:7.  
         [0098]    [0098]FIG. 9 is a schematic diagram showing the frame structure of the image signal  101  when the intensity filters have an area ratio of 3:7:7. In FIG. 9, G 1 , G 2  and G 3  indicate the color filter G of the color wheel  41  and the intensity filters F 1 , F 2  and F 3  of the intensity wheel  42 , respectively. Similarly, R 1 , R 2  and R 3  indicate the color filter R and the respective intensity filters F 1 , F 2  and F 3 , and B 1 , B 2  and B 3  indicate the color filter B and the respective intensity filters F 1 , F 2  and F 3 .  
         [0099]    With the frame of FIG. 9, the minimum unit time in the pulse-width modulation is 1 frame time/3/(7+7+3). On the other hand, in the case of the conventional display system having no intensity switch filter unit shown in FIG. 1, it is assumed that n=1. Thus, the minimum unit time in the pulse-width modulation is 1 frame time/3/255 as illustrated in FIG. 4. From this it is to be understood that in the display system according to the embodiment of the present invention, the minimum unit time in the pulse-width modulation is dramatically prolonged as compared to the conventional display system, thereby enabling the use of the spatial light modulator with low response speed.  
         [0100]    Additionally, as it is required to send data corresponding to the number of pixels within the span of the minimum unit time for sending modulated data to the spatial light modulator, it takes 1 frame time/3/255/the number of pixels to transfer data per pixel in the conventional display system. On the other hand, the display system according to the embodiment of the present invention allows 1 frame time/3/(7+7+3)/the number of pixels of data transfer time per frame. That is, time allocated for data transfer per frame is prolonged as compared to the conventional display system, and thereby the operation speed of electric circuits can be reduced. This contributes to reductions in electric power consumption, parts cost and circuit design cost as well as improvement in reliability, besides allowing margins in circuit design.  
         [0101]    Incidentally, while the intensity switch filter unit  32  is disposed behind the color switch filter unit  31  in the display system depicted in FIG. 6, the unit  32  may be situated in front of the unit  31 .  
         [0102]    Colors switched by the color switch filter unit  31  are not limited to red, green and blue, and not necessarily three in number.  
         [0103]    The color switch filter unit  31  shown in FIG. 6, the color switch timing information table  65  and color switch control circuit  68  shown in FIG. 8 are dispensable.  
         [0104]    In addition, the present invention is applicable to three panel systems having three spatial light modulators.  
         [0105]    Besides, the display system in FIG. 6 may be provided with an intensity wheel like the one depicted in FIG. 10 instead of the intensity wheel  42 . The intensity wheel  43  in FIG. 10 includes three intensity filters F 1 , F 2  and F 3  which are arranged in the circumferential direction at a prescribed rate. The intensity filters are in the same ratio shown by the intensity filters F 1 , F 2  and F 3  of the intensity wheel  42 . In this case, the rotational speed of the intensity wheel  43  is set at triple the rotational speed of the color wheel  41 . With this, the intensity wheel  43  makes one complete rotation with respect to each color filter of the color wheel  41 , thereby achieving the same effect as described previously.  
         [0106]    The ColorQuad™ architecture presented by M. G. Robinson et al. in “High Contrast Color Splitting Architecture Using Color Polarization Filters” SID &#39;00 Digest, Vol. 31, p.92 (April 2000) may be adopted as the color switch filter unit  31  in FIG. 6. In this case, the color switch control signal  104  is used for controlling the color switch operation.  
         [0107]    A twisted nematic liquid crystal panel may be employed as the intensity switch filter unit  32  in FIG. 6. In this case, the intensity switch control signal  105  is used for controlling the transmittance of the twisted nematic liquid crystal panel.  
         [0108]    Further, the intensity wheel  42  in FIG. 6 is not limited to the one comprised of intensity filters covering the whole visible light range. It is possible to use, for example, filters that are capable of intensity switch regarding the wavelength band of the light output from the color switch filter unit  31 .  
         [0109]    The intensity levels may be switched at the light source instead of using the intensity switch filter unit  32 . That is, the display system shown in FIG. 6 may have a configuration without the intensity switch filter unit  32  in which the intensity switch control signal  105  is input to the control circuit of the light source  1  so that the control circuit carries out the intensity switch operation.  
         [0110]    The intensity levels may also be switched by using a stop-down feature well known for cameras. For example, it is possible to omit the intensity switch filter unit  32  from the display system shown in FIG. 6 and provide the system with the stop-down feature for changing the aperture of the projection lens  3  instead. In. this case, the intensity switch control signal  105  is input to the control circuit of the stop-down feature for adjusting the aperture of the projection lens  3  so that the control circuit carries out the intensity switch operation.  
         [0111]    The colors may be switched by using plural light sources emitting lights in different colors instead of using the color switch filter unit  32 . That is, the display system shown in FIG. 6 may be provided with the light sources emitting lights in different colors instead of the light source  1  as substitute for the color switch filter unit  32 . In this case, the light sources are switched according to the color switch control signal  104 .  
         [0112]    Further, the numbers of the intensity levels may be different for the respective colors. That is, while there are provided three intensity filters F 1 , F 2  and F 3  for each of the color filters R, G and B of the color wheel  41  in the display system shown in FIG. 6, different numbers of the intensity filters may be provided for the respective colors.  
         [0113]    As set forth hereinbefore, in accordance with the present invention, it is possible to reduce the processes of manufacturing the wheel as compared to the case of producing the conventional display system. Thus, display systems can be provided at low cost.  
         [0114]    Moreover, it is possible to dispense with the intensity wheel and the color wheel, since the intensity levels may be switched at the liquid crystal panel or the light source, or by adjusting the aperture of the projection lens, and besides the colors may be switched by using plural light sources emitting lights in different colors. This facilitates control operation and reduces costs.  
         [0115]    While the present invention has been described with reference to the particular illustrative embodiment, it is not to be restricted by the embodiment but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiment without departing from the scope and spirit of the present invention.