Abstract:
A projection television has a unit for reflecting OFF light not advancing toward a screen of light emitted by a reflection type light modulating element and a unit for controlling the amount of the reflecting OFF light and returns the OFF light which has not been used in a conventional device to the light source to reuse the OFF light. This configuration increases the utilization efficiency of the light and, as a result, can realize a powerful projection television having a bright projection image.

Description:
BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The present invention relates to a projection television and, in particular, to a light source device relating to an image display (optical system, driving system and the like) and the configuration thereof. 
   In recent years, there have been realized large-size displays having a screen size larger than a direct-view-type television of a cathode ray tube (hereinafter referred to as CRT) system. 
   One of them is a projection television (projection type display). Basically, the projection televisions are broadly divided into two types of configurations: 
   that is, (1) a projection television using a CRT as a light source and having a projection optical system for enlarging and projecting light emitted by the CRT and a screen for receiving the projected light; and (2) a projection television for applying light (white light or a color filter output light of this white light) from a light source emitting white light (white light source) to a light modulating element such as a light valve or the like and having a projection optical system for enlarging and projecting the light modulated by the light modulating element and a screen for receiving the projected light. 
   In the light modulating element used in the case (2) of the above-mentioned two types of projection televisions, various types of elements such as a liquid crystal light valve, a DMD (Digital Multi-mirror Device) and the like are adopted (DMD is a trademark of Texas Instrument Co. Ltd). 
   The best element of these light modulating elements is the DMD which is a reflection type light modulating element. The DMD has a large number of micro mirrors (mirror elements) which are two-dimensionally arranged and can be independently driven. The DMD is more advantageous in utilization efficiency of light than the other light modulating elements. 
     FIG. 1  is an illustration to show the operation of the DMD as a light modulating element used in a projection type display, which is disclosed in Japanese Patent Publication Laid-open No. 8-21977 (This is equivalent to U.S. Pat. No. 5,467,146). The DMD will be hereinafter described with reference to FIG.  1 . 
     FIG. 1  shows a mirror element constituting the DMD and schematically shows the relationship between its operation and optical path. 
   In this respect, the words of “ON” and “OFF” in the following description means as follows: “ON” means a state in which light is projected from a mirror element to a screen (or a state of the mirror element at that time) and “OFF” means a state in which light is not projected from a mirror to a screen (or a state of the mirror element at that time). 
   In  FIG. 1 , a reference numeral  1  designates a mirror element and the mirror element  1  is “ON” in a state of tilt shown in FIG.  1 ( a ). A reference numeral  2  shows a state of tilt in which the mirror element  1  is “OFF”. A reference numeral  3  is a light receiving plane which incident light enters. 
   A reference character L 1  designates incident light. A reference character L 2  designates reflecting light (ON light) from the mirror element  1  in the case where the mirror element  1  “ON”. A reference character L 3  designates reflecting light (OFF light) from the mirror element  1  in the case where the mirror element  1  is “OFF”. 
   A reference character L 4  designates reflecting light (undesired reflecting light) from the mirror element  1  in the case where the mirror element  1  is in an intermediate state (for example, in the case where a power source is not turned on or in a standby state in which a driving signal is not applied, that is, in the case where all the mirror elements  1  constituting the DMD can be treated as one plane mirror as a whole). 
   An image display in a device using the DMD is realized by projecting light (ON light) reflected by mirror elements  1  in the “ON” state, among the many number of mirror elements  1  arranged two-dimensionally, to a screen (therefore, OFF light or undesired reflecting light does not relate to the image display). 
   When a power source is not turned on or in a standby state in which a driving signal is not applied, the mirror elements  1  are in the intermediate state in which they are along a plane (as described above, in the intermediate state, all the mirror elements  1  constituting the DMD can be treated as one plane mirror as a whole). 
   When the mirror element  1  is controlled so as to be “ON”, it is tilted to a state of 1 shown in FIG.  1 ( a ) (for example, at 10 degrees in the clockwise direction). Further, when the mirror element  1  is controlled so as to be “OFF”, it is tilted to a state of 2 (for example, at 10 degrees in the counterclockwise direction, that is, θ=10 degrees in FIG.  1 ( a )). 
   Accordingly, in the case where the mirror element  1  is “ON”, the incident light L 1  is reflected in the direction of ON light L 2  in FIG.  1 ( a ) by the mirror element  1  tilted at 10 degrees in the clockwise direction and is enlarged and projected to a screen by a projection optical system (not shown). 
   Further, in the case where the mirror element  1  is “OFF”, the incident light L 1  is reflected in the direction of OFF light L 3  in FIG.  1 ( a ) by the mirror element  1  tilted at 10 degrees in the counterclockwise direction (in a state of tilt 2). The reflecting light is not entered into the projection optical system but is absorbed by a black mask (not shown, light absorber such as a metal coated with black). 
   In this respect, in the case where the mirror element  1  is in the intermediate state, the incident light L 1  is reflected in the direction of undesired light L 4  in FIG.  1 ( a ) by the mirror element  1 . The reflecting light is not entered into the projection optical system but is absorbed by a black mask (not shown, light absorber such as a metal coated with black). 
   FIG.  1 ( b ) is a schematic side view of the DMD. A reference character  4  designates a mirror arrangement region (expanding two-dimensionally) in which a large number of mirror elements  1  are arranged. In an actual projection type display, the incident light L 1 , the ON light L 2  and the OFF light L 3  are entered into and emitted from the whole mirror arrangement region  4 . In FIG.  1 ( b ), for the sake of simplification, the state of entrance and emission of the light is shown by one light beam. In this regard, a reference character  400  designates an optical deflector in which a large number of mirror elements  1 , each of which can be independently set in the “ON” states and the “OFF” states, are two-dimensionally arranged in the mirror arrangement region  4 . 
   A large number of mirror elements  1  which are two-dimensionally arranged in the mirror arrangement region  4  included in the optical deflector  400  are independently set in the “ON” states and the “OFF” states, whereby the incident light entered into the whole mirror arrangement region  4  is reflected as the ON light L 2  and the OFF light L 3  in correspondence with the “ON” states and the “OFF” states of the mirror elements  1 . 
   In the case where a moving image is displayed by a projection type display, for example, the average intensity of the ON light L 2  is determined by a ratio of the period of the “ON” state to the period of one field or one frame of an inputted image signal. Then, the moving image of gradation is displayed by changing the ratio of the period of the “ON” state to the period of one field or one frame of an inputted image signal. 
   In this respect, brightness of the whole screen in one field or one frame can be displayed by the number of mirror elements  1 (area ratio) in the “ON” state of all the mirror elements  1  belonging to the mirror arrangement region  4 . 
   The average ON ratio of the mirror elements  1  (which shows the utilization efficiency of the incident light L 1 ) is defined as follows from the average intensity of the above-mentioned ON light L 2  and the brightness of the whole screen. 
   That is, the average ON ratio P of the mirror elements  1  is defined as the following equation (1) in the period of one field or one frame (hereinafter referred to as on screen period).
 
 P= (the ratio of a time period during which the mirror elements  1  are in the “ON” state to one screen period)×(the ratio of the mirror elements  1  in the “ON” state to all the mirror elements  1 )  (1)
 
   For example, in the case where the mirror elements  1  are in the “ON” state in 20% of the area of the whole mirror arrangement region  4  (the whole screen) and the ratio of the time period during which the mirror elements  1  are in the “ON” state to one field period or one frame period is 50%, the average ON ratio P is given by
 
 P= 0.5×0.2=0.1 
 
   This can be thought to be equivalent to that 10% of all the mirror elements  1  are in the “ON” state on the average. Therefore, assuming that both of the reflectance factor and the vignetting factor of the mirror element  1  are 100% (the mirror elements  1  produce no loss in the entrance or reflection of light) and that optical power of the incident light L 1  is 100%, 10% of the optical power is projected to the screen as reflecting light L 2  and remaining 90% of the optical power is made OFF light L 3 . Therefore, in this case, the utilization factor of the incident light L 1  is 10%. 
   In the brightest portion of an image of high contrast (a Highlight portion in which luminance is at a maximum level; this luminance is referred to as peak luminance), 100% of the mirror elements  1  are in the “ON” state. In this case, ideally, the incident light L 1  has the same light intensity as the ON light L 2 . That is, the maximum value of the ON light L 2  is constant irrespective of the area ratio of the highlight portion and the image. 
   For example, in the case where it is intended to more brightly display an image of a large screen size of 200 inches, a conventional projection television needs a lamp or a light source of large power. This makes the device expensive and increases power consumption. 
   In actual ordinary image display, however, it is very rare that the average brightness (luminance level) of the image is always high. Further, in a movie or the like, there are many dark scenes and hence an average luminance level is low in many cases. Therefore, most of light generated by the lamp is made the OFF light L 3  which is not projected to the screen, which reduces the utilization efficiency of light. 
   SUMMARY OF THE INVENTION 
   The present invention has been made to solve the above-mentioned problems. It is the object of the present invention to greatly increase the peak luminance on a screen of an image without increasing the power of a lamp by improving the utilization efficiency of light. 
   Another object of the present invention is to provide a light source device and a projection television capable of greatly increasing luminance even in an image display of average brightness. 
   Still another object of the present invention is to produce a stable color balance in an image display even if luminance is increased. 
   A device in accordance with the present invention mainly has an optical element for uniforming a light intensity distribution of the emitting light of a lamp in a plane perpendicular to the direction of propagation, an optical deflector for changeably reflecting the emitting light of the optical element into either one of two directions, and an optical reflector for reflecting light in said one of two directions, along an axis of said one of two directions. 
   This returns the reflecting light in the one direction of the two directions produced by the optical deflector to the lamp, which can increase the utilization efficiency of light, as described in detail in the following. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of the present invention will be described in detail with reference to the following figures in which: 
       FIG. 1  is an illustration to show an optical path of a unit mirror element constituting a reflection type light modulating element; 
       FIG. 2  is an illustration to show the optical path of a mirror element and optical elements in the vicinity thereof of a projection television of a preferred embodiment 1 in accordance with the present invention; 
       FIG. 3  is a configurational view to show the optical elements of a projection television of a preferred embodiment 1 in accordance with the present invention; 
       FIG. 4  is a functional block diagram to show the signal processing of a projection television of a preferred embodiment 1 in accordance with the present invention; 
       FIG. 5  is an example of a screen display and an enlarged view of display pixels of a projection television of a preferred embodiment 1 in accordance with the present invention; 
       FIG. 6  is an illustration to show a light transfer rate of the respective optical elements of a projection television of a preferred embodiment 1 in accordance with the present invention; 
       FIG. 7  is an illustration to show an operation in which a control unit of a projection television of a preferred embodiment 1 in accordance with the present invention controls the incident luminous flux I of a projection type light modulating element; 
       FIG. 8  is an illustration to show the optical path of a mirror element and optical elements in the vicinity thereof of a projection television of a preferred embodiment 2 in accordance with the present invention; 
       FIG. 9  is a functional block diagram to show the optical elements of a projection television of a preferred embodiment 3 in accordance with the present invention; 
       FIG. 10  is a functional block diagram to show the signal processing of a projection television of a preferred embodiment 3 in accordance with the present invention; and 
       FIG. 11  is an illustration to show an operation in which a control unit of a projection television of a preferred embodiment 3 in accordance with the present invention controls the incident luminous flux I of a projection type light modulating element. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will be specifically described in the following based on the preferred embodiments thereof with reference to the figures. 
   (First Embodiment) 
     FIG. 2  is an illustration to show a mirror element and an OFF light reflecting unit of a projection television of a preferred embodiment 1 in accordance with the present invention. Here, for the sake of simplification, an optical system of monochromatic light will be described. Also, the description of the same members as the conventional ones will be omitted. 
   In  FIG. 2 , a reference character  5  designates a polarization converting element, a reference character  6  designates a liquid crystal shutter element, a reference character  7  designates a mirror, and reference characters L 1 , L 2 , and L 3  designate incident light, ON light, OFF light, respectively. 
   The polarization converting element described in Japanese Patent Publication Laid-open No. 7-294906 and Japanese Patent Publication Laid-open No.11-183848 can be used as the polarization converting element  5  described above. The polarization converting element is an element for converting light having random polarization directions (non-polarized light) into light having one polarization direction. 
   The polarization converting element will be described in brief. The polarization converting element comprises a polarization beam splitter array in which polarization beam splitters having polarization separating films and prisms are alternately arranged. A lens array is added to the light entrance plane of the polarization beam splitter and a λ/2 phase plate is added to the light emission plane thereof. 
   The incident light having random polarization directions includes so-called s-polarized light and p-polarized light. The incident light entering the lens array enters polarization beam splitters arranged in correspondence with the individual lens portions. 
   The incident light is separated by the polarization separating film into the s-polarized light reflected by the polarization separating film and the p-polarized light transmitting through the polarization separating film. The reflected s-polarized light is refracted and emitted in the direction of emission of light by a neighboring prism. 
   Further, the transmitted p-polarized light is transmitted through the λ/2 phase plate provided on the light emission plane of the polarization beam splitter and is converted into s-polarized light and is emitted in the direction of emission of light. 
   Therefore, the incident light transmitting through the polarization converting element and having random polarization directions is converted into emitting light most of which is s-polarized light (if light having polarizing component in the first and second polarization directions enters, the polarizing component of the light is converted into a polarizing component in the first or second polarization direction). 
   Further, a reference character L 301  designates light transmitted through the polarization converting element  5  (hereinafter referred to as transmitted light L 301 ), a reference character L 302  designates light transmitted through the liquid crystal shutter element  6  (hereinafter referred to as transmitted light L 302 ), and a reference character L 303  designates light reflected by the mirror  7  (hereinafter referred to as reflecting light L 303 ). 
   A reference character L 304  designates the light  303  after transmission through the liquid crystal shutter element  6  (hereinafter referred to as transmitted light L 304 ), a reference character L 305  designates the light  304  after transmission through the polarization converting element  5  (hereinafter referred to as transmitted light L 305 ), and a reference character L 306  designates the light  305  which is reflected by a light receiving plane  3  and propagates in the direction opposite to the direction of the incident light L 10 (that is, the respective mirror elements are arranged so that their surfaces reflect light in the direction of the OFF light L 30 , hereinafter referred to as reflecting light L 306 ). 
   In this respect, these transmitted lights L 301  and L 302 , the reflecting light L 303 , the transmitted light L 304  and L 305 , and the reflecting light L 306  are generated when the OFF light L 30  is generated. When the ON light L 20  is generated, these transmitted lights L 301  and L 302 , the reflecting light L 303 , the transmitted lights L 304  and L 305 , and the reflecting light L 306  are not generated. 
   Operations will be described in the following. Light emitted from a lamp not shown (a monochromatic light source for emitting monochromatic light or a white light source for emitting white light including three primary colors (in the case of color display)) enters a light receiving plane  3  as incident light L 10 . The incident light L 10  is separated into the ON light L 20  and the OFF light L 30  by the tilts of the individual mirror elements two-dimensionally arranged in the mirror arrangement region  4 . 
   In the case where the ON light L 20  is generated, the incident light L 10  is reflected and polarized in the direction of the ON light L 20  in the figure in the vicinity of the light receiving plane  3  and passes through the projection optical system and becomes a light point on a screen (not shown). 
   Further, in the case where the OFF light L 30  is generated, the incident light L 10  is reflected and polarized in the direction of the OFF light L 30  in the figure in the vicinity of the light receiving plane  3  and enters the polarization converting element  5 . Here, the mirror elements arranged in the mirror arrangement region  4  are not in the intermediate state but in the “ON” state or in the “OFF” state. 
   The polarization direction of the OFF light L 30  is not constant. The polarization direction of the OFF light L 30  basically includes two kinds of straight polarizing components of a first direction and a second direction perpendicular to the first direction (hereinafter simply referred to as a first polarizing component and a second polarizing component). 
   The polarization converting element  5  transmits the first polarizing component of the two kinds of polarizing components of the OFF light L 30  as it is. Also, the polarization converting element  5  converts the second polarizing component into the first polarizing component. 
   Therefore, the transmitted light after the polarization converting element  5  makes the transmitted light L 301  of a combination of the first polarizing component passing through the polarization converting element  5  as it is and the second polarizing component converted into the first polarizing component by the polarization converting element  5  (the polarization converting element  5  has a function of aligning the polarization directions). 
   The liquid crystal shutter element  6  has polarization transmitting filters for transmitting the first polarizing component on both faces thereof. The liquid crystal included in the liquid crystal shutter element  6  can change polarization rotating angle by a known control (for example, voltage control or the like). 
   Therefore, the liquid crystal shutter element  6  having the polarization transmitting filters for transmitting the first polarizing component on both planes thereof can change the light intensity of the transmitted light L 302  transmitted the liquid crystal shutter element  6  by giving a control signal to the liquid crystal shutter element  6  from the outside. 
   That is, in the case where the liquid crystal shutter element  6  is in the state where it can transmit the first polarizing component with no loss, the transmitted light L 301  entering the liquid crystal shutter element  6  is emitted as the transmitted light L 302  without attenuation. 
   The mirror  7  reflects the transmitted light L 302  entering the mirror  7  (this light is the reflecting light L 303 ). Since the reflecting light L 303  is the first polarizing component, it passes through the liquid crystal shutter element  6  (this light is the transmitted light L 304 ). 
   Further, since the polarization converting element  5  transmits the first polarizing component, the transmitted light L 304  is transmitted through the polarization converting element  5  (this light is the transmitted light L 305 ). 
   The transmitted light L 305  enters the light receiving plane  3  and is reflected thereby (this light is the reflecting light L 306 ). The reflecting light L 306  propagates in the direction of the lamp (not shown) which is opposite to the direction of the propagation of the incident light L 10 . 
     FIG. 3  is a configurational view to show the whole optical system of a projection television of the preferred embodiment  1  in accordance with the present invention. 
   In  FIG. 3 , a reference character  8  designates a lamp, a reference character  9  designates the light source of the lamp  8 , a reference character  10  designates a reflecting plate for reflecting light emitted by the light source  9  provided in the lamp  8  in the right direction in the figure, a reference character  12  designates an emitting luminous flux emitted by the lamp  8 . A reference character  11  designates a beam shaping optical system into which the emitting luminous flux  12  emitted by the lamp  8  is entered to change the diameter of the emitting luminous flux  12  (diameter of the emitting luminous flux  12  emitted by the lamp  8  when it is a circular luminous flux). 
   A reference character  13  designates the emitting luminous flux emitted by the beam shaping optical system  11  and a reference character  14  designates a driving signal for controlling the light emission of the light source  9 . 
   A reference character  15  designates a driving signal for driving and/or controlling the individual mirror elements included in the optical deflector  400  and a reference character  16  designates a driving signal for driving and/or controlling the liquid crystal shutter element  6 . 
   In this respect, a reference character  100  designates an optical unit and comprises the lamp  8 , the beam shaping optical system  11 , the light deflector  400 , the polarization converting element  5 , the liquid crystal shutter element  6  and the mirror  7 . 
   The optical deflector  400 , as described above, changeably reflects the emitting light in two directions. An optical reflector comprises the polarization converting element  5 , the liquid crystal shutter element  6  and the mirror  7 . 
   This optical reflector reflects the reflecting light (OFF light) emitted by the optical deflector  400  in one direction of the two directions (or the reflecting light is reflected in either one of two directions) in the one direction (The optical reflector for reflecting light reflected in said one of two directions, along an axis of said one of two directions. Accordingly, the reflecting light is returned to the light source). 
   In this respect, an optical system such as projection lens and the like provided in the part to which the ON light L 20  is to be propagated and a screen to which an image is projected are not shown in the figure. 
   In a conventional projection type display, a metal halide lamp or an arc lamp which can emit strong light is used as a light source. In the configuration shown, for example, an arc lamp is used as the light source  9  included in the lamp  8 . 
   In the case where the arc lamp is used as the light source  9 , light is emitted from a micro point (light emitting point) in a space by a discharging arc. The reflecting plate  10  is constituted so as to reflect the light emitted by the light source  9  in a predetermined direction. 
   A parabolic mirror, for example, is used as the reflecting plate  10 , and in the case where the parabolic mirror is used, by disposing the light emitting point of the light source  9  at the focal point of the parabolic mirror, the light emitted by the reflecting plate  10  is made the nearly parallel luminous flux  12 . 
   The emitting luminous flux  12  emitted by the reflecting plate  10  enters the beam shaping optical system  11  as the emitting light from the lamp  8 . In this case, the emitting luminous flux  12  makes a collimating luminous flux. 
   The beam shaping optical system  11  shapes the emitting luminous flux  12  emitted by the lamp  8  into an emitting luminous flux  13  having a diameter suitable for irradiating the light receiving plane  3  of the optical deflector  400 . The beam shaping optical system  11  is constituted in the same way as an ordinary beam magnifying/diminishing unit for varying the diameter of the emitting luminous flux  12  by a desired magnification. 
   The emitting luminous flux  13  emitted by the beam shaping optical system  11  irradiates the light receiving plane  3  of the optical deflector  400  as the incident light L 10  entering the optical deflector  400 . The incident light L 1  irradiating the light receiving plane  3  is the ON light L 20  or the OFF light L 30 , as described above. 
   In the case where the OFF light L 30  is generated, as described with reference to  FIG. 2 , the transmitted light L 305  is generated as the returning light of the OFF light L 30  entering the polarization converting element  5  by the operation of the polarization converting element  5 , the liquid crystal shutter element  6  and the mirror  7  and enters the light receiving plane  3  of the optical deflector  400  (this generates the reflecting light L 306 ). 
   Since the process from the entrance of the OFF light L 30  into the polarization converting element  5  to the generation of the reflecting light L 306  is conducted by a light speed, most of the mirror elements included in the optical deflector  400  are tilted to the angle to produce the OFF light L 30 . 
   Therefore, the reflecting light L 306  is made returned to the vicinity of the light emitting point of the light source  9  by the beam shaping optical system  11  and the reflecting plate  10 . The light returned to the vicinity of the light emitting point of the light source  9  is propagated back and forth on the above-mentioned propagation path comprising the reflecting plate  10 , the beam shaping optical system  11 , the optical deflector  400 , the polarization converting element  5 , the liquid crystal shutter element  6 , and the mirror  7 . 
   In this regard, the optical path from the lamp  8  to the optical deflector  400  of the configuration shown in  FIG. 3  (which are arranged in the beam shaping optical system  11  in the actual configuration) includes an optical element shaped like a column or having a reflecting wall on the inner surface of a hollow space. 
   This optical element is provided so as to uniform the light intensity of the emitting luminous flux  12  or  13  in a plane normal to an optical axis (these are described in detail, for example, in U.S. Pat. Nos. 5,625,738, 5,634,704, 507,613). That is, this optical element is a light uniforming element. The light uniforming element is used for uniforming the light intensity of the emitting light of the lamp  8  in the plane normal to the propagation direction of the emitting light. 
   As described in the above-mentioned U.S. Patents, for example, in the optical element shaped like a column, the incident light satisfies the total reflection conditions on the outer surface of the column-shaped optical element and is repeatedly reflected plural times in the optical element. 
   In this manner, the emitting light from the end face of the emitting side of the optical element is emitted in the state in which the intensities of a lot of light entering the optical element are mixed, whereby the light intensity distribution is made uniform in a plane normal to the optical axis. 
   Further, in the optical element having the reflecting wall on the inner surface of the hollow space, light is repeatedly reflected plural times in the hollow space. 
   These operations makes the light intensity distribution of the emitting light of the light entering the optical element uniform in a plane normal to the optical axis. 
   The light returning to the vicinity of the light emitting point of the light source  9  is made uniform by this unit for making light intensity uniform. Accordingly, in addition to the mirror elements included in the optical deflector generating the OFF light, the light returning to the vicinity of the light emitting point is applied also to the mirror elements generating the ON light (the returned OFF light is utilized as the ON light.) 
   According to this configuration, the light returning to the vicinity of the light emitting point of the light source  9  is again utilized in the optical unit  100 . Therefore, the light intensity in the vicinity of the light emitting point of the light source  9  in this configuration is made larger than that in the configuration in which the reflecting light L 304  is not returned to the vicinity of the light emitting point of the light source  9  (this is because the returned light is added to light newly emitted by the light source  9 ). 
   If the operation to be accomplished by the configuration described above is utilized, desired light can be obtained without increasing light newly emitted by the light source  9 . Further, desired light intensity can be obtained even if light newly emitted by the light source  9  is reduced as compared with the conventional configuration. 
     FIG. 4  is a functional block diagram to show a signal processing unit for processing the signal of the projection television of the preferred embodiment 1 in accordance with the present invention. 
   In  FIG. 4 , a reference character  20  designates a television image signal, a reference character  21  designates an image memory (memory unit) for storing image information of one frame/field (one screen) of the image signal, a reference character  22  designates a feature detecting unit for detecting the features of an image such as a maximum value, a minimum value, an average value and the like of the luminance level of the television image signal  20 , and a reference character  23  designates a unit for calculating the average ON ratio P in the optical deflector  400 . In this case, the average ON ratio P is a ratio of the ON period of the average light on the display screen to a predetermined period when the display screen is displayed in sequence of time. 
   A reference character  24  designates an image quality adjusting unit for adjusting an image quality, a reference character  25  designates a timing generating unit for generating ON/OFF timings to drive the mirror elements in the optical deflector  400 , and a reference character  26  designates an inverted gamma-correction table for correcting the beam current characteristics of a CRT. 
   A reference character  27  designates a timing randomizing unit for varying ON/OFF timings for the respective mirror elements in the optical deflector  400  and a reference character  28  designates a mirror driving unit for driving the mirror elements in the optical deflector  400 . 
   A reference character  29  designates a control unit for outputting a control signal based on the output of the average on ratio calculating means  23  for adjusting an image quality and driving a shutter and a lamp. 
   A reference character  30  designates a shutter driving unit, a reference character  31  designates a lamp driving unit, a reference character  32  designates a feature signal for representing the features of the image, a reference character  33  designates an ON ratio P signal for representing an ON ratio P in the optical deflector  400 , and a reference character  34  designates an image quality adjusting controlling signal applied to the image quality adjusting unit  24 . 
   Operations will be described in the following. 
   The image memory  21  stores an inputted television image signal  20  as a digital signal (image data) per one frame/field (one screen). 
   The image data stored in the image memory  21  is outputted to the feature detecting unit  22  and the average ON ratio calculating unit  23 . The feature detecting unit  22  detects the features of the image such as the maximum value, the minimum value, the average value and the like of a luminance level in the inputted image data. 
   The average ON ratio calculating unit  23  reads out a coefficient necessary for performing an inverted gamma correction corresponding to the luminance levels of the respective pixels in the inputted image data from the inverted gamma-correction table  26  to perform the inverted gamma correction. Then, the average ON ratio calculating unit  23  calculates an average ON ratio P in the optical deflector  400  based on the corrected image data. 
   The image quality adjusting unit  24  reads out the image data stored in the image memory  21  and adjusts (corrects) the image date so as to make it a desired image quality. Then, the image quality adjusting unit  24  outputs image data after an image quality adjustment to the timing generating unit  25 . 
   The timing generating unit  25  reads out a coefficient necessary for performing the inverted gamma correction corresponding to the luminance levels of the respective pixels in the inputted image data from an inverted gamma-correction table  26  and performs the inverted gamma correction. 
   In addition, the timing generating unit  25  generates and outputs a timing signal for setting the individual mirror elements included in the optical deflector  400  in the “ON” state based on the image data subjected to the inverted gamma correction. 
   The timing signal outputted by the timing generating unit  25  is inputted to the timing randomizing unit  27  where the inputted timing signal is subjected to a shift operation on a time axis to make a new timing signal such that it provides a different timing between pixels and is outputted. 
   The new timing signal outputted by the timing randomizing unit  27  is inputted to a mirror driving unit  28 . The mirror driving unit  28  outputs a driving signal for driving the mirror elements included in the optical deflector  400  based on the new timing signal to drive the mirror elements. 
   A feature signal  32  and an average ON ratio P signal  33  are inputted to the control unit  29 . The control unit  29  outputs control signals to the shutter driving unit  30  and the lamp driving unit  31  based on these inputted signals. Further, the control unit  29  outputs an image adjusting controlling signal  34  to the image adjusting unit  24 . 
   The optical deflector  400 , the optical shutter  6  and the lamp  8  which are included in the optical unit  100  are driven, respectively, as described above. 
   The operation of the timing randomizing unit  27  will be further described in detail with reference to FIG.  5 . 
     FIG. 5  shows the relationship between one example of a screen display of a projection type display and mirror elements. In FIG.  5 ( a ), a reference character  40  designates a display screen and a reference character  41  designates a rectangular bright portion displayed near the center of the display screen  40  as one example of the display of the display screen  40 . 
   A reference character  42  designates a portion which is displayed outside the bright portion  41  and is darker than the bright portion  41 . 
   Further, reference characters m 1  and m 2  shown in FIG.  5 ( b ) are bright pixels which are in the vicinity of the boundary of the bright portion  41  and the dark portion  42  and are included in the bright portion  41 . Reference characters m 3  and m 4  are dark pixels which are in the vicinity of the boundary of the bright portion  41  and the dark portion  42  and are included in the dark portion  42  (description will be made assuming that the bright pixels m 1  and m 2  have the same brightness and the dark pixels m 3  and m 4  have the same brightness). 
   Here, the words “bright” and “dark” mean that in the case where the brightness of displayed pixels are compared with each other, a relatively bright pixel is called a bright pixel and a relatively dark pixel is called a dark pixel. 
   In this respect, for the sake of simplification, the following assumption will be made. That is, the assumption will be made that mirror elements  1  corresponding to the dark pixels m 3  and m 4  are in the “OFF” state and that a ratio of a time period during which the mirror elements  1  corresponding to the bright pixels m 1  and m 2  are in the “ON” state to one screen period is 50%. 
   FIG.  5 ( c ) and ( d ) are examples of the driving state into which the mirror elements  1  corresponding to the respective bright pixels m 1  and m 2  are brought (in FIG.  5 ( c ) and ( d ), a vertical axis represents projection light intensity (relative value) displaying the respective pixels and a lateral axis represents time, respectively). 
   Reference characters T 1  and T 2  represent a starting time and an ending time of one screen period. A reference character  43  designates one screen period and if the mirror elements  1  are in the “ON” state in this period, a ratio of a time period during which the mirror elements  1  are in the “ON” state to one screen period is 100%. 
   Reference characters  44 ,  45 ,  46 , and  47  represent time periods during which the mirror elements  1  corresponding to the respective pixels m 1  and m 2  are in the “ON” state. 
   In the case of FIG.  5 ( c ) and ( d ), as described above, the assumption is made that the time periods during which the mirror elements  1  are in the “ON” state (that is, the sum of the time periods  44  and  45 , and the sum of the time periods  46  and  47 ) are 50% of one screen period. 
   Here, the time periods  44  and  45  are different in timing on a time axis from the time periods  46  and  47 , as shown in the figures. Different timings can be given to arbitrary two pixels in the same way. 
   Here, as shown in the figures, the above-mentioned different timings are given to two time periods in one screen period such as the periods  44  and  45 , and the periods  46  and  47 . However, this is not limited to two time periods in one screen period (the number of time periods in one screen periods may be three or more). 
   The timing randomizing unit  27  varies ON timings for the respective mirror elements. Usually, as to pixels constituting a screen, a screen in one screen period is represented by several hundred thousand pixels (corresponding to the total number of the mirror elements  1  in the case where the mirror elements  1  included in the optical deflector  400  are two-dimensionally arranged). 
   Therefore, the timing may be determined at any time period or timing if the timing can reduce a variation in the average ON ratio P during one screen period. 
   In the above-mentioned preferred embodiment, it has been described that the light intensity of the incident light L 10  can be substantially increased by returning light emitted by the lamp  8  to the lamp  8 . In the following, this will be further described in a quantitative aspect. 
     FIG. 6  shows the transfer of light in the configuration of the optical unit  100  shown in FIG.  3 . In  FIG. 6 , a reference character a designates the reflectance factor of a reflecting plate  10 , a reference character n designates the light transfer rate of the beam shaping optical system  11 , and a reference character P designates the average ON ratio of the optical deflector  400 . 
   A reference character K designates a ratio that the OFF light emitted by the mirror elements  1  of the optical deflector  400  passes through the polarization converting element  5 , the liquid crystal shutter element  6  and the mirror  7  and returns to the mirror elements  1  of the optical deflector  400 . In the case where such a liquid crystal shutter element  6  is used, the ratio K can be, controlled by changing a polarizing angle by the liquid crystal shutter element  6 . 
   A reference character I 0  designates a light intensity emitted by the light source  9  of the lamp  8  (that is, the intensity of light generated by the light source  9  and not including the intensity of light returning to the light source  9 ) of the emitting luminous flux  12  shown in  FIG. 3. A  reference character I 1  designates the intensity of light emitted by the light source  9  of the lamp  8  and passing through the beam shaping optical system  11  (that is, the intensity of light generated by the light source  9  and not including the intensity of light returning to the light source  9  and passing through the beam shaping optical system  11 ) of the emitting luminous flux  13  shown in FIG.  3 . 
   Considering the whole optical unit  100 , light corresponding to the average ON ratio P of the light intensity I 1  is the ON light L 20 . The ON light L 20  is projected to the screen by the projection optical system disposed after the optical deflector  400 . 
   When the average ON ratio is P, the ratio of the OFF light L 30  is given by 1−P. The OFF light L 30  is returned to the mirror elements  1  of the optical deflector  400  at the ratio K by the polarization converting element  5 , the liquid crystal shutter element  6  and the mirror  7  by the operation described above. 
   In this respect, in this case, the optical position and arrangement of the optical deflector  400  are adjusted so that the OFF light of the mirror elements  1  is returned to the mirror elements  1  in the OFF state of the optical deflector  400 . 
   This arrangement returns the OFF light to the vicinity of the light emitting point of the light source  9  of the lamp  8  via the beam shaping optical system  11  by the operation described above. 
   Letting a ratio that the emitting luminous flux  13  having a light intensity I 1  returns to the light source  9  be m, the ratio m is expressed by the following equation by the use of the average ON ratio P and ratio K.
 
 m= (1− P )× K 
 
   FIG.  6 ( b ) is an illustration in which the propagation of the OFF light is simplified. Simplifying the optical system from the light source  9  to the mirror  7 , in this optical system, the reflecting plate  10  and the mirror  7  can be regarded as an optical system including two opposed mirrors having certain light transfer rates. This is shown in FIG.  6 ( b ). 
   In FIG.  6 ( b ), a reference character  50  designates a mirror having a reflectance factor a corresponding to the reflecting plate  10 , a reference character  51  designates a mirror having a reflectance factor m (equivalent to the ratio m described above) corresponding to the mirror  7 . 
   The light in the optical system shown in FIG.  6 ( a ), like the simplified configuration shown in FIG.  6 ( b ), propagates back and forth between the mirrors  50  and  51  via the beam shaping optical system  11  sandwiched between the mirrors  50  and  51 . 
   In FIG.  6 ( b ), a reference character I r  designates the intensity of light propagating back and forth r times between the mirrors  50  and  51 . This light intensity I r  is derived from the study of the r-time propagation of light between the mirrors  50  and  51 . The light intensity I r  is expressed by the following equation (1) (which is expressed by a geometric progression having an initial term of I 0 .n and a common ratio a.m.n 2 ) 
                       I   1     =       ⁢       I   0     ·   n                   I   2     =       ⁢       I   1     ·   m   ·   n   ·   a   ·   n                 =       ⁢       I   1     ·   a   ·   m   ·     n   2                     I   3     =       ⁢       I   2     ·     (     a   ·   m   ·     n   2       )                   =       ⁢       I   1     ·     (     a   ·   m   ·     n   2       )                     I   r     =       ⁢       I     r   -   1       ·     (     a   ·   m   ·     n   2       )                   =       ⁢       I   1     ·       (     a   ·   m   ·     n   2       )       r   -   1                             EQUATION   ⁢           ⁢     (   1   )               
 
   Further, the light intensity I caused by the round propagation of light can be calculated by assuming that the number of the round propagations r is infinite, that is, can be calculated as shown in the following equation (2) by calculating the sum of the infinite series shown by the above equation (1) (can be expressed by the sum of infinite series having an initial term of I 0 .n and a common ratio a.m.n 2 ). 
               I   =         ∑     r   =   1     ∞     ⁢     I   r       =           I   1     ·     {     1   +     a   ·   m   ·     n   2       +       (     a   ·   m   ·     n   2       )     2     +   …     }       ⁢       (     a   ·   m   ·     n   2       )     ·   I       =           I   1     ·     {       a   ·   m   ·     n   2       +       (     a   ·   m   ·     n   2       )     2     +   …     }       ⁢       (     1   -     a   ·   m   ·     n   2         )     ·   I       =     I   1             ⁢     
     ⁢     I   =       I   1     /     (     1   -     a   ·   m   ·     n   2         )                 EQUATION   ⁢           ⁢     (   2   )               
 
   An increase rate of the light intensity I with respect to the light intensity I 1  can be calculated by dividing both side of the equation (2) (here, m=(1−P).K) by the light intensity I 1 , as shown by the following equation (3).
 
 I/I   1 =1/(1− a. (1− P ) K.n   2 )  EQUATION (3)
 
   Letting the average ON ratio P be a variable and a ratio K be a parameter, the equation (3) are calculated as shown in FIG.  7 . 
   In  FIG. 7 , a lateral axis designates an average ON ratio P and a vertical axis designates a light intensity I. In  FIG. 7 , curves  60  to  63  show variations in the light intensity I in the case where the reflectance factor a of the mirror  50  is 0.95 and a light transfer rate of the beam shaping optical system  11  is 0.9 and a ratio K is varied 1.0, 0.8, 0.5 and 0.2, respectively. A reference character  64  designates the light intensity I in the case the OFF light is not returned to the light source  9 . 
   A reference character  65  designates a point of the light intensity I in the case where the average ON light P is 100% (that is, the OFF light is not emitted). Reference characters  66  to  68  designate points of the light intensity I when the ratio K is 1 and the average ON light P is varied about 0.75, 0.375 and 0.075. 
   A reference character  69  designates a point of the light intensity I in the case where the light intensity at the point  68  is reduced by adjusting the liquid crystal shutter element  6  (by reducing a light transmission rate) to reduce the ratio K to about 0.65 (the light intensity I is expressed by a relative intensity in which when the average ON ratio P is 100% (average ON ratio P=1), the light intensity I=1). 
   As can be seen from the curve  60 , the light intensity I gradually decreases as the average ON ratio P increases and becomes  1  when the average ON ratio P becomes 100% (point  65  in FIG.  7 ). 
   In the case where the average ON ratio P is 100%, when all the reflecting light of the incident light to the optical deflector  400  are the ON light, light intensity=1. In this case, there is no OFF light (that is, no light returns to the vicinity of the light emitting point of the light source  9 ). 
   When the average ON ratio P is about 0.375, the light intensity I becomes 2 (point  67  in FIG.  7 ). In this state, the intensity of the incident light when the OFF light returning to the vicinity of the light emitting point of the light source  9  again enters the optical deflector  400  is 2 times the light intensity when the average ON ratio P is 100%. 
   When the average ON ratio P is further reduced to about 0.075, the light intensity I becomes 3.5 (point  68  in FIG.  7 ). In this state, the intensity of the incident light when the OFF light returning to the vicinity of the light emitting point of the light source  9  again enters the optical deflector  400  is 3.5 times the light intensity when the average ON ratio P is 100%. 
   In order to display an image with high fidelity, ideally, it is desirable that the brightness of a screen is never varied. From this viewpoint, for example, in a CRT type television, in the case of an image having an average luminance higher than an ordinary luminance level, the beam current of the CRT is limited to limit the luminance of the image. 
   Accordingly, also in a projection type display like a projection television, if the projection type display can stably display an image of the ordinary average luminance level, it can be thought to have a sufficient performance. 
   In the case where the average ON ratio P is equal to or smaller than a required value Q corresponding to required light intensity I=2, in  FIG. 7 , regarding the image as having an almost average luminance, stabilizing the light intensity at 2 can be realized by controlling the ratio K. 
   Conversely, in the case where the average ON ratio P is larger than the required value Q, by maximizing the ratio K, the light intensity I with respect to the average ON ratio P is gradually varied (limited) along the curve  60  shown in FIG.  7 . 
   This can make the limit characteristics of the luminance similar to those of the CRT type television. Here, the required value Q can be determined based on the quality of the image and the extent to which the light intensity I is to be increased. 
   Into the control unit  29  in  FIG. 4  are inputted the average ON ratio P signal  33  outputted by the average ON ratio calculating unit  23  and the feature signals  32  outputted by the image feature detecting unit  22  and including the maximum value Vmax, the minimum value Vmin, and the average value Vave of the luminance levels of the respective pixels which indicate the features of the image. 
   The control unit  29  compares the initial value Q 0  of the required value Q which is previously stored therein with the respective average ON ratio P signals  33  inputted and outputs various kinds of control signals according to the characteristics of the image. 
   When the value given by the average ON ratio P signal  33  is smaller than the initial value of the average ON ratio P, the control unit  29  outputs a control signal to control the ratio K so that the shutter driving unit  30  for driving the liquid shutter element  6  makes the light intensity I constant. 
   In addition, in the case where the difference between the maximum value Vmax and the minimum value Vmin of the luminance levels of the respective pixels included in the feature signal  32  is close to the difference between the maximum luminance and the minimum luminance of the image, the image is judged as having a high contrast and the above required initial value Q 0  is changed into a required value Q 1  smaller than the initial value Q 0 . 
   According to the change of the initial required value Q 0  to the required value Q 1 , the control unit  29  gives a control signal to increase the contrast of the image to the image quality adjusting unit  24  to increase the extent of the increase of the light intensity I. 
   In this respect, the control unit  29  controls the luminance levels of the respective pixels so that they keep the average luminance level Vave thereof during at least from several fields to several frames. When the control unit  29  judges that the average luminance level Vave is small during from several fields to several frames, it judges that the OFF light is much generated to set the ratio K at a value close to the maximum value and gives a control signal for reducing the brightness of the lamp  8  to the lamp driving unit  31 . 
   The case where the average luminance level Vave is very small corresponds to the case where the average ON ratio P shown in  FIG. 7  is almost zero. Therefore, the light intensity I can be increased by a factor of about 4 by setting the ratio K at the maximum value. 
   Controlling the light intensity I generated by the lamp  8  to half in this state is equivalent to controlling the light intensity I to two times. 
   In this respect, needless to say, the values of the reflectance factor a of the reflecting plate  10 , the light transfer rate n of the beam shaping optical system  11 , the average ON ratio P of the optical deflector  400 , and the ratio K, which have been described up to this point, vary with the optical parts constituting the optical system. 
   There may be various choices in the optical parts used in the optical systems. For example, it is not always necessary that the ratio K can vary from 1 to 0. That is, in the example shown in  FIG. 7 , if the ratio K is larger than about 0.6, the increase rate of the light intensity I can be made 2. 
   Further, even when the ratio K can not be increased, if a part of OFF light is entered into the liquid crystal shutter element  6  and the other part of OFF light is reflected by a mirror having a high reflectance factor, the ratio K can be substantially increased. 
   In this respect, as to the response speed of the liquid crystal shutter element  6 , a liquid crystal shutter element which can be operated within a few milliseconds has been put into practical use. Therefore, it is possible to control the ratio K for each video field/frame time period (about {fraction (16/32)} millisecond). 
   (Preferred Embodiment 2) 
   The preferred embodiment 2 in accordance with the present invention has the following features as compared with the above-mentioned preferred embodiment 1. Here, the description of the same configuration and operation as the preferred embodiment 1 will be omitted. 
     FIG. 8  is an illustration to describe a mirror element and an OFF light reflector of a projection television in a preferred embodiment 2 in accordance with the present invention. Here, for the sake of simplification, a monochromatic optical system will be described. The description of the same things as the conventional one or the preferred embodiment 1 will be omitted. 
   In  FIG. 8 , a reference character  410  designates a second optical deflector, a reference character  31  designates an incident plane of the second optical deflector  410 , a reference character L 11  designates an incident light, a reference character L 21  designates an ON light, a reference character L 31  designates an OFF light, a reference character L 311  designates a reflecting light reflected by the second optical deflector  410  and propagating in the direction opposite to the direction of the OFF light L 31 . 
   A reference character L 312  designates a reflecting light reflected by the second optical deflector  410  and not propagating in the direction opposite to the direction of the OFF light L 31  (OFF light from the second deflector  410 ). The configuration and the operation of the second deflector  410  are the same as those previously described with reference to FIG.  1 ( a ). 
   As shown in  FIG. 8 , the incident light L 31  enters the incident plane of the second deflector  410  at an angle θ with respect to a direction perpendicular to the incident plane. When the mirror elements of the second optical deflector  410  are tilted at the angle θ in the anticlockwise direction in the figure (the mirror elements of the second optical deflector  410  are in the ON state), the reflecting light L 311  is generated as a reflecting light with respect to the incident light L 31 . 
   When the mirror elements of the second optical deflector  410  are tilted at the angle θ in the clockwise direction, the reflecting light L 312  tilted at an angle  3 θ in the clockwise direction with respect to the direction perpendicular to the incident plate of the second optical deflector  410  is emitted as reflecting light with respect to the incident light L 31  (the mirror elements of the second optical deflector  410  are in the OFF state). 
   Therefore, by controlling the ratio of the mirror elements in the ON state of the second optical deflector  410 , the ratio of the reflecting light L 311  to the incident light L 31  (or, the ratio of the reflecting light to the light source  9 ) can be controlled. Here, the reflecting light L 312  of the second optical deflector  410  is absorbed by a black mask or the like. 
   Here, the second optical deflector  410  is not always required to be of the same shape as the optical deflector  400 , but light collecting unit such as a collective lens or the like may be provided in the optical path through which the incident light L 31  propagates. 
   This makes it possible to use the second optical deflector  410  which has a small number of mirror elements and hence is small in size and inexpensive. 
   Assuming that a optical deflector having the same shape as the optical deflector  400  is used as the second optical deflector  410 , even if the second optical deflector  410  has about 1% of defective elements, there is no problem in practice. 
   The second optical deflector  410  is driven in the same way as the optical deflector  400  by the same configuration (not shown) as drives the optical deflector  400 , as described with reference to  FIG. 4 , the configuration including the timing generating unit  25 , the timing randomizing unit  27  and mirror driving unit  28 . 
   Further, since the second optical deflector  410  is not related to an image itself projected to the screen, it is essential only that the mirror elements are changed between the ON state and the OFF state for each one field time period or one frame time period and the states of the mirror elements are not required to be changed at high speeds which are required in the display of the pixels. 
   According to the present preferred embodiment 2, since the OFF light from the optical deflector  400  does not propagates back and forth through an optical path including the polarization converting element  5  and the liquid crystal shutter element  6 , it causes a small loss of light. Therefore, it is possible to increase the ratio of light returning to the light source  9  as compared with the preferred embodiment 1. 
   In the present preferred embodiment, the optical reflector in the preferred embodiment 1 is constituted of the second optical deflector  410 . 
   That is, the optical reflector in the present preferred embodiment is constituted such that it changeably reflects the incident light to the optical reflector into two directions and that the reflecting light (OFF light) in one direction of the two directions from the optical deflector  400  is reflected in the direction of the incident light. 
   This produces an advantage that the second optical deflector  410  is not required to be arranged relatively to the optical deflector  400  with high accuracy, and the like. 
   (Preferred Embodiment 3) 
     FIG. 9  is a configurational view to show the general optical system of a color type projection television of a preferred embodiment 3 in accordance with the present invention. Here, in the present preferred embodiment, the description of the same configuration and operation as the above-mentioned preferred embodiments will be omitted. 
   In  FIG. 9 , a reference character  200  designates an optical unit, a reference character  82  designates a total reflecting prism, a reference character  83  designates the reflecting plane of the total reflecting prism  82 , a reference character  84  designates a color separating prism (dichroic prism, color separating element, which separates white light into a plurality of color lights), a reference character  85  designates the red reflecting plane of the color separating prism  84 , and a reference character  86  designates the blue reflecting plane of the color separating prism  84 . 
   Reference characters  210 G,  210 R, and  210 B designate a green light modulating unit, a red light modulating unit and a blue light modulating unit, respectively. 
   In the light modulating unit  210 G, a reference character  400 G designates an optical deflector,  5 G designates a polarization converting element,  6 G designates a liquid shutter element,  7 G designates a mirror and  81 G designates an optical sensor. 
   A reference character  15 G designates a driving signal of the optical deflector  400 G,  16 G designates a driving signal of the liquid crystal shutter  6 G, and  90 G designates an output signal of the optical sensor  81 G. 
   A reference character L 10 G designates an incident light (green) entering the light modulating unit  210 G, L 20 G designates an emitting light (green) emitted from the light modulating unit  210 G. 
   Similarly, in the light modulating unit  210 R, a reference character  400 R designates an optical deflector,  5 R designates an polarization converting element,  6 R designates a liquid crystal shutter element,  7 R designates a mirror, and  81 R designates an optical sensor. 
   A reference character  15 R designates a driving signal of the optical deflector  400 R,  16 R designates a driving signal of the liquid crystal shutter  6 R, and  90 R designates a output signal of the optical sensor  81 R. 
   A reference character L 10 R designates an incident light (red) entering the light modulating unit  210 R, L 20 R designates an emitting light (red) emitted from the light modulating unit  210 R. 
   Similarly, in the light modulating unit  210 B, a reference character  400 B designates an optical deflector,  5 B designates an polarization converting element,  6 B designates a liquid crystal shutter element,  7 B designates a mirror, and  81 B designates an optical sensor. 
   A reference character  15 B designates a driving signal of the optical deflector  400 B,  16 B designates a driving signal of the liquid crystal shutter  6 B, and  90 B designates an output signal of the optical sensor  81 B. 
   A reference character L 10 B designates an incident light (blue) entering the light modulating unit  210 B, L 20 B designates an emitting light (blue) emitted from the light modulating unit  210 B. 
   A reference character L 20 C designates a synthetic light of the emitting lights L 20 G, L 20 R, and L 20 B of the light modulating units  210 G,  210 R, and  210 B of the respective colors of green, red, and blue. Here, in  FIG. 9 , a projection lens and a screen are not shown which are to be arranged at some later points in the direction of propagation of the synthetic light L 20 C. 
   A reference character L 10 C is the emitting light of a beam shaping optical system  11  and an emitting luminous flux (white luminous flux) including the respective colors of green, red, and blue lights. 
   The emitting luminous flux L 10 C enters the total reflecting prism  82  and, as shown in  FIG. 9 , is totally reflected by the reflecting plane  83  and entered into the color separating prism  84 . 
   In the color separating prism  84  are provided the red reflecting plane  85  made of a dielectric film or the like and reflecting red light and the blue reflecting plane  86  made of a dielectric film or the like and reflecting blue light. The emitting luminous flux L 10 C entering the color separating prism  84  is separated into incident lights L 10 G, L 10 R, L 10 B which enter the respective light modulating units  210 G,  210 R, and  210 B. 
   The emitting luminous flux L 10 C emitted from the beam shaping optical system  11  includes so called three primary colors of red (R), green (G), and blue (B). This emitting luminous flux L 10 C enters the total reflecting prism  82  and is totally reflected and bent downward in the figure by the reflecting plane  83  provided in the total reflecting prism  82 . 
   The bent emitting luminous flux L 10 C enters the color separating prism  84 . In the color separating prism  84  are provided the red reflecting plane  85  for reflecting red light and the blue reflecting plane  86  for reflecting blue light. 
   Accordingly, the red light included in the emitting luminous flux L 10 C entering the color separating prism  84  is reflected in the left direction in the figure by the red reflecting plane  85  and is entered into the red light modulating unit  210 R (incident light L 10 R). 
   Further, similarly, the blue light included in the emitting luminous flux L 10 C entering the color separating prism  84  is reflected in the right direction in the figure by the blue reflecting plane  86  and is entered into the blue light modulating unit  210 B (incident light L 10 B). 
   Since the green light included in the emitting luminous flux L 10 C is not reflected by the red reflecting plane  85  and the blue reflecting plane  86 , it propagates downward and enters the green light modulating unit  210 G (incident light L 10 G). 
   The incident light L 10 R entering the light modulating unit  210 R enters the optical deflector  400 R and generates the ON light (corresponding to the emitting light L 20 R) and the OFF light L 30 R in the same way as the optical deflector  400  described above. 
   The emitting light L 20 R enters the color separating prism  84  as the emitting light of the light modulating unit  210 R and is reflected upward in the figure by the red reflecting plane  85  provided in the color separating prism  84  (becomes the red optical component of the synthetic light L 20 C). 
   The OFF light L 30 R returns to the light emitting point of the light source  9  through a path opposite to the path through which the incident light L 10 R propagates by the same operations of the polarization converting element  5 R, the liquid crystal shutter element  6 R and the mirror  7 R as those of the polarization converting element  5 , the liquid crystal shutter element  6  and the mirror  7  which have been described above. 
   The light returned to the light source  9  is reflected by the reflecting plate  10  and again enters the light modulating unit  210 R and thereafter the same operation is repeated. 
   The incident light L 10 B entering the light modulating unit  210 B enters the optical deflector  400 B to generate the ON light (corresponding to the emitting light L 20 B) and the OFF light L 30 B by the same operation as the optical deflector  400  described above. 
   The emitting light L 20 B enters the color separating prism  84  as the emitting light of the light modulating unit  210 B and is reflected upward in the figure by the blue reflecting plane  86  provided in the color separating prism  84  (becomes the blue light component of the synthetic light L 20 C). 
   The OFF light L 30 B returns to the light emitting point of the light source  9  through a path opposite to the path through which the incident light L 10 B propagates by the same operations of the polarization converting element  5 B, the liquid crystal shutter element  6 B and the mirror  7 B as those of the polarization converting element  5 , the liquid crystal shutter element  6  and the mirror  7  which have been described above. 
   The light returned to the light source  9  is reflected by the reflecting plate  10  and again enters the light modulating unit  210 B and thereafter the same operation is repeated. 
   The light incident light L 10 G entering the light modulating unit  210 G enters the optical deflector  400 G to generate the ON light (corresponding to the emitting light L 20 G) and the OFF light L 30 G by the same operation as the optical deflector  400  described above. 
   The emitting light L 20 G enters the color separating prism  84  as the emitting light of the light modulating unit  210 G, but is not reflected by the red reflecting plane  85  and the blue reflecting plane  86  and propagates straight and transmits upward in the figure (becomes the green light component of the synthetic light L 20 C). 
   The OFF light L 30 G returns to the light emitting point of the light source  9  through a path opposite to the path through which the incident light L 10 G propagates by the same operations of the polarization converting element  5 G, the liquid crystal shutter element  6 G and the mirror  7 G as those of the polarization converting element  5 , the liquid crystal shutter element  6  and the mirror  7  which have been described above. 
   The light returned to the light source  9  is reflected by the reflecting plate  10  and again enters the light modulating unit  210 G and thereafter the same operation is repeated. 
   As described above, the emitting lights L 20 R, L 20 B, and L 20 G enter the total reflecting prism  82 . Since these emitting lights L 20 R, L 20 B, and L 20 G do not satisfy the total reflection conditions at the reflecting plane  83 , they are not reflected by the reflecting plane  83 , but are transmitted therethrough and are emitted from the total reflecting prism  84  as synthetic light L 20 C. 
   The emitting synthetic light L 20 C is projected to a screen through a projection optical system (not shown) provided at the later position. 
   In this respect, it is also recommended that the light modulating units  210 R,  210 B, and  210 G be provided with optical sensors  81 R,  81 B, and  81 G and receive parts of the OFF lights L 30 R, L 30 B, and L 30 G and monitor the light intensities of the respective OFF lights by the use of the output signals  90 R,  90 B, and  90 G which are outputted by the respective optical sensors. 
   According to this configuration, it is possible to detect the ratios of the respective colors of the OFF light returning to the vicinity of the light emitting point of the light source  9 . That is, since the ratios of the respective colors returning to the vicinity of the light source  9  can be varied by the use of the liquid crystal shutter elements  6 R,  6 B, and  6 G based on the respective output signals  90 R,  90 B, and  90 G, basically, it is possible for white light to return. 
     FIG. 10  is a functional block diagram to show a signal processing device for processing the signal of the projection television of the preferred embodiment 3 in accordance with the present invention. 
   In  FIG. 10 , a reference character  20 C designates a color television image signal,  21 C designates an image memory,  22 C designates a unit for detecting features or qualities of an image,  23 C designates a unit for calculating the average ON ratio P of the mirror elements of the respective color lights. 
   A reference character  24 C is an image quality adjusting unit,  29 C designates a control unit, and  300 G,  300 R, and  300 B designate signal processing units of green, red, and blue. 
   In the configuration of the signal processing unit  300 G, a reference character  25 G designates a timing generating unit for generating ON/OFF timings to be given to the mirror elements of the optical deflector  400 G, and  26 G designates an inverted gamma-correction table to represent the beam current characteristics of a CRT. 
   A reference character  27 G designates a timing randomizing unit for varying ON/OFF timing for each of the mirror elements,  28 G designates a driving unit for outputting a driving signal for driving the mirror elements,  30 G designates a driving unit for outputting a driving signal for driving the liquid crystal shutter element  6 G. 
   The operation of the signal processing unit  300 G will be described in the following. Here, since the configurations and operations of the signal processing units  300 R and  300 B are basically the same as those of the signal processing unit  300 G, the description thereof will be omitted. 
   An image memory  21 C stores an inputted color television signal  20 C as a digital signal (image data) per one frame/field (one screen). 
   The image data stored in the image memory  21 C is outputted to the feature detecting unit  22 C and the average ON ratio calculating unit  23 C. The feature detecting unit  22 C detects the features of the image such as maximum value, the minimum value and average value of the luminance level of the inputted image data. 
   The average ON ratio detecting unit  23 C reads out the coefficient for the inverted gamma-correction corresponding to the luminance level of the respective pixels in the inputted image data from the inverted gamma-correction table  26 G and performs the inverted gamma correction. Further, the average ON ratio detecting unit  23 C calculates an average ON ratio Pg in the optical deflector  400 G based on the corrected image data (in the signal processing unit  300 R and  300 B, an average ON ratios Pr and Pb are calculated) 
   The image quality adjusting unit  24 C reads out the image data stored in the image memory  21 C and adjusts (corrects) the image data so as to make a desired image quality. Thereafter, the image quality adjusting unit  24 C outputs the image data after adjustment to the timing generating unit  25 G. 
   The timing generating unit  25 G reads out the coefficient for the inverted gamma correction corresponding to the luminance level of the respective pixels of the inputted image data from the inverted gamma-correction table  26 G and performs the inverted gamma correction (the signal processing units  30 OR and  300 B read out the coefficients from the inverted gamma-correction tables  26 R  26 B and perform the inverted gamma correction). 
   Further, the timing generating unit  25 G generates and outputs a timing signal to set the individual mirror elements included in the optical deflector  400 G in the “ON” state based on the image data subjected to the inverted gamma correction (in the signal processing units  300 R and  300 B, timing signals are outputted by the respective timing generating units  25 R and  25 B). 
   The timing signal outputted by the timing generating unit  25 G is inputted into the timing randomizing unit  27 G. The inputted timing signal is subjected to a shift operation on a time axis so that timings vary among the pixels to make a new timing signal and is outputted (in the signal processing units  300 R and  300 B, the new timing signals are outputted by the timing randomizing units  27 R and  27 B). 
   The new timing signal outputted by the timing randomizing unit  27 G is inputted into the mirror driving unit  28 G. The mirror driving unit  28 G outputs a driving signal for driving the mirror elements included in the optical deflector  400 G to the optical deflector  400 G based on the new timing signal to drive the mirror elements (in the signal processing units  300 R and  300 B, the new timing signals are inputted into the mirror driving units  28 R and  28 B to drive the mirror elements of the optical deflectors  400 R and  400 B). 
   Into the control unit  29 C are inputted a feature signal  32 C and an average ON ratio Pg signal  33 C. The control unit  29 C outputs control signals to the shutter driving units  30 G and lamp driving units  31 G based on these inputted signals, respectively. Further, the control units  29   c  outputs an image quality adjusting control signal  34 C to the image quality adjusting unit  24 C (in the signal control units  300 R and  300 B, the control unit  29 C outputs control signals to the shutter driving units  30 R and  30 B and lamp driving unit  31 R and  31 B). 
   As described above, the optical deflectors  400 G,  400 R and  400 B and liquid crystal shutter elements  6 G,  6 R and  6 B, and the lamp  8  which are included in the optical unit  200  are driven, respectively. 
   When the respective light modulating units  210 G,  210 R and  210 B are provided with the optical sensors  81 G,  81 R and  81 B, the ratios of light of the respective colors returning to the vicinity of the light source  9  are varied by the liquid crystal shutter elements  6 G,  6 R and  6 B, as described above, based on the output signals  90 G,  90 R and  90 B outputted by these optical sensors  81 G,  81 R and  81 B. 
   According to this configuration, it is possible to detect the ratios of the respective colors of the OFF light returning to the vicinity of the light emitting point of the light source  9  and to basically make white light return. 
   It is possible to predict an increase ratio in luminous flux caused by the re-use of the OFF light by applying the equation (3) in the description of the preferred embodiment 1 in accordance with the present invention to the respective colors. 
     FIG. 11  shows the results of calculation of the luminous fluxes of the respective colors by the equation (3) and the lateral axis designates the average ON ratio Pg, Pr and Pb (Pg is an average ON ratio corresponding to green, Pr is an average ON ratio corresponding to red, and Pb is an average ON ratio corresponding to blue, respectively) and the vertical axis designates the light intensity Ig, Ir, Ib (Ig is a light intensity corresponding to green, Ir is a light intensity corresponding to red, and Ib is a light intensity corresponding to blue). 
   Curves  60  to  63  show changes in characteristics of the light intensities Ig, Ir and Ib in the case where the ratio K is varied 1.0, 0.8, 0.5 and 0.2, for the sake of simplification, under the conditions that the reflectance factor a of the reflecting plate  10  is 0.95 and the light transfer rate n of the beam shaping optical system  11  is 0.90 for the respective colors. Further, a reference character  64  designates the light intensity in the case where the OFF light is not returned to the light source  9 . 
   A reference character  65  designates the point of the light intensities Ig, Ir and Ib when the average ON ratios Pg, Pr and Pb are 100% (that is, there is no OFF light). Further, axes  92  and  93  designate the results of detection of the average ON ratios of the respective colors for the two different kinds of image signals. 
   In the example shown by the axis  92 , the average ON ratio is small for any color. The average ON ratio Pg for green can be increased to the light intensity level shown by a point  94  in the case where the ratio K is 1.0. Therefore, it is possible to adjust an increase rate of the light intensity to 2 for all colors by controlling the ratio K. 
   On the other hand, in the example shown by the axis  93 , since the average ON ratio Pg of green is large, the light intensity can be increased only to the level shown by a point  95 . That is, the average ON ratio Pb of blue, one of the other colors (red, blue), for example, can be increased to a level shown by a point  97  by itself, but it is adjusted to the level shown by a point  98  of the same level as the green. In this way, it is possible to correctly display the hue of a display image. 
   The control unit  29 C in  FIG. 10 , into which the ON ratio signals  33 C of the respective colors outputted by the average ON ratio calculating unit  23 C, the maximum value Vmax (maximum value of the luminance of the image), the minimum value Vmin (minimum value of the luminance of the image), and the average value Vave (average value of luminance of the image) of the luminance outputted as the features of the image by the feature detecting unit  22 C, and the output signals  90 G,  90 B and  90 R outputted in correspondence with the respective optical sensors  81 G,  81 B and  81 R are inputted, performs the following control according to a combination of these data and a change in time. 
   That is, the control unit  29 C calculates an increase rate of the maximum light intensity Gg, Gr and Gb by the equation (3), based on the average ON ratios Pg, Pr and Pb of the respective inputted colors, the specific values of the reflectance factor a of the reflecting plate  10 , the light transfer rate n of the beam shaping optical system  11 , the average ON ratio P and the ratio K which are determined by the optical components. Here, the minimum value of the increase rates of Gg, Gr and Gb is made Gmin. 
   For example, in the case where the initial value Glim (upper limit value) of the increase rate is 2 and the minimum value Gmin of the increase rate of the increase rate is less than the initial value Glim, the control unit  29 C controls the ratio K by driving the optical shutter elements  6 G,  6 B and  6 R so that the increase rates of all colors become the minimum value Gmin. 
   In the case where the minimum value Gmin of the increase rate is larger than the initial value Glim of the increase rate, the control unit  29 C controls the ratio K by driving the optical shutter elements  6 G,  6 B and  6 R so that the increase rates of all colors become the initial value Glim. 
   In this respect, in the case where the minimum value Gmin of the increase rate exceeds the initial value Glim of the increase rate and the difference between the maximum value Vmax of the luminance and the minimum value Vmin of the luminance is large and close to the maximum range of change in the image, the control unit  29 C judges that the image has a high contrast and changes the initial value Glim of the increase rate into a value larger than the minimum value Gmin of the increase rate. 
   The control unit  29 C increases the upper limit of the increase rate in this way and controls the image quality adjusting unit  24 C to adjust the quality of the image so that the image has a higher contrast. 
   Further, the control unit  29 C keeps the average value Vave of the luminance at least for several fields (or several frames). In the case where the average luminance is small in sequence for plural fields (or plural frames), the control unit  29 C judges that a lot of OFF light is generated. 
   The control unit  29 C sets the ratio K of the respective colors at a value close to the maximum value and controls the lamp driving unit  31  to lower the brightness of the lamp  8 . 
   Further, a case where the average value Vave of the luminance is small corresponds to a case where all of the average ON ratios Pg, Pr and Pb are small and nearly close to 0. Therefore, it is possible to increase the increase rate of light by about 4 times by maximizing the ratio K corresponding to the respective colors. 
   In this respect, in the case of this condition, for example, it is also possible to increase the increase rate of light by 2 times by halving the intensity of light generated by the lamp  8 . 
   In general, a lack in a color balance in a color image display presents a problem that an image to be displayed in white in itself is displayed in color. 
   By the way, in the control unit  29 C of the present preferred embodiment can also independently set the light intensities Ig, Ir and Ib in correspondence with the average ON ratios Pg, Pr and Pb. 
   Therefore, for example, when a color balance is lost by the variations in the optical characteristics of the optical parts or by the changes in the optical characteristics of the optical parts, the control unit  29 C can also adjust the lost color balance to an original color balance. 
   For example, first, the ratio K (the reflectance factor of the OFF light) corresponding to the respective colors is set at the maximum value during a display of a black screen which is in a mute state at the start of the television. In this state, the output signals  90 G,  90 R, and  90 B from the optical sensors  81 G,  81 R and  81 B are inputted into the control unit  29 C. 
   The control unit  29 C judges a color balance based on the output signals  90 G,  90 R and  90 B and adjusts control signals to be given to the liquid crystal shutter elements  6 G,  6 R and  6 B so that the light intensities of the respective colors are adjusted to a predetermined ratio to provide an original color balance. According to this adjustment, it is possible to vary the ratio of the light intensities of the respective colors and to adjust the color balance. 
   In this respect, the configuration to return the OFF light to the light source  9 , which comprises a combination of the polarization converting element, the liquid crystal shutter element and the mirror, may comprise the optical deflector  410  as described in the preferred embodiment 2. This can simplify the configuration of the device. 
   Further, it is possible to determine the light intensities Ig, Ir and Ib with respect to the average ON ratios Pg, Pr and Pb corresponding to the respective colors by a look-up table or a predetermined function equation. The light intensities Ig, Ir and Ib may be predicted for several fields or several frames. 
   The invention may be embodied in other specific forms without departing from the spirit or essential parts thereof. The above embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. 
   The entire disclosure of the Japanese Application No. 2001-092114 filed on Mar. 28, 2001 including specification, claims, drawings and summary are incorporated herein by reference in its entirety.