Single-panel color projector

Light emitted from a white light source is separated into red, green and blue light beams by dichroic mirrors. The resulting light beams are directed onto a lenticular lens system having large-pitch lenticular lenses and small-pitch lenticular lenses formed on opposed surfaces. This lens system converts the red, green and blue light beams incident on each of the large-pitch lenticular lenses into a color band set consisting of a set of separated subbands of the red, green and blue light beams the diameter of which is narrowed down to one-third or less of that of the incident light beam through the small-pitch lenticular lenses. The resulting color band sets emerging from the small-pitch lenticular lenses are focused onto a DMD panel through a mirror galvanometer and a coupling lens. The mirror galvanometer is rotated in such a way as to move the color band sets over the DMD panel up and down by the amount corresponding to the pitch of the color band sets while the DMD panel is being driven. Modulated light from the DMD panel is projected onto a screen through a projection lens.

BACKGROUND OF THE INVENTION
 The present invention relates to a single-panel color projector which uses
 only a single light valve for light modulation.
 The current mainstream of color projector is in three-panel projectors that
 use three light valves such as liquid crystal panels. This is because they
 are easy to install, have good portability, and provide high brightness.
 However, the light valve is costly. Thus, the overall cost of apparatus
 that uses three light valves is increased.
 There is also known a single-panel color projector that uses only one light
 valve such as a liquid crystal panel. This type of color projector is
 lower in cost than those using three light valves. A conventional
 single-plate color projector, for example, one which uses a mosaic color
 filter for the light valve as disclosed in Japanese Unexamined Patent
 Publication No. 59-230383 has a problem that image brightness is lowered
 because about two-third of illumination light is absorbed or reflected by
 that color filter. Moreover, the resolution required with the single light
 valve is three times as high as that of the light valves in the
 three-plate color projector. Thus, the cost of the light valve will become
 very high and a considerable reduction in overall cost cannot therefore be
 expected.
 A color projector as disclosed in Japanese Unexamined Patent Publication
 No. 4-60538 is arranged to separate three primary colors of light in
 angular directions by means of three dichroic mirrors and cause the three
 primary colors of light to focus onto their respective target areas by
 microlenses, thereby performing the function of a color filter. This
 significantly improves utilization of illumination light. However, in this
 type of color projector as well, as with the color projector using color
 filter, the resolution is required to be three times as high as the
 resolution of each of three light valves in the three-panel color
 projector and hence a considerable reduction in overall cost cannot be
 expected.
 As another single-panel type of color projector, a color sequential display
 type of color projector is also known which uses a rotating color filer
 disc. In this color projector, one pixel is irradiated sequentially with
 red, green and blue colors of light to provide full color display. In this
 system, the resolution required with the light valve remains unchanged
 from that in the three-plate color projector. However, this system
 requires a light valve with a short response time. Therefore, liquid
 crystal panels as used in normal color projectors cannot be used because
 they have too long a response time. For this reason, light valves, such as
 DMDs (Digital Micromirror Devices) or ferroelectric liquid crystal panels,
 that have a short response time will be used. However, such light valves
 are bistable devices which are switched between on and off states. For
 example, the representation of gray scales by the valve is realized by the
 use of PWM (Pulse Width Modulation), i.e., by varying its on time. In this
 system, about two-third of illumination light is lost because of
 absorption or reflection by the color filter disk, which will result in a
 problem of low image brightness.
 As a high-speed light valve, a reflective liquid crystal panel oriented
 toward micro-display (head-mounted display) has been developed which
 sequentially turns on red, green and blue LEDs to provide color sequential
 display. In application of this liquid crystal panel in a liquid crystal
 projector, red, green and blue light will be taken out of white light from
 a white light source by the use of a rotating color filter disk, as used
 in a single-panel color projector using DMD, and then projected
 sequentially onto the liquid crystal panel. This system needs a
 rapid-response liquid crystal light valve.
 The responsiveness of the liquid crystal light valve depends on the
 response characteristic of a liquid crystal material and the response
 characteristic of switching devices. As rapid-response liquid crystal
 materials, ferroelectric liquid crystal materials and anti-ferroelectric
 liquid crystal materials have been developed. These materials perform a
 digital-like operation of blocking light or letting it pass and the
 representation of gray scales is carried out by controlling the length of
 time that light passes through. Therefore, these materials must be used in
 combination with very fast switching devices. Amorphous silicon as the
 material of switching devices widely used in transmissive liquid crystal
 light valves is low in electron mobility and is not therefore suitable for
 fast switching. On the other hand, crystalline silicon used in reflective
 liquid crystal valves is high in electron mobility and hence allows fast
 switching required with the color sequential system. This system allows
 full color display with one pixel of liquid crystal and can solve the
 problem of the conventional single-panel system that there is the need for
 a light valve whose resolution is at least three times higher than with
 the three-panel system in which three light valves are used. However, it
 is impossible to solve the other problem that only one-third of light from
 a light source is utilized because of the use of the rotating color filter
 disk and bright image display cannot therefore be provided.
 Moreover, a color-sequential projector is also known which employs a
 rotating prism. For example, in U.S. Pat. No. 5,528,318 there is disclosed
 a system which decomposes white light from a white light source into red,
 green and blue color band sets and scans or moves these color band sets
 over a light valve using the rotating prism. This system allows the panel
 resolution to be the same as with the three-plate system. In addition,
 this system has the advantage of being high in image brightness. However,
 this system has the following problems: (1) Color nonuniformity occurs
 because the color band scanning speed is not constant with respect to the
 rotation of the prism, and (2), since the color band sets are narrow in
 width, the length of time that a band moves across a certain line of
 pixels is very short and hence the existing DMDs and ferroelectric liquid
 crystal panels cannot represent required gray scales within that length of
 time with the use of PWM control.
 As described above, the conventional single-panel color projector cannot
 increase image brightness using an existing light valve and cannot realize
 the representation of sufficient gray scales at low cost.
 It is therefore an object of the present invention to provide a
 single-panel color projector which permits image brightness to be
 increased, cost to be reduced and sufficient gray scales to be represented
 through the use of an existing light valve.
 BRIEF SUMMARY OF THE INVENTION
 According to an aspect of the present invention, there is provided a
 single-panel color projector comprising: a white light source; color
 separating/ reflecting means for separating white light emitted from the
 white light source into a plurality of color light beams and reflecting
 each of the light beams at a different angle; conversion means for
 converting the color light beams reflected by the color
 separating/reflecting means into color band sets arranged in a
 predetermined pitch, each color band consisting of a set of subbands of
 the color light beams arranged in a predetermined sequence; focusing means
 for focusing the color band sets from the conversion means onto a light
 valve for light modulation; scanning means for moving the color band sets
 from the conversion means over the light valve by the amount corresponding
 to the pitch of the color band sets; and projection means for projecting
 light modulated in the light valve.
 According to the present invention, image brightness can be increased and
 cost can be reduced using an existing light valve. In addition, sufficient
 gray scales can be represented.
 Further, a loss attendant the reversal of direction in which the color band
 sets are moved over the light valve can be reduced.
 Additional objects and advantages of the invention will be set forth in the
 description which follows, and in part will be obvious from the
 description, or may be learned by practice of the invention. The objects
 and advantages of the invention may be realized and obtained by means of
 the instrumentalities and combinations particularly pointed out
 hereinafter.

DETAILED DESCRIPTION OF THE INVENTION
 The preferred embodiments of the present invention will be described with
 reference to the accompanying drawings.
 First Embodiment
 First, the outline of a first embodiment of the present invention will be
 given. White light emitted from a white light source, such as a metal
 halide lamp, is separated into red, green and blue light beams which are
 not parallel with one another by three non-parallel dichroic mirrors.
 These light beams are then converted by a lenticular lens system into a
 plurality of color band sets each consisting of a set of three red, green
 and blue-light subbands. The width of each color subband is set a little
 smaller than one third of the pitch of the color band sets. The image of
 all the color band sets is formed by a coupling lens onto a DMD panel that
 is a high-speed light valve. The image is formed to overfill the effective
 area of the DMD panel by the width of one color band set.
 All the color band sets are moved by a scanning optical system consisting
 of a galvanometer over the DMD panel at equal velocity by the amount
 corresponding to the width of one color band set (the pitch of the color
 band sets). At the completion of the movement of the color band sets by
 the amount corresponding to their pitch, the galvanometer reverses rapidly
 to make a backward scan. In this manner, forward and backward scans are
 repeated.
 When each color subband scans a certain line of pixels on the DMD panel,
 the on/off time of each of the pixels is controlled appropriately by an
 electronic circuit in the projector in accordance with the cross-sectional
 shape of color and brightness of that subband and a color component for
 that pixel in an image to be displayed.
 Thus, the width of each color subband can be set to about one-third of the
 scanning width and the length of time that each color subband scans a
 pixel can be made sufficiently long, which allows the DMD panel to
 represent sufficient gray levels with PWM control. Light modulated by the
 DMD panel is projected onto a screen through a projection lens.
 Thus, using a single DMD panel, most of the red, green and blue light can
 be employed simultaneously, image brightness can be increased, and cost
 can be reduced. Moreover, the use of a single panel eliminates the
 necessity of correction for misalignment among three-color pixels as
 occurs in the three-plate color projector. Furthermore, there is no need
 for a dichroic mirror or dichroic prism for color composition, thus
 allowing a further reduction in cost.
 Specific examples will be described hereinafter.
 Referring now to FIG. 1, 1 denotes a white source such as a metal halide
 lamp. The white light source 1 is placed in the position of a first focal
 point of an elliptic reflecting mirror 2. A concave lens 3 is placed short
 of a second focal point of the reflecting mirror 2 so that its focal point
 conforms to that second focus, i.e., on the side of the white light
 source.
 Thus, white light emitted from the white light source 1 is reflected by the
 elliptic reflecting mirror 2 and then passes through the concave lens 3,
 so that it has its diameter narrowed down and is converted into a
 substantially parallel beam of light. Note that a convex lens can be used
 instead of the concave lens 3, in which case the convex lens is placed
 behind the second focal point of the elliptic reflecting mirror 2 so that
 its focal point conforms to the second focal point.
 Behind the concave lens 2 is placed a dichroic mirror system 4 serving as
 color separating and reflecting means and consisting of three dichroic
 mirrors 4R, 4G and 4B. Each of the dichroic mirrors 4R, 4G and 4B is
 placed tilted with respect to the incident light beam. The dichroic mirror
 4R situated in the front selectively reflects light within the red
 waveband. The central dichroic mirror 4G selectively reflects light within
 the green waveband. The rearmost dichroic mirror 4B selectively reflects
 light within the blue waveband.
 The light reflected by the dichroic mirror or mirrors situated near to the
 white light source 1 will not reach the dichroic mirror or mirrors
 situated farther away from the white light source. Thus, each dichroic
 mirror need not necessarily be formed to reflect light reflected by the
 dichroic mirror or mirrors situated in front of it. That is, the dichroic
 mirror 4G may or may not reflect the red waveband component. Likewise, the
 dichroic mirror 4B may or may not reflect the red and green waveband
 components.
 The light valve can be prevented from heating by causing each of the
 dichroic mirrors 4R, 4G and 4B to have the property of transmitting
 infrared radiation.
 The dichroic mirror 4G is placed at an angle of 45 degrees with respect to
 the direction of incident light beam. The dichroic mirror 4R is oriented
 at an angle of, for example, +4 degrees with respect to the dichroic
 mirror 4G. The dichroic mirror 4B is oriented at an angle of -4 degrees
 with respect to the dichroic mirror 4G. Thereby, the green light beam is
 bent 90 degrees by reflection by the dichroic mirror 4G. The red and blue
 light beams reflected by the dichroic mirrors 4R and 4B are oriented at
 angles of -8 and +8 degrees, respectively, with respect to the green light
 beam.
 The red, green and blue light beams emerging from the dichroic mirror
 system 4 are directed to a lenticular lens system 5 as conversion means.
 The lenticular lens system consists, as shown in FIG. 2, of a combination
 of lenticular lenses 5a arranged in a large pitch and lenticular lenses 5b
 arranged in a small pitch. For example, the large-pitch lenticular lenses
 5a and the small-pitch lenticular lenses 5b are formed on opposite sides
 of a sheet of glass by press working. This lens system is formed such that
 three small-pitch lenses are opposed to one of large-pitch lenticular
 lenses 5a.
 The green light beam directed vertically to a certain lenticular lens 5al
 of the large-pitch lenticular lenses 5a converges at the focal point of
 that lens 5a1 and is then converted into a light beam of diameter of 1/3
 or less by a small lenticular lens 5bG1 whose focal point is common to the
 large-pitch lens 5a1 and whose focal length is 1/3 or less of that of the
 lens 5a1.
 The red light beam incident on the lens 5a1 at an angle of -8 degrees
 converges on a point located at some distance from the focal point of that
 lens and is then converted into a light beam of diameter of 1/3 or less by
 the lens 5bR1 whose focal point is that point on which the red light beam
 converges.
 The blue light beam incident on the lens 5a1 at an angle of +8 degrees
 converges on a point that is located at some distance from the focal point
 of that lens and opposed to the red-light convergence point with the focal
 point of the lens 5a1 therebetween. It is then converted into a light beam
 of diameter of 1/3 or less by the lens 5bB1 whose focal point is that
 point on which the blue light beam converges.
 In this manner, the lenticular lens system 5 produces color band sets each
 consisting of a set of three separate red, green and blue subbands. The
 number of the color band sets thus produced is equal to the number of
 effective large-pitch lenticular lenses on which incoming light falls.
 The outgoing color band sets from the lenticular lens system 5 are directed
 onto the reflecting surface of a mirror galvanometer associated with an
 encoder 6. The mirror galvanometer and the encoder constructs scanning
 means that deflects the color band sets by the amount corresponding to
 their pitch. The reflected light from the mirror 6a are focused onto the a
 DMD (Digital Micromirror Device) panel 8 as a light valve by a coupling
 lens 7 constituting image formation means. The mirror galvanometer 6a is
 rotatable upon its axis of rotation, i.e., in the directions indicated by
 arrows.
 In the initial state of the mirror galvanometer 6a, the image of the color
 band sets projected onto the DMD panel 8 is set to cover the width of the
 effective area of the DMD panel in the horizontal direction (in FIG. 1,
 the direction normal to the drawing paper) and, in the vertical direction,
 overflow the DMD panel effective area down by the amount corresponding to
 the pitch of the color band sets.
 The topmost line of pixels on the DMD panel 8 is numbered 0. In the initial
 state of the mirror galvanometer 6a, the color band sets directed to the
 DMD panel over a distance of X mm from the topmost line form a repeating
 pattern of f(x) of three red, green and blue subbands as shown in FIG. 3.
 In FIG. 1, when the mirror galvanometer 6a is rotated counterclockwise at a
 constant angular velocity, all the color band sets on the DMD panel 8 move
 vertically up at a constant velocity V. Although, in practice, a certain
 period of time is needed before the constant velocity V is reached, the
 point of time at which the uniform motion is started is taken to be the
 initial state, i.e., t=0.
 The topmost line on the DMD panel 8 is successively exposed to red, green
 and blue light beams in the pattern of f(0+V.times.t) as shown in FIG. 4.
 The N-th line counting from the topmost line is successively exposed, as
 shown in FIG. 5, to red, green and blue light beams in the pattern of
 f(N.times.p+V.times.t), i.e., in the pattern which differs only in phase
 from f(0+V.times.t). Here, t is the movement time of color band sets.
 Therefore, while all the color band sets are moved by the distance
 corresponding to their pitch, all the pixels on the DMD panel 8 will be
 irradiated with light in the same pattern but in different phases
 depending on their respective location. When the color band sets are moved
 by their pitch, the mirror galvanometer 6a switches rapidly its direction
 of rotation to start a uniform motion in the reverse direction. The mirror
 galvanometer 6a reciprocates repeatedly in this way. The DMD panel 8 is
 switched off during the interval when the direction of rotation of the
 mirror is being reversed.
 If the overall width of the color band sets in the vertical direction is
 made M (&gt;1) times larger than the pitch of the color band sets and the
 color band scanning width in the vertical direction is made (M-1) times
 larger, then the mirror galvanometer 6a will not be required to switch its
 direction of rotation until the color band sets are moved vertically up or
 down by (M-1) times the pitch. Thus, the number of times the mirror
 switches its direction of rotation is reduced, allowing brighter images to
 be obtained. In this case, however, since the area which is exposed to the
 color band sets increases, the amount of light that is effectively
 directed onto the DMD panel 8 will be somewhat reduced.
 The color and intensity (brightness) of light to which a certain pixel is
 exposed can be known by reading a pattern f(x) of colors and brightness in
 each color band with sensors, for example, at the time of assembly of the
 color projector, storing resulting data in the form of a table into a
 nonvolatile memory, detecting the amount of movement of the color band
 sets with the encoder, and referencing the data in the nonvolatile memory.
 A full color image can be produced on the DMD panel 8 and then projected
 onto a screen through the projection lens 9 by controlling the timing of
 turning on the corresponding pixel on the DMD panel while the color band
 sets are being moved so that gray scale levels of three primary-color
 components for the pixel are obtained.
 For example, by controlling the timing of turning on and off a certain
 pixel on the N-th line as shown in FIG. 6, in the first forward scan the
 pixel is displayed in 100% of green G, 50% of blue B, and 25% of red R and
 in the next backward scan it is displayed in 0% of green G, 100% of blue
 B, and 50% of red R.
 Thus, using a single DMD panel, most of the red, green and blue light can
 be employed simultaneously, image brightness can be increased, and cost
 can be reduced. Moreover, the width of each color subband in each color
 band set can be set to about one-third of the scanning width and hence the
 length of time that each color subband scans a pixel can be made
 sufficiently long, which allows the DMD panel to represent sufficient
 shades with PWM control.
 Furthermore, the use of a single panel eliminates the necessity of
 correction for misalignment among three-color pixels as occurs in the
 three-plate color projector. In addition, there is no need for a dichroic
 mirror or dichroic prism for color composition, thus allowing a further
 reduction in cost.
 Second Embodiment
 In the second embodiment, like reference numerals are used to denote
 corresponding components to those in the aforementioned first embodiment
 and detailed description thereof is omitted.
 Although, in the first embodiment described above, the dichroic mirror
 system is used as the decomposing/reflecting means for decomposing white
 light into red, green and blue light beams, in the second embodiment a
 phase volume diffraction grating 11 is used which consists of a phase
 volume hologram having a lens function. This grating has a function of
 separating incoming light into red, green and blue light beams that are
 tilted slightly with respect to one another. In addition, the grating has
 the function of large-pitch lenticular lenses.
 Therefore, a lenticular lens system 12 for producing color band sets each
 consisting of a set of red, green and blue subbands from incoming white
 light is only required to have small-pitch lenticular lenses.
 As shown in FIG. 7, white light from the concave lens 3 is directed onto
 the phase volume grating 11 consisting of a phase volume hologram and then
 separated into red, green and blue light beams at some angles with one
 another. The red, green and blue light beams then fall on the lenticular
 lens system 12.
 The lenticular lens system 12 produces a plurality of color band sets each
 consisting of a set of red, green and blue subbands. The width of each
 subband is set to about one-third of the scanning width.
 The image of the color band sets emerging from the small-pitch lenticular
 lenses of the lenticular lens system 12 is reflected by the mirror
 galvanometer 6a through the coupling lens 7 and then focused through a
 polarizing beam splitter 13 onto a light valve 14 consisting of a
 reflective liquid crystal panel.
 The color image emerging from the light valve 14 is projected by the
 projection lens 9 through the polarizing beam splitter 13 onto a screen.
 In this arrangement, as in the aforementioned arrangement, image brightness
 can be increased and cost can be reduced. In addition, sufficient shades
 can be represented.
 In the aforementioned embodiments, a mirror galvanometer is used as the
 scanning means that moves the color band sets by the distance
 corresponding to their pitch; however, this is not restrictive. For
 example, use may be made of a variable-apex-angle prism whose apex angle
 can be adjusted electrically or a mechanism that allows control of the
 movement of the coupling lens.
 Third Embodiment
 In the third embodiment, like reference numerals are used to denote
 corresponding components to those in the aforementioned embodiments and
 detailed description thereof is omitted.
 In this embodiment, as shown in FIG. 8, a plate-like lenticular lens system
 5 is used as the conversion means and a linear motor/encoder 21 that moves
 the lenticular lens system forward and backward as indicated by arrows is
 used as the scanning means.
 The lenticular lens system 5 produces a plurality of color band sets each
 consisting of a set of red, green and blue subbands from incoming white
 light. The width of each subband is set to about one-third of the scanning
 width.
 The image of the color band sets emerging from the small-pitch lenticular
 lenses of the lenticular lens system 5 is reflected by a reflecting mirror
 22 and then focused by the coupling lens 7 onto the DMD panel 8 as a light
 valve.
 The length of the lenticular lens system 5 is set so that the image of the
 color band sets projected onto the DMD panel 8 in the initial state of the
 linear motor/encoder 21 covers the width of the effective area of the DMD
 panel in the horizontal direction (in FIG. 8, the direction normal to the
 drawing paper) and, in the vertical direction, overflow the DMD panel
 effective area down by the amount corresponding to an integral multiple of
 the pitch of the color band sets.
 The topmost line of pixels on the DMD panel 8 is numbered 0. In the initial
 state of the linear motor/encoder 21, the color band sets directed onto
 the DMD panel over a distance of X mm from the topmost line form a
 repeating pattern of f(x) of three red, green and blue subbands as shown
 in FIG. 9.
 When the lenticular lens system 5 is moved forward at a constant velocity
 by the linear motor/encoder 21, all the color band sets on the DMD panel 8
 move vertically up at a constant velocity V. Although, in practice, a
 certain period of time is needed before the constant velocity V is
 reached, the point of time at which the uniform motion is started is taken
 to be the initial state, i.e., t=0.
 The topmost line of pixels on the DMD panel 8 is successively exposed to
 red, green and blue light beams in the pattern of f(0+V.times.t) as shown
 in FIG. 10. When the color band sets are moved by the distance
 corresponding to their pitch, the incident light on the DMD panel 8
 becomes equal to that in the initial state.
 Since the length of the lenticular lens system 5 is set larger than the
 width of incident light by the amount corresponding to an integral
 multiple of the pitch of the color band sets, the scanning of the color
 band sets can be made a plurality of times in succession. FIG. 10 shows a
 case where the scanning is made twice in succession.
 Thus, by increasing the length of the lenticular lens system 5, a forward
 or backward scan can be made with a plurality of color band sets, allowing
 the number of times switching between forward and backward scans is made
 to be reduced.
 When the near end of the lenticular lens system 5 reaches the near end of
 the incident light as the result of forward movement of the lens system,
 the linear motor/encoder 21 starts pulling back the lenticular lens system
 at a constant velocity. In this way, the linear motor/encoder 21 repeats
 reciprocation of the lenticular lens system 5. During the interval when
 the direction of movement of the lenticular lens system is being reversed,
 the driving of the DMD panel 8 is interrupted, resulting in a loss.
 The N-th line counting from the topmost line is successively exposed, as
 shown in FIG. 11, to red, green and blue light beams in the pattern of
 f(N.times.p+V.times.t), i.e., in the pattern which differs only in phase
 from f(0+V.times.t).
 The color and intensity (brightness) of light to which a certain pixel is
 exposed can be known by reading a pattern f(x) of colors and brightness in
 each color band set with sensors, for example, at the time of assembly of
 the color projector, storing resulting data in the form of a table into a
 nonvolatile memory, detecting the amount of movement of the color band
 sets with the encoder, and referencing the data in the nonvolatile memory.
 A full color image is produced on the DMD panel 8 and then projected onto a
 screen through the projection lens 9 by controlling the timing of turning
 on the corresponding pixel on the DMD panel while the color band sets are
 being moved so that gray scale levels of three primary-color components
 for the pixel are obtained.
 For example, by controlling the timing of turning on and off a certain
 pixel on the N-th line as shown in FIG. 12, in the first forward scan the
 pixel will be displayed in 100% of green G, 50% of blue B, and 25% of red
 R and in the next backward scan it will be displayed in 0% of green G,
 100% of blue B, and 50% of red R.
 By switching the scanning direction after a forward scan corresponding to a
 plurality of color band sets is made and switching again the scanning
 direction after a backward scan corresponding to the plurality of color
 band sets, the number of times the direction switching is made is reduced,
 allowing the loss attendant the direction switching to be reduced.
 As in the aforementioned embodiments, in this embodiment, using a single
 DMD panel, most of the red, green and blue light can be employed
 simultaneously, image brightness can be increased, and cost can be
 reduced. Moreover, the width of each color subband in each color band set
 can be set to about one-third of the scanning width and hence the length
 of time that each color subband scans a pixel can be made sufficiently
 long, which allows the DMD panel to represent sufficient shades with PWM
 control.
 Fourth Embodiment
 In the fourth embodiment, like reference numerals are used to denote
 corresponding components to those in the aforementioned embodiments and
 detailed description thereof is omitted.
 Although the third embodiment uses the plate-like lenticular lens system as
 the conversion means and the linear motor/encoder that moves forward and
 backward the lenticular lens system as the scanning means, the fourth
 embodiment uses a cylinder-shaped lenticular lens system 31 formed on the
 surfaces of a cylinder as the conversion means and a driving means, such
 as a motor, that rotates the lenticular lens system in the direction
 indicated by an arrow as the scanning means.
 Within the cylinder are housed the white light source 1, the elliptic
 reflecting mirror 3, the concave lens 3, the dichroic mirror system 4, and
 a reflecting mirror 32 that directs white light from the concave lens 3 to
 the dichroic mirror system 4. The lenticular lens system 31 has
 large-pitch lenticular lenses formed on its inside surface and small-pitch
 lenticular lenses formed on its outside surface.
 In this embodiment, red, green and blue light beams separated by the
 dichroic mirror system 4 is converted by the lenticular lens system 31
 into color band sets each consisting of a set of red, green and blue
 subbands. The color subbands are then moved over the DMD panel 8 like
 R.fwdarw.G.fwdarw.B.fwdarw.R.fwdarw.G.fwdarw.B.fwdarw.. . . without
 direction reversal by the lenticular lens system being rotated at a
 constant speed by the driving means.
 The rotating of the lenticular lens system in one direction allows the
 color band sets to scan over the DMD panel 8 continuously without
 direction reversal, thus eliminating the loss attendant scanning direction
 reversal. In this embodiment, as in the aforementioned embodiments, image
 brightness can be increased and cost can be reduced. In addition,
 sufficient shades can be displayed.
 Fifth Embodiment
 In the fifth embodiment, like reference numerals are used to denote
 corresponding components to those in the aforementioned embodiments and
 detailed description thereof is omitted.
 Although, in the third embodiment described above, the dichroic mirror
 system is used as the color separating/reflecting means for separating
 white light into red, green and blue light beams, in the fifth embodiment
 the phase volume diffraction grating 11 is used which consists of a phase
 volume hologram having a lens function. This grating has a function of
 separating incoming light into red, green and blue light beams at some
 angles with one another. In addition, the grating has the function of
 large-pitch lenticular lenses.
 Therefore, the lenticular lens system 12 for producing color band sets each
 consisting of a set of red, green and blue subbands from incoming white
 light is only required to have small-pitch lenticular lenses.
 As shown in FIG. 14, white light passed through the concave lens 3 is
 directed onto the phase volume grating 11 consisting of a phase volume
 hologram and then separated into red, green and blue light beams at some
 angles with one another. The red, green and blue light beams then are
 directed to the lenticular lens system 12.
 The lenticular lens system 12 produces a plurality of color band sets each
 consisting of red, green and blue subbands from the incident red, green
 and blue light. The width of each subband is set to about one-third of the
 scanning width. The phase volume grating 11 and the lenticular lens system
 12, which are of integral construction, are moved forward and backward as
 indicated by arrows by the linear motor/encoder 21. The phase volume
 grating 11 and the lenticular lens system 5 have their length set larger
 than the width of incident light by the amount corresponding to an
 integral multiple of the pitch of the color band sets. Thus, the scanning
 of the color band sets can be made a plurality of times in succession.
 The image of the color band sets emerging from the small-pitch lenticular
 lenses of the lenticular lens system 12 is reflected by the coupling lens
 7, then reflected by the reflecting mirror 41 and formed through the
 polarizing beam splitter 13 onto the light valve 14 consisting of a
 reflective liquid crystal panel.
 The full color image emerging from the light valve 14 is projected by the
 projection lens 9 through the polarizing beam splitter 13 onto a screen.
 In this arrangement as well, the number of times of the direction switching
 is reduced by switching the scanning direction after a forward scan
 corresponding to an integral multiple of the pitch of the color band sets
 is made and switching again the scanning direction after a backward scan
 corresponding to the integral multiple of the pitch of the color band
 sets. Thus, the loss attendant the direction switching can be reduced. In
 this embodiment, as in the aforementioned embodiments, image brightness
 can be increased and cost can be reduced. In addition, sufficient gray
 scale levels can be displayed.
 Although the third and fifth embodiments have been described as using a
 linear motor as the scanning means for moving continuously the color band
 sets over the DMD panel by the amount corresponding to an integral
 multiple of their pitch, this is not restrictive. For example, a motor and
 a cam may be used to reciprocate the lenticular lens system.
 Further, although the aforementioned embodiments have been described as
 using a DMD panel or reflective liquid crystal panel as the light valve,
 this is not restrictive. Any light valve can be used provided that it has
 a response fast enough to make color-sequential display.
 Additional advantages and modifications will readily occur to those skilled
 in the art. Therefore, the invention in its broader aspects is not limited
 to the specific details and representative embodiments shown and described
 herein. Accordingly, various modifications may be made without departing
 from the spirit or scope of the general inventive concept as defined by
 the appended claims and their equivalents.