Double-pass projection displays with separate polarizers and analyzers

A double-pass projection display system with separate polarizers and analyzers. One embodiment of the invention includes an input polarizer, a color separation/recombination device, and a number of image producing sections. The input polarizer receives unpolarized white light and polarizes this light. The color separation/recombination device receives the polarized white light and separates this light into a number of component color bands. The color separation/recombination device then supplies a component color band to each image producing sections. Each section includes a spatial light modulator and an output analyzer. The spatial light modulator modulates the color band that the image producing section receives from the color separation/recombination device. The output analyzer then (1) discards the light, in the modulated band, that has a first polarization state, and (2) directs the light, in the modulated band, that has a second polarization state to the color separation/recombination device. The color separation/recombination device then produces a single color light by combining the component color bands that it receives from the output analyzers of the image producing sections.

BACKGROUND OF THE INVENTION
 To date, a variety of optical projection systems have been proposed. Each
 of these display systems typically includes (1) an input polarizer, (2)
 one or more spatial light modulators, and (3) one or more output
 analyzers. An input polarizer linearly polarizes unpolarized light. One
 type of input polarizer is a polarizing beam splitter ("PBS"), which
 polarizes unpolarized light by splitting it into transmitted P-polarized
 light and reflected S-polarized light. P-polarized light is light that is
 parallel to the plane of incidence (which is defined by the incident and
 reflected rays), while S-polarized light is light that is perpendicular to
 the plane of incidence.
 A spatial light modulator (SLM) receives the light that an input polarizer
 linearly polarizes. An SLM often includes an array of picture elements
 (also called pixels) that the SLM individually controls to modulate the
 light passing through the pixels. An SLM is typically formed by
 positioning a layer of liquid crystal material between two electrodes. One
 of the electrodes is segmented into an array of pixel electrodes to define
 the pixels of the SLM, while the other electrode is usually not segmented.
 There are two varieties of SLM's: reflective and transmissive. In both
 varieties, the direction of an electric field applied between each pixel
 electrode and the other electrode determines whether the corresponding
 pixel changes the polarization of light falling on the pixel. Hence, in
 both varieties, the incident light is modulated by changing the
 polarization of light falling on certain pixels while leaving unchanged
 the polarization of the light falling on other pixels.
 An output analyzer receives the light transmitted or reflected by an SLM.
 Output analyzers are polarization-selective devices similar to the input
 polarizers. Polarizing filters and PBS's are two types of output
 analyzers. An output analyzer allows a certain polarization state of the
 light to pass, while discarding the remaining polarization states. Hence,
 output analyzers are placed at the outputs of SLM's to obtain the pattern
 of modulation of the SLM's, and thereby generate images. An observer will
 not perceive an image unless an analyzer follows an SLM, because the SLM
 does not attenuate the incident beam of light, but rather simply modulates
 the lights polarization state.
 Projection displays generate color images by modulating, analyzing, and
 combining component color bands. Display devices typically use a few
 component colors (such as the primary additive colors, red, green or blue)
 to generate a multitude of colors for display. A component color band is a
 portion of the light spectrum corresponding to a component color. When all
 the component color bands are added, they produce white light. Conversely,
 component color bands can be extracted from white light.
 To generate color images, projection displays not only use input
 polarizers, SLM's, and output analyzers, but they also use other devices.
 For instance, color projection systems often either use (1) a light source
 for each component color band (e.g., three light sources for the three
 primary additive colors, red, green, and blue), or (2) a single source of
 white light with a prism or other color separation device that separates
 incident white light into component color bands (e.g., into red, green,
 and blue light).
 The component color bands are then used to illuminate one or more SLM's,
 which modulate the incident light for each color band. The modulated color
 bands are then recombined to produce a full-color image. The recombination
 may take place sequentially or simultaneously.
 I. Color-Field Sequential Display Systems.
 Color-field sequential systems create an image by sequentially projecting
 red ("R"), green ("G"), and blue ("B") images. FIG. 1 presents one prior
 art color-field sequential system. This display system 100 uses a
 mechanical color filter wheel 105 positioned between a light source 110
 and a light valve 115 (which includes an SLM and an analyzer).
 As shown in FIG. 2, the filter wheel 105 is divided into three filter
 sections, each acting as a pass filter for one of the three primary
 additive colors. By rotating the filter wheel, successive red, green, and
 blue light are generated to illuminate the light valve. The light valve is
 then modulated to generate successive red, green, and blue images. The
 eye-brain system fuses the successively-projected color images into a
 single blended polychromatic image, if the eye is stationary and the
 successive color patterns are projected at a high rate.
 The eye, however, is not always stationary and often moves, and this
 movement can cause the viewer to see artifacts, called color sequential
 artifacts ("CSA"). For instance, the viewer might see spurious images
 (such as flashes of red, green, or blue light). CSAs are not only
 annoying, but also present safety concerns (e.g., they may cause epileptic
 attacks).
 Increasing the projection rate of the images can minimize color sequential
 artifacts. However, at high rates, the mechanical color filter 105 does
 not operate reliably and introduces noise and vibration into the system.
 Electronic color switches can be used in place of the mechanical filter
 105, but the electronic switches require complicated processing and
 driving circuitry, and are somewhat inefficient at their high switching
 rates. Finally, sequential system 100 does not generate good color
 contrast because its light valve 115 cannot be cost-effectively designed
 to operate perfectly for each of the three generated color bands.
 II. Simultaneous Projection Display Systems.
 Simultaneous projection display systems create a color image by optically
 superimposing multiple partial-color images to the same location. In
 addition to using light sources, input polarizers, color-separating
 devices, SLM's, and output analyzers, simultaneous projection systems also
 use color-recombining devices (such as dichroic prisms) to recombine each
 of the component color images in a coordinated way.
 Simultaneous projection systems may be divided into single-pass and
 double-pass systems. Double-pass systems use the same device for both the
 color separation and recombination operations, while single-pass systems
 use different devices for these operations.
 A. Single-Pass Systems.
 FIG. 3 presents one prior art single pass system. The light from the light
 source is separated into three color bands using dichroic filters. A
 separate light valve (formed by an SLM and an output analyzer) modulates
 each color band. The modulated color bands are then recombined using
 dichroic filters.
 There are several disadvantages to this architecture. For example, this
 system is somewhat bulky and relatively expensive since it uses many
 components. Also, its projection lens is complex and costly since it needs
 a projection lens with large back-focal length due to the relatively large
 distance from the panel to the lens. The dichroic filters used for the
 recombination operations also introduce aberrations and distortion in the
 generated images.
 FIG. 4 presents another prior art single-pass system. This system receives
 R, G, and B light either from three sources of light (as shown in FIG. 4),
 or from a color-separator (not shown) that separates these different color
 bands from white light. System 400 also utilizes three PBS's 420, 425, and
 430. These PBS's serve as input and output polarizers. Specifically, the
 PBS's initially receive unpolarized light from light sources 405, 410, and
 415. They transmit the P-polarized light out of the system, while
 reflecting the S-polarized light towards the SLM's 435, 440, and 445.
 The SLM's then modulate and reflect the received light back to the PBS. On
 the second pass through, the PBS's serve as output polarizers (i.e.,
 output analyzers). The analyzers (1) reflect and thereby reject the
 S-polarized light (corresponding to the light having a polarization that
 the SLM's did not change), and (2) transmit the P-polarized light
 (corresponding to the light having a polarization that the SLM's changed).
 The dichroic prism 450 receives the color images output from the analyzers
 and combines these images into a single polychromatic image. Projection
 lens 460 then projects this image on a screen.
 The design and construction constraints on this system are considerably
 relaxed because each pair of PBS's and SLM's is tightly coupled and
 operates over a narrow color spectrum. Also, the recombination prism does
 not convert the polarization of the light because the light passing
 through it only has a single polarization orientation--in this case,
 P-polarized.
 This system, however, uses many components. For instance, it either needs
 three different light sources, or it needs a color-separating device
 different than the recombination prism. As a result, this system is
 somewhat bulky and relatively expensive.
 B. Double-Pass Systems.
 Unlike single-pass systems, double-pass systems use one device (e.g., one
 dichroic prism) for both the color separation and recombination
 operations. Hence, double-pass systems are typically smaller and less
 expensive.
 FIG. 5 presents one prior art double-pass projection display system. This
 system 500 includes a light source 505, a PBS 510, a prism 515, and three
 SLM's 520, 525, and 530. The light source 505 supplies unpolarized light
 to the PBS 510. This PBS serves as both the input polarizer and the output
 analyzer. As the input polarizer, the PBS polarizes the unpolarized white
 light that it receives from the light source 505 by transmitting
 P-polarized light out of the system (and thereby discarding this
 polarization), while reflecting S-polarized light towards the prism.
 The dichroic prism 515 then separates the S-polarized white light into its
 color components, and directs each color light to the corresponding color
 SLM. The SLM's then modulate and reflect the received light. The reflected
 light includes both S-polarized light (corresponding to light having a
 polarization that the SLM's did not change) and P-polarized light
 (corresponding to light having a polarization that the SLM's changed).
 The light reflected by the SLM's then enters the prism, which now combines
 the modulated color light and supplies the combined light to the PBS. On
 the second pass through, the PBS serves as the output analyzer that (1)
 reflects and thereby rejects the S-polarized light, and (2) transmits the
 P-polarized light. The projection lens then receives the P-polarized light
 from the analyzer and focuses this light on the screen.
 The design and construction constraints on this system are considerable
 because the PBS operates as the analyzer for all three-color bands, and
 therefore must meet exacting performance requirements over the entire
 color spectrum. It is quite difficult to have the PBS perform optimally
 over the entire color spectrum. The PBS typically is optimized for one or
 two of the additive colors, which causes the PBS to offer poor contrast
 and poor dark states for the third additive color.
 A high degree of scattered light also exists in the dichroic prism because
 all the light reflected by the SLM's is directed through the prism. This,
 in turn, increases the performance requirements on the prism. In addition,
 the light passing through the prism has both S and P polarization. This
 causes the prism to introduce polarization conversion. Specifically, when
 both S and P light traverse through the prism, the prism rotates the
 polarization of the S and P light and/or introduces ellipticity into the
 polarization state.
 Polarization conversion then contaminates the analyzing operation performed
 by the PBS on the second pass. For instance, if the polarization
 conversion causes S-polarized light from a pixel (an "OFF" or dark pixel)
 to slightly rotate so that it now has a P-polarized component, then the
 PBS on the second pass does not reject all the light for that pixel and
 allows the P-polarization component to pass. Hence, the polarization
 conversion causes light to leak into the dark pixels and reduces the
 contrast and brightness of the bright pixels.
 Therefore, there is a need in the art for a double-pass system that
 generates good dark states and good color contrast. There is also a need
 for a double-pass system that avoids leakage of light into dark pixels.
 The double-pass system should also ideally have analyzers that operate
 over narrower bands and closely couple to their respective SLM's.
 SUMMARY OF THE INVENTION
 This invention is directed towards double-pass projection displays with
 separate polarizers and analyzers. One embodiment of the invention
 includes an input polarizer, a color separation/recombination device, and
 a number of image producing sections. The input polarizer receives
 unpolarized white light and polarizes this light. The color
 separation/recombination device receives the polarized white light and
 separates this light into a number of component color bands.
 The color separation recombination device then supplies a component color
 band to each image producing sections. Each section includes a spatial
 light modulator and an output analyzer. The spatial light modulator
 modulates the color band that the image producing section receives from
 the color separation/recombination device. The output analyzer then (1)
 discards the light, in the modulated band, that has a first polarization
 state, and (2) directs the light, in the modulated band, that has a second
 polarization state to the color separation/recombination device. The color
 separation/recombination device then produces a single color light by
 combining the component color bands that it receives from the output
 analyzers of the image producing sections.

DETAILED DESCRIPTION OF THE INVENTION
 This invention is directed towards double-pass projection displays with
 separate polarizers and analyzers. In the following description, numerous
 details are set forth for purpose of explanation. However, one of ordinary
 skill in the art will realize that the invention may be practiced without
 the use of these specific details. In other instances, well-known
 structures and devices are shown in block diagram form in order not to
 obscure the description of the invention with unnecessary detail.
 FIG. 6 presents one embodiment of the invention. This embodiment is a
 double-pass projection display system 600. This display system includes
 (1) a light source 605, (2) an input polarizer 610, (3) a color
 separation/recombination device 615, and (4) three image-producing
 sections 670, 675, and 680. Each image producing section pairs one output
 analyzer (620, 625, or 630) with one transmissive SLM (650, 655, or 660)
 to produce an image for a green, blue, or red component color. Each image
 producing section also includes one set of mirrors (635, 640, or 645).
 The light source 605 initially supplies unpolarized light to the input
 polarizer 610. In the embodiment shown in FIG. 6, the input polarizer is a
 PBS, such as a MacNeille polarizing cube. Other embodiments utilize
 different input polarizers. The PBS polarizes the unpolarized light that
 it receives from the light source 605 by allowing the P-polarized light to
 pass through it and out of the system (and thereby discarding this
 polarization component of the light). The PBS 610 reflects the remaining
 S-polarized light towards the color separation/recombination device 615.
 The color separation/recombination device is a dichroic prism 615 (such as
 a Philips prism) in the embodiment shown in FIG. 6. Other embodiments of
 the invention, however, use different color separation/recombination
 devices. For example, as shown in FIG. 8, one embodiment of the invention
 uses a dichroic cube 805 (such as the X-cube manufactured by Balzers,
 Inc). A dichroic cube can be used to implement the invention because the
 invention ensures that only one polarization is transmitted in a
 particular direction through the color separation/recombination device,
 and this is a precondition for using dichroic cubes.
 The dichroic prism 615 separates the S-polarized white light that it
 receives into component red, green, and blue light. The prism directs the
 green color light to output analyzer 620, the blue color light to output
 analyzer 625, and the red color light to output analyzer 630.
 In the embodiment shown in FIG. 6, the output analyzers are PBS's that are
 similar to PBS 610. Other embodiments, however, use different analyzing
 devices. PBS's 620, 625, and 630 reflect their respective incident
 S-polarized component light to mirrors 635, 640, and 645 respectively.
 These mirrors then reflect and direct the light to pass through the
 transmissive SLM's 650, 655, and 660.
 Each SLM modulates its incident S-polarized component light by changing the
 S-polarization of light passing through a first set of pixels (the "ON"
 pixels) to a P-polarization, while leaving the S-polarized light passing
 through a second set of pixels (the "OFF" pixels) unchanged. In some
 embodiments of the invention, the SLM's 650, 655, and 660 are formed by
 (1) placing a layer of liquid crystal material (such as surface-stabilized
 ferroelectric liquid crystal (SSFLC)) between two electrodes, and (2)
 segmenting one of the electrodes into an array of pixels electrodes that
 define the pixels of the SLM. Other embodiments of the invention use
 different liquid crystal materials, different electrode structures, and/or
 different SLM's.
 The electric field applied between each pixel electrode and the other
 electrode, in conjunction with the structure and orientation of the SLM's
 liquid crystal material, determine how each pixel rotates the polarization
 of light falling on it. In the embodiment shown in FIG. 6, SLM's 650, 655,
 and 660 are structured as half-wave plates because light passes through
 them only once.
 In addition, the orientation of the each half-wave plate with respect to
 the polarization of the incident light is chosen to provide an optical
 phase shift of 90.degree. when the appropriate potential difference exists
 between the pixel electrode and the transparent electrode. An optical
 phase shift of 90.degree. changes the incident S-polarized light to
 P-polarized light. Hence, each SLM modulates its component-color light by
 changing the polarization state of certain regions in this light (i.e.,
 rotating light falling on a first set of "ON" pixels of the SLM) while
 leaving unchanged the polarization state of other regions in the light
 (i.e., not rotating light falling on a second set of "OFF" pixels of the
 SLM).
 Output analyzers 620, 625, and 630 then (1) receive their modulated,
 component-color light from their respective SLM's 650, 655, and 660, and
 (2) create component color images by filtering out (i.e., rejecting) the
 S-polarized light from their component color light. More specifically, the
 output analyzers receive P-polarized light from the "ON" pixels of the
 SLM's, and S-polarized light from the "OFF" pixels of the SLM's. The
 "ON-state" P-polarized light passes through the output analyzers and into
 the color separation/recombination prism, while the "OFF-state"
 S-polarized light reflects off the output analyzers and out of the system.
 Hence, the output analyzers discard the S-polarized light to create dark
 regions in the component color light, which correspond to the "OFF" state
 pixels. By creating these dark regions in their component color light, the
 output analyzers 620, 625, and 630 create color images respectively for
 their green, blue, and red color bands.
 The dichroic prism 615 receives the component color images output from the
 analyzers (i.e., receives the "ON-state" P-polarized component color light
 from the analyzers). On the second pass through, the prism serves as a
 color recombination device that superimposes and combines the received
 P-polarized green, blue, and red light to generate a full-color image.
 The prism then directs the combined color light to PBS 610. On the second
 pass through, the PBS 610 acts as an auxiliary or "clean-up" analyzer that
 reflects and thereby rejects any S-polarized light that might have leaked
 through analyzers 620, 625, and 630 and/or might have inadvertently been
 created in the prism 615. The PBS 610 then transmits the P-polarized light
 to the projection lens 690, which focuses this light on the screen.
 Projection display system 600 has numerous advantages. This system provides
 the superior image quality of a single pass system with the compactness,
 robustness and low cost of a double-pass system. Double-pass system 600 is
 small and relatively inexpensive because it uses one device for both the
 color separation and color recombination operations.
 This system also limits the polarization of light flowing through the color
 separation/recombination device 615 to a single polarization in each
 direction of flow. The light flowing from the PBS 610 to the output
 analyzers 620, 625, and 630 is only S-polarized, while the light flowing
 from the output analyzers 620, 625, and 630 to the PBS 610 is P-polarized.
 Limiting the light flowing through the prism 615 to a single polarization
 in each direction has a number of advantages. It enables display 600 to
 use a dichroic cube as the color separation/recombination device. It also
 significantly relaxes the design constraints on the
 separation/recombination device, because the separation/recombination
 device 615 does not need to preserve the phase relationship between the S
 and P polarized components.
 Limiting the light flow to a single polarization also prevents the prism
 from introducing unwanted polarization conversion. Preventing polarization
 conversion improves image quality. Specifically, prior art color
 recombination devices suffer from polarization conversion, which causes
 light to leak into the dark pixels and reduces the contrast and brightness
 of the bright pixels. By preventing polarization conversion from occurring
 in color recombination device 615, system 600 enjoys superior dark states
 and color contrast.
 In addition, each output analyzer in system 600 operates over a narrow
 band. Hence, each output analyzer can be specifically designed to operate
 perfectly over its targeted narrow band. Each output analyzer also tightly
 couples to an SLM. This tight coupling in conjunction with the narrow band
 operation of the output analyzer allow the image-producing sections to
 generate sharp "ON" and "OFF" states.
 System 600 is also advantageous because it minimizes the amount of light
 flowing through the color recombination device. Reducing the total amount
 of light in the color recombination device reduces scattered light, thus
 improving the contrast of the image. It accomplishes this because the
 output analyzers discard the unused "OFF" state S-polarized light.
 Mirror assemblies 635, 640, and 645 are also beneficial because they allow
 the display system to operate even when the polarization states are
 inverted. In other words, if the position of the projection lens and the
 light source were swapped, and PBS 610 directed P-polarized light towards
 prism 615, the mirror assemblies would enable the display system 600 to
 continue operating. The only difference would be that the "ON" polarized
 light (directed by the output analyzers back into the prism) would be
 S-polarized light.
 FIG. 7 presents another embodiment of the invention. Double-pass projection
 system 700 is analogous to projection system 600 of FIG. 6, except that
 system 700 (1) uses reflective SLM's rather than the transmissive SLM's of
 system 600, and (2) uses only one mirror (735, 740, or 745) to direct the
 light between each pair of output analyzer and SLM. Consequently, the
 discussion above regarding the operation of the light source 605, the
 input polarizer 610, the color separation/recombination device 615, and
 the output analyzers 620, 625, and 630 in system 600, is equally
 applicable to the operation of these devices in system 700.
 The structure and operation of reflective SLM's 735, 740, and 745 will now
 be described. In some embodiments of the invention, each SLM is composed
 of a layer of a liquid crystal material (such as SSFLC) positioned between
 two electrodes. The first electrode is a transparent electrode, while the
 second electrode is a reflective electrode. Other embodiments of the
 invention use different liquid crystal materials, different electrode
 structures, and/or different SLM's.
 The reflective electrode is divided into a two-dimensional array of pixel
 electrodes that define the pixels of the SLM. Each pixel electrode
 reflects the portion of the incident polarized light that falls on the
 pixel electrode towards the SLM's corresponding output analyzer. The
 potential difference between the pixel electrode and the transparent
 electrode establishes an electric field across the part of the liquid
 crystal layer between the pixel and transparent electrodes. The electric
 field applied between each pixel electrode and the other electrode, in
 conjunction with the structure and orientation of the SLM's liquid crystal
 material, determine how each pixel rotates the polarization of light
 falling on it.
 The reflective SLM is structured as a quarter-wave plate because light
 passes through the reflective SLM twice, once before and once after the
 reflection. The orientation of the quarter-wave plate with respect to the
 polarization of the incident light is chosen to provide a double-pass
 optical phase shift of 90.degree. when the appropriate potential
 difference exists between the pixel electrode and the transparent
 electrode.
 As mentioned above, system 700 operates analogously to system 600, with the
 exception of the reflective SLM's and their corresponding mirror
 assemblies. System 700 also enjoys the same advantages as system 600.
 Consequently, the analogous operations and advantages are not repeated
 below in order not to obscure the description of the invention with
 unnecessary detail.
 While the invention has been described by reference to numerous specific
 details, one of ordinary skill in the art will recognize that the
 invention can be embodied in other specific forms without departing from
 the spirit of the invention. For instance, the embodiments described above
 separate white light into the red, green, and blue component color bands.
 Other embodiments of the invention, however, utilize different component
 color bands.
 In addition, the SLM's of systems 600 and 700 change the polarization of
 the light falling on the "ON" state pixels while they leave the
 polarization of the light falling on the "OFF" state pixels unchanged. The
 SLM's of other embodiments of the invention, however, use different
 conventions in order to modulate the light. Thus, one of ordinary skill in
 the art would understand that the invention is not to be limited by the
 foregoing illustrative details, but rather is to be defined by the
 appended claims.