Liquid crystal display system and light projection system

A projection system used in a liquid crystal display (LCD) system is provided. The projection system has a light source, a polarization set which further includes a double dove prism, a vertical prism and a half-wave plate, a polarization beam splitter (PBS), a dichroic prism, and a projection lens. The light source emits a white light, which enters the polarization set and is split into a P-state polarized blue light and a mixed light. The mixed light includes an S-state polarized red light and an S-state polarized green light. The PBS allows the P-state polarized blue light to transmit and enter onto a blue light LCD panel, and deflects the mixed light by 90.degree.. The dichroic prism splits the S-state polarized red light and the S-state polarized green light of the mixed light, which respectively enter a red light LCD panel and a green light LCD panel. The projection lens collects light from the red, green, blue LCD panels and project the lights onto a screen.

CROSS-REFERENCE TO RELATED APPLICATION
 This application claims the priority benefit of Taiwan application serial
 no. 87108197, filed May 26, 1998, the full disclosure of which is
 incorporated herein by reference.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to an electronic optical system, and more
 particularly to a light projection system suitable for a use in a liquid
 crystal display system (LCD) to display image.
 2. Description of Related Art
 Recently, a LCD device is widely used in TV, computer, monitor or other
 display system. Comparing with a conventional display apparatus with a
 picture tube, the LCD system is lighter and has a smaller dimension. It
 becomes a necessary part to display system, such as a notebook computer.
 FIG. 1 is a structure diagram, illustrating a polarization light projection
 system used in a reflection-type color display board as disclosed by U.S.
 Pat. No. 5,530,489. In FIG. 1, a reading light source 100 can emit a white
 light. The white light is polarized by a polarization beam splitter (PBS)
 102 and split into an S-state polarized beam and a P-state polarized beam,
 both which are also reflected so that both light polarized beams are
 deflected by 90 degrees. The S-state polarized beam forms the WS beam. The
 P-state polarized beam is converted into an S-polarization beam WS'
 through a half-wave plate 106. The WS and WS' beams are incident to a
 polarization analyzer 108 and are deflected by 90 degrees again, in which
 the polarization analyzer 108 further ensures that the Ws and the WS'
 beams are polarized into an S polarized state.
 The WS and the WS' beams enter a dichroic prism 110, which deflects a blue
 light BS of the WS and WS' beams by 90.degree. and allows a red light RS
 and a green light GS to continuously transmit. The blue light BS passes a
 light path compensation plate 112 and enters a blue liquid crystal light
 valve (LCLV) 114, which converts the blue light BS into a blue light BP
 with P-state polarization and reflect the blue light BP back to the
 dichroic prism 110 through the light path compensation plate 112. The blue
 light BP is deflected to a projection lens 122 through the polarization
 analyzer 108. The projection lens 122 project the blue light BP onto an
 image screen (not shown). For the red light RS and the green light GS,
 they continuously travel to a color filter prism 116, which deflects the
 green light GS by 90.degree. into a red LCLV 118, and allows the red light
 RS to pass and reach a green LCLV 120. The red LCLV 118 reflects the green
 light GS back and converts it into a green light GP with P-state
 polarization. Similarly, the green LCLV 120 reflects the red light GS back
 and converts it into a red light GP with P-state polarization. The red
 light RP and the green light GP are also deflected to the projection lens
 122 and projected to the image screen like the blue light BG.
 In this system shown in FIG. 1, the system includes two light splitters and
 several prisms, resulting in a large system dimension. This display system
 cannot be efficiently applied in a large displaying area and is not
 portable. Moreover, a poor focusing quality severely occurs due to a too
 large distance between the LCLVs and the projection lens. This further
 limits its applications.
 Another system is disclosed by U.S. Pat. No. 5,153,752 to reduce the
 distance of the projection lens and the system dimension. FIG. 2 is a
 structure diagram, illustrating a polarization light projection system
 used in a reflection-type color display board as disclosed by U.S. Pat.
 No. 5,153,752. In FIG. 2, a light source 200 can emit a white unpolarized
 light S+P, which enters a PBS 201 and is split into an S-state polarized
 beam S1 and a P-state polarized beam P2. The S-state polarized beam S1 is
 deflected by 90.degree. and enters a dichroic prism set 204, which
 includes several dichroic prisms 204a, 204b, 204c, and 204d. After passing
 the dichroic prism set 204, the S-state polarized beam S1 are split into a
 red light RS, a green light GS, and a blue light BS, which respectively
 travel to LCD panels 205R, 205G and 205B. The LCD panels 205R, 205G and
 205B respectively convert the red light RS, the green light GS, and the
 blue light BS into a red light RP, a green light GP, and a blue light BP
 with P-state polarization, and reflect the lights RP, GP, and BP onto a
 projection lens 206, which projects passing light onto a screen (not
 shown).
 For the P-state polarized beam P2, as it passes the PBS 201, it enters a
 half-wave plate 202 and is polarized to an S-state polarized beam S2. The
 S-state polarized beam S2, similar to the S-state polarized beam S1, is
 reflected by the LCD panels 205R, 205G and 205G and reach the screen at
 the end.
 In this conventional projection system of FIG. 2, the dimension and the
 light focusing issues of the conventional projection system of FIG. 1 is
 reduced. However, since the system of FIG. 2 is very complicate,
 production yield rate is low and fabrication cost is high. Moreover, since
 several prisms are used in the system, a little misalignment may cause a
 large error. Its requirement of alignment precision is much higher that a
 usual level.
 SUMMARY OF THE INVENTION
 It is at least an objective of the present invention to provide a LCD
 system, particularly suitable for a color LCD system. The LCD system
 includes a projection system with a denser layout so that system dimension
 and back focal length are effectively reduced. A light path needs no a
 complicate reflection set so that there is no need of high alignment
 precision. Fabrication cost is also effectively reduced.
 In accordance with the foregoing and other objectives of the present
 invention, a projection system used in a LCD system is provided. The
 projection system includes a light source, a polarization set which
 further includes a double dove prism, a vertical prism and a half-wave
 plate, a polarization beam splitter (PBS), a dichroic prism, and a
 projection lens. The light source emits a white light, which enters the
 polarization set and is split into a P-state polarized blue light and a
 mixed light. The mixed light includes an S-state polarized red light and
 an S-state polarized green light. The PBS allows the P-state polarized
 blue light to transmit and enter onto a blue light LCD panel, and deflects
 the mixed light by 90.degree.. The dichroic prism splits the S-state
 polarized red light and the S-state polarized green light of the mixed
 light, which respectively enter a red light LCD panel and a green light
 LCD panel. The projection lens collects lights from the red, green, blue
 LCD panels and project the lights onto a screen.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
 The conventional projection system used in a LCD system includes issues of,
 for example, large dimension, poor focusing quality, complicate system, or
 high fabrication cost. The invention introduces a projection system used
 in a LCD system so as to at least solve the above issues.
 FIG. 3 is a structure diagram, schematically illustrating a polarization
 light projection system using reflection-type LCD panels, according to a
 preferred embodiment of the invention. In FIG. 3, a greatly simplified
 projection system is designed so that dimension is greatly reduced, a back
 focal length is also reduced. The system has a large tolerance of
 misalignment. Fabrication cost is also reduced.
 The projection system of the invention includes a light source 300, a
 double dove prism 301A, a vertical prism 301B, a half-wave plate 305, a
 polarization beam splitter (PBS) 306, a color filter prism 309, and a
 projection lens 311. The double dove prism 301A further includes optical
 films 302 and 304, in which the optical film 302 behaves as a PBS for blue
 light but behaves as a dichroic mirror for red light and green light. The
 light source 300, such as a lamp, emits a light, such as a white light,
 which is incident on the optical film 302. A red light R and a green light
 G of the white light are reflected and a blue light B is split into a blue
 light BS with S-state polarization and a blue light BP with P-state
 polarization. The blue light BS is also reflected by the optical film 302
 so as to form a mixed light RG+BS.
 The blue light BP continuously travels to a reflection mirror 303 so that
 the blue light BP is deflected by 90.degree., travelling in parallel to
 the mixed light RG+BS. The blue light BP then directly travels to a blue
 LCD panel 310B through the optical film 302 and the BPS 306.
 The mixed light RG+BS resulting from the optical film 302 keep travelling
 to the optical film 304, which behaves as a PBS for red light and green
 light but behaves as a dichroic mirror for blue light. So, the red/green
 light RG of the mixed light RG+BS is split into a red/green light RPGP
 with P-state polarization and a red/green light RSGS with S-state
 polarization. The red/green light RPGP continuously travels in the same
 direction but the red/green light RSGS is deflected by 90.degree. onto the
 optical film 302. Due to the property of the optical film 304, the blue
 light BS just transmits the optical film 304 and combines with the
 red/green light RPGP to form a mixed light RPGP+BS, which enters the
 half-wave plate 305 through the vertical prism 301B. The purpose of the
 vertical prism 301B is used to compensate a corner of the double dove
 prism 301A to form a box corner for easy assembling. The half-wave plate
 305, for example, located on the vertical prism 301B so as to only allow
 the mixed light RPGP+BS to pass. The half-wave plate 305 changes
 polarization state of passing light so that the red/green light RPGP is
 converted into a red/green light RS'GS' with S-state polarization, and the
 blue light BS is converted into a blue light BP'. The blue light BP'
 travels to the blue LCD panel 310B through the PBS 306 but the red/green
 light RS'GS' is deflected by the PBS 306 by 90.degree. onto the dichroic
 prism 309.
 As the red/green light RSGS is incident on the optical film 302, it is
 deflected again by 90.degree. so that the red/green light RSGS is
 reflected to the PBS 306, which deflects the red/green light RSGS again.
 As a result, the red/green light RS'GS' and the red/green light RSGS do
 travel in parallel and both enter the dichroic prism 309. The red/green
 light RS'GS' and the red/green light RSGS are respectively split by the
 dichroic prism 309 into a green-content light G and a red-content light.
 The red-content light R travels to a red LCD panel 310R, and the
 green-content light G travels to a green LCD panel 310G. All the red LCD
 panel 310R, the green LCD panel 310G, and the blue LCD panel 310B
 respectively reflect incident lights back along the same light path to the
 PBS 306, in which all polarization states are inverted also. Through the
 PBS 306, a mixed light RPGP+BS and a mixed light RP'GP'+BS' therefor are
 formed and travel to the projection lens 311. The projection lens 311 can
 project incident light onto a screen (not shown).
 In this projection system of the invention, the optical film 302 can be
 formed to include, for example, a stacked-layer structure by alternatively
 depositing layers of materials with high and low refractive indices as
 listed in Table 1. Similarly, the optical film 304 can also be formed, for
 example, like the optical film 302 with two different film materials as
 listed in Table 2. Their reflection factors are shown in FIG. 4 and FIG.
 5. FIG. 4 is a reflection factor distribution, schematically illustrating
 a reflection spectrum of the optical film 302 in FIG. 3. FIG. 5 is a
 reflection factor distribution, schematically illustrating a reflection
 spectrum of the optical film 304 in FIG. 3. An incident angle is
 45.degree. so as to deflect light by 90.degree.. Each S and P curves in
 FIG. 4 and FIG. 5 respectively represent an S-state polarized light and a
 P-state polarized light. An observable range of wavelength (.lambda.) is,
 for example, about between 400 nm and 690 nm. Both the optical films 302
 and 304 may also be formed by deposition of more than two film materials.
 For example, some layers with medium refractive index may be further
 included in the stack-layer structure of the optical films 302 and 304.
 The invention has been described using an exemplary preferred embodiment.
 However, it is to be understood that the scope of the invention is not
 limited to the disclosed embodiment. On the contrary, it is intended to
 cover various modifications and similar arrangements. The scope of the
 claims, therefore, should be accorded the broadest interpretation so as to
 encompass all such modifications and similar arrangements.