Polarized light source device for liquid crystal projector

A high-efficiency polarized illuminating system for a liquid crystal projector is disclosed. The illuminating system is easy to fabricate and set up, with a low cost since, unlike in the prior art, no particular component is needed in the illuminating system and the components used in the system does not require a highly precise alignment. The illuminating system comprises: A light source device for providing a light beam; a polarizing beam-splitting device for converting the light beam into a p-polarized light beam and an s-polarized light beam; a first lens plate for converging the p-polarized light beam and the s-polarized light beam into a plurality of light spots; a polarization rotating device for optionally passing the plurality of light spots formed by the p-polarized light beam or the s-polarized light beam to output the plurality of light spots with a certain polarization; and a second lens plate for projecting the plurality of light spots to form a plurality of light beams having the certain polarization, in which the plurality of light beams are overlapped on a converging lens to form a polarized light beam that is converged on a liquid crystal panel.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a light source and, more particularly, to
 a high-efficiency polarized light source device for a liquid crystal
 projector.
 2. Description of Prior Art
 In a conventional liquid crystal (hereinafter referred to as LC) projector,
 the LCd is play panel is normally rectangular, but the cross-section of
 the projecting beam emitted from the light source is circular. Therefore,
 the light energy distributed in the circumferential areas is sacrificed in
 order to irradiate the whole LC display panel. Moreover, since an LC
 projector requires polarized light, half the light energy is lost while
 the non-polarized light emitted from the light source is being polarized.
 Because of the above problems, the brightness of the display in a
 conventional LC projector is not sufficient for image projection. One
 solution is to provide a light source with a higher power. However, this
 causes some other problems in that this approach not only consumes much
 more electricity, but also generates undesirable heat that will cause
 further problems.
 In order to overcome such problems, other optical systems have been
 developed in the prior art. For example, and referring to FIG. 1, U.S.
 Pat. No. 5,098,184 discloses an illumination system for an image
 projection apparatus. The illumination system comprises a radiation source
 22, a concave reflector 24 and a first and a second lens plate 26, 28 each
 being provided with a matrix of lenses for forming superimposed images of
 the radiation source on the object to be illuminated. The first lens plate
 26 and the second lens plate 28 are utilized to redistribute the light
 intensity. Furthermore, the shape of each lens 27 and lens 29 corresponds
 to the shape of the LC display panel 20. Thus, this invention can provide
 a uniform brightness and efficiently make use of the light energy.
 However, that half the light energy is lost while converting the
 non-polarized light into polarized light still remains a problem.
 In order to improve the efficiency of the LC projector, it is important to
 reduce the light energy lost while generating polarized light. A prior art
 entitled "Ultra-High-Efficiency LC Projector Using a Polarized Light
 Illuminating System" has been disclosed in SID 97 DIGEST, pp. 993 to 996,
 by Nakamura et al.
 Referring to FIG. 2, the illuminating system includes a light source 30; a
 reflector 31; a first lens plate 35; a second lens plate 38; a polarizing
 beam-splitter array 140; a plurality of half wave plates 145; and a
 condenser lens 50. The first lens plate 35 includes a plurality of
 rectangular lenses 36 having a geometrical shape similar to the liquid
 crystal panel 5.
 The second lens plate 38 includes a plurality of lenses 139 corresponding
 to the lenses 36 included in the first lens plate 35.
 The polarizing beam-splitter array 140 includes a plurality of beam
 splitters, which is placed in the rear of the second lens plate 38 for
 splitting and polarizing the light beams into s-polarized light beams and
 p-polarized light beams.
 The plurality of half wave plates 145 corresponding to the polarizing
 beam-splitter array 140 are placed on the paths of the s-polarized light
 beams or the paths of the p-polarized light beams to output alight beam
 having the same polarization. And the condenser lens 50 projects the light
 beam onto the liquid crystal panel 5.
 In the illuminating system described above, the non-polarized light beam is
 converted into p-polarized light or s-polarized light by using a plurality
 of polarizing beam-splitters 140. Each polarizing beam-splitter can
 optionally pass the p-polarized light or the s-polarized light. The half
 wave plates 145 are alternately disposed at the output of the polarizing
 beam-splitter. Refer to FIG. 3, for example, while the non-polarized light
 beam P+S is incident into the polarizing beam splitter 141 through the
 lens 139, the p-polarized light beam P1 is transmitted through the
 polarizing beam splitter 141 and the s-polarized light beam S1 is
 reflected. The p-polarized light beam P1 is then passed through the half
 wave plate 145 and converted into an s-polarized light beam S2. Thus the
 light beam output from the polarizing beam splitter is s-polarized light
 beam S1+S2. In other words, the light energy of the light source device
 being inputted into the polarizing beam splitter is totally converted into
 a light beam having the same polarization. The performance of the LC
 projector can be markedly raised. However, the fabrication of the
 illuminating system is too complex. A plurality of tiny polarizing beam
 splitters have to be cemented together. It is very difficult to exactly
 align the surfaces coated with a semi-reflecting coating for each
 polarizing beam splitter to be parallel with each other. Furthermore, the
 position of the halfwave plate has to exactly correspond to the polarizing
 beam splitter. That is, only one of the light beams split by the
 polarizing beam splitter passes through the half wave plate, while the
 other one does not. Moreover, the alignment of the polarizing beam
 splitter in the LC projector must be precise. This causes some
 inconvenience to make use of such an illuminating system.
 SUMMARY OF THE INVENTION
 Accordingly, in order to overcome the problems of the prior art, an object
 of the present invention is to provide a polarizing light source device
 for a liquid crystal projector that simplifies the fabrication and set-up
 of the liquid crystal projector and significantly improves its luminous
 efficiency.
 To achieve the above object, this invention utilizes a first lens plate and
 a second lens plate to redistribute the light energy generated by the
 light source device. Furthermore, a half wave plate is utilized to make
 uniform the polarization of the light beams split by a beam splitter.
 However, in order to simplify the fabrication of the LC projector, the
 polarizing beam splitter mentioned above is not used in the present
 invention. This invention employs common polarizing beam splitting devices
 such as a Wallaston prism or a wedge having a polarizing semi-reflector
 formed thereon.
 Unlike the polarizing beam splitter in the prior art, the polarizing
 beam-splitting device used in this invention can be placed before or after
 the second lens plate. That is, the configuration of the LC projector is
 variable.

Other objects and further features of the present invention will become
 more apparent from the following detailed description when read in
 conjunction with the accompanying drawings.
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring to FIG. 4, according to the first embodiment of this invention,
 the liquid crystal projector comprises: A light source 10 for providing a
 linearly polarized light beam; a condenser 20, placed after the light
 source 10, for converging the polarized light beam; and a liquid crystal
 panel 30 being illuminated by the polarized light beam, so as to project
 images displayed on the liquid crystal panel 30 on a screen.
 The optical layout of the illumination system, i.e., the light source 10,
 is also shown in FIG. 4. The illumination system includes a lamp 40, a
 reflector 42, a Wallaston prism 44, a first lens plate 46, a polarization
 direction rotator 48 and a second lens plate 50. The lamp 40 emits light
 in the direction of the display panel as well as in the rearward
 direction. The reflector 42 receives the rearwardly emitted light and then
 forms a parallel beam. The parallel beam is incident on the Wallaston
 prism 44. The Wallaston prism 44 can convert the unpolarized parallel beam
 into two linearly polarized light beams: A p-polarized light beam and an
 s-polarized light beam.
 For the sake of simplicity, the first lens plate 46 only includes four
 lenses in FIG. 4. In practice, however, the array 46 might comprise, for
 example, 16.times.9 lenses. Each of the lenses of the first lens plate 46
 images the lamp 40 on a corresponding lens of a second lens plate 50. In
 this embodiment, since the p-polarized light beam and the s-polarized
 light beam are incident to the first lens plate 46 at different angles,
 the p-polarized light spot and the s-polarized light spot formed by the
 first lens plate 46 are spaced apart at a certain distance. As a result, a
 plurality of p-polarized light spots and a plurality of s-polarized light
 spots are alternately formed on the polarization direction rotator 48,
 since that the first lens plate 46 consists of a plurality of lenses.
 The configuration of the polarization direction rotator 48 has to be
 designed corresponding to the first lens plate 46. For example, the
 polarization direction rotator 48 includes a plurality of half wave
 plates. The number of half wave plates equals the number of lenses on the
 first lens plate 46. Corresponding to each lens of the first lens plate
 46, each half wave plate is located at a certain position on the
 polarization direction rotator 48 to receive only p-polarized light spots
 or only s-polarized light spots. Referring to FIG. 5, the p-polarized
 light beam p and the s-polarized light beam s are respectively focused on
 the upper portion and the lower portion of the corresponding lens 51 of
 the second lens plate 50 by the lens 47 of the first lens plate 46.
 Therefore, the p-polarized light spot is directly incident to the lens 51.
 On the other hand, the s-polarized light spot formed by the lens 47 passes
 through the half wave plate 49 and then is converted into a p-polarized
 light spot. That is, all of the light spots become p-polarized after
 passing through the polarization direction rotator 48. The polarization
 direction rotator 48 can be disposed in front of or at the back of the
 second lens plate 50 in a manner such that the polarization direction
 rotator 48 can control whether p-polarized light or s-polarized light
 passes through the half wave plates.
 Each lens of the second lens plate 50 ensures that a radiation spot formed
 on the corresponding lens is imaged on the display panel 30. The condenser
 20 ensures that all re-images are superimposed on one another in the plane
 of the display panel 30 and is arranged behind the second lens plate 50.
 This results in a desired uniformity for the illumination intensity
 distribution in this plane.
 The liquid crystal display panels which are used when displaying
 conventional video images have an aspect ratio of 4:3. In this embodiment,
 the lenses of the plates have the same aspect ratio. As a result, all
 radiation coming through the first lens plate passes through the display
 panel and the illumination system has a high collection efficiency.
 Furthermore, the first lens plate consists of a plurality of rectangular
 lenses having a geometrical cross section similar to the display panel.
 Referring to FIG. 6, according to the second embodiment of this invention,
 the liquid crystal projector comprises: A light source 60 for providing a
 linearly polarized light beam; a condenser 70, placed after the light
 source 60, for converging the polarized light beam; and a liquid crystal
 panel 80 being illuminated by the polarized light beam, so as to project
 images displayed on the liquid crystal panel 80 on a screen.
 In this embodiment, the optical layout of the illumination system, i.e.,
 the light source 60, is also shown in FIG. 6. The illumination system
 includes a lamp 90, a reflector 92, a wedge 94, a first lens plate 96, a
 polarization direction rotator 98 and a second lens plate 100.
 The main difference of this embodiment and the previous embodiment is that
 the beam-splitting polarizer used in the previous embodiment is the
 Wallaston prism and in this embodiment is the wedge.
 The lamp 90 emits light in the direction of the display panel as well as in
 the rearward direction. The reflector 92 receives the rearwardly emitted
 light and then forms a parallel beam. The parallel beam is incident to the
 wedge 94.
 Referring to FIG. 7, the wedge 94 has a polarized beam splitting coating on
 the first surface 94a and a mirror on the second surface 94b. While the
 unpolarized light beam passes the first surface 94a of the wedge 94, for
 example, the s-polarized light beam S is reflected by the beam-splitting
 polarizer and the p-polarized light beam P transmits across the
 beam-splitting polarizer. Then, the p-polarized light beam is reflected by
 the mirror as it reaches the second surface 94b. Both the p-polarized
 light beam and the s-polarized light beam are incident to the first lens
 plate 96. The degree of polarization for the polarized beam-splitting
 coating is operated at an angle of (.alpha..sub.1 -.alpha..sub.2), where
 .alpha..sub.1 is the angle between the first surface 94a and the optical
 axis of the LCD panel 80, and .alpha..sub.2 is the angle between the
 second surface 94b and the optical axis of the LCD panel 80. Furthermore,
 the angle between the p-polarized light beam and the s-polarized light
 beam equals (.alpha..sub.1 -.alpha..sub.2)/2.
 With reference to FIG. 8, an alternate embodiment of the arrangement
 illustrated in FIG. 5 and discussed in relation thereto is shown. Like the
 arrangement of FIG. 5, the first lens plate 46 (provided with the lens 47)
 and the second lens plate 50 (provided with the lens 51) are provided in a
 spaced-apart arrangement. However, unlike the arrangement of FIG. 5, the
 half wave plate 49 is positioned in front of the first lens plate 46.
 In the above two embodiments, the reflector is used to produce a parallel
 light beam. Therefore, the reflector can be a paraboloid mirror or an
 ellipsoid mirror.
 The beam-splitting polarizer used is a Wallaston prism in the first
 embodiment and is a wedge in the second embodiment. However, according to
 the scope of this invention, the beam-splitting polarizer is not limited
 to the Wallaston prism or the wedge, but can be any device that can
 convert an unpolarized light beam into a p-polarized light beam and an
 s-polarized light beam, in which the p-polarized light beam and the
 s-polarized light beam are not parallel to each other. The angle between
 the p-polarized light beam and the s-polarized light beam is determined
 according to the following equation:
EQU tan.theta.=D/2f,
 where D is the diameter of the aperture of a lens at the first lens plate,
 and f is the focal length of a lens at the second lens plate.
 The first lens plate and the second lens plate can be designed in the
 following process. First, the vertex point of each lens on the first lens
 plate can be determined in accordance with the converging point of the
 light source and the center of each lens on the second lens plate. Second,
 the vertex point of each lens on the second lens plate can be determined
 in accordance with the center of the LCD panel and the center of each lens
 on the first lens plate. Then, the focal length of each lens on the first
 lens plate can be determined to focus the converging point of the parallel
 light beam on the second lens plate. Thereafter, the focal length of each
 lens at the second lens plate is determined to image a corresponding lens
 of the first lens plate on the LCD panel.
 Regarding the beam-splitting polarizer, the angle A between the p-polarized
 light beam and the s-polarized light beam can be worked out with the
 diameter D of each lens at the first lens plate and the distance between
 the first lens plate and the second lens plate.
EQU tan A=D/2f,
 The prism angle B of the beam-splitting polarizer is then determined,
 wherein
EQU tan A=2(n.sub.e -n.sub.o)tan B
 in case that the Wallaston prism is used as the beam-splitting polarizer,
 and
 B=A/2
 in case that the wedge is used. Where n.sub.e is the refractive index of
 the extraordinary ray and n.sub.o is the refractive index of the ordinary
 ray.
 According to the above description, this invention possesses the advantages
 of ease in fabrication and set up of the illuminating system with a low
 cost since, unlike the prior art, no particular component is needed and
 the components used in this invention do not require a highly precise
 alignment.
 While the present invention has been particularly shown and described with
 reference to preferred embodiments, it will be readily appreciated by
 those of ordinary skill in the art that various changes and modifications
 may be made without departing from the spirit and scope of the invention.
 It is intended that the claims be interpreted to cover the disclosed
 embodiment, those alternatives which have been discussed above and all
 equivalents thereto.