LIQUID CRYSTAL PROJECTOR

Abstract: In a liquid crystal projector wherein a light source is arranged behind a transmission liquid crystal display panel having a rectangular display portion, an image displayed on the liquid crystal display panel is enlarged by a projection lens, and an enlarged image is projected on the screen located in front of the liquid crystal display panel, a linear Fresnel lens (9) is arranged between the light source (2) and the liquid crystal display panel (l) to narrow illumination light from the light source (2) along only the widthwise direction of the rectangular display portion (la). When the illumination light from the light source (2) is narrowed along only the widthwise direction of panel (l), the amount of unused light emitted outside the long sides of the display portion (la) can be reduced, and the light from the light source (2) can be effectively utilized for image projection.

Description: The present invention relates to a liquid crystal projector according to the preamble of claim 1. In a conventional liquid crystal projector, a liquid crystal display panel displays an image, and the image on the panel is projected and enlarged on a screen surface. WO 85/05192 discloses a liquid crystal projector comprising optical means including a projection lens for projecting light to form an image that is a function of selective operation of said liquid crystal display responsive to a selecting device. The projected light may be coloured by using selectively operable liquid crystal filter. The conventional liquid crystal projector has the following arrangement. Fig. 1 shows a typical conventional liquid crystal projector. Reference numeral 1 in Fig. 1 denotes a transmission type dot-matrix liquid crystal display panel; and 2, a light source arranged behind panel 1. Light source 2 comprises lamp 3 and parabolic reflector 4 for reflecting light from lamp 3 in a direction parallel to optical axis O. Light source 2 thus emits illumination light A parallel to axis O toward panel 1. Reflector 4 in light source 2 in the liquid crystal projector comprises a parabolic reflector in Fig. 1. However, in a conventional liquid crystal projector using a conventional elliptical reflector as the light source reflector, a relay lens is arranged in the light source, and the light from the lamp is corrected by the relay lens to obtain collimated light. The collimated light is then incident on the liquid crystal display panel. Reference numeral 5 denotes a projection lens arranged in front of panel 1. The light passing through panel 1, i.e., an image displayed on panel 1 is enlarged by lens 5, and an enlarged image is projected onto screen S. Panel 1 is of a TN (Twisted Nematic) type. Panel 1 is inclined at a predetermined angle with respect to optical axis O such that illumination light A from the rear side of the panel has the highest efficiency. Reference numeral 7 denotes a frame-like light-shielding plate for supporting panel 1. Some of the beams of illumination light A from light source 2, which are incident on display portion (i.e., a display area) 1a of panel 1 pass through panel 1. However, the beams incident outside display portion 1a are shielded by panel 1 and light-shielding plate 7. Therefore, only the beams passing through display portion 1a of panel 1 are projected onto screen S. The conventional liquid crystal projector enlarges and projects the image displayed on liquid crystal display panel 1 onto screen S. According to the liquid crystal projector, slide films need not be inserted or extracted, unlike in the conventional projector using slide films. In addition, the conventional liquid crystal projector can project an animation such as a television image onto the screen surface. In the conventional liquid crystal projector described above, in order to perfectly project an image onto screen S, illumination light A must be incident on the entire surface of display portion 1a of liquid crystal display panel 1. The normal shape of display portion 1a of panel 1 is rectangular, while the shape of the cross section of light A from light source 2 is circular. If the beam spot of light A is small, the illumination beam is not incident on the corners of display portion 1a of panel 1, so that an incomplete circular image is projected onto the surface of screen S. In the conventional liquid crystal projector described above, the diameter of reflector 4 of light source 2 is selected to be large enough to cover the circumcircle of rectangular display portion 1a of panel 1, so that the entire display portion 1a of panel 1 can be illuminated. With the above arrangement, a perfect image without corner omissions can be projected onto screen S. In the conventional liquid crystal projector, however, since circular illumination light A is directly incident from light source 2 onto liquid crystal display panel 1, the amount of unused light (i.e, light shielded by the portion outside display portion 1a and by light-shielding plate 7) illuminating the portion outside portion 1a of panel 1 is increased. Therefore, light A from light source 2 cannot be effectively utilized. As described above, since the cross section of light A emitted from light source 2 and reflected by reflector 4 is circular while the shape of display portion 1a of panel 1 is rectangular, the amount of unused light emitting the portion outside display portion 1a is increased at portions above and below display portion 1a if panel 1 is located such that the longitudinal direction of display portion 1a extends horizontally, as shown in Fig. 2. Most of the beams of illumination light A from light source 2 are wasted. The object underlying the subject matter of claims 1 and 10 is to provide a liquid crystal projector wherein the amount of unused light emitted on portions excluding a display portion of the liquid crystal display panel is reduced to effectively utilize the illumination light from the light source for image projection, and at the same time the intensity of the light incident on the display portion of the liquid crystal display panel is increased to project a bright image onto the screen. It is a further object of the invention to provide a liquid crystal projector wherein the light beams incident on said display portion of the liquid crystal panel are parallel to the optical axis. The above object is solved by the characterizing features of the subject matter of claim 1 and 10. Further embodiments are subject to the sub-claims. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. l is a longitudinal sectional view of an optical system of a conventional liquid crystal projector; Fig. 2 is a sectional view of the optical system shown in Fig. l taken along the line II - II thereof; Fig. 3 is a longitudinal sectional view of an optical system of a liquid crystal projector according to a first embodiment of the present invention; Fig. 4 is a sectional view of the optical system shown in Fig. 3 taken along the line IV - IV thereof; Fig. 5 is a front view of linear Fresnel lens 9 shown in Fig. 3; Fig. 6 is a side view of linear Fresnel lens 9 shown in Fig. 3; Fig. 7 is a front view of a primary color filter arranged on liquid crystal display panel l; Fig. 8 is a view of an optical path of light transmitted through linear Fresnel lens 9 when viewed in the lateral direction; Fig. 9 is a view of the optical path of light transmitted through linear Fresnel lens 9 when viewed from the top; Fig. l0 is a longitudinal sectional view of an optical system of a liquid crystal projector according to a second embodiment of the present invention; Fig. ll is a sectional view of the optical system shown in Fig. l0 taken along the line XI - XI thereof; Fig. l2 is a longitudinal sectional view of an optical system of a liquid crystal projector according to a third embodiment of the present invention; Fig. l3 is a front view of second linear Fresnel lens l0 shown in Fig. l2; Fig. l4 is a side view of second linear Fresnel lens l0 shown in Fig. l2; Fig. l5 is a view of an optical path of light passing through linear Fresnel lenses 9 and l0 in the lateral direction; Fig. l6 is a plan view of the optical system showing paths of light; Fig. l7 is a view of an optical path of light passing through a color filter formed on the front surface of liquid crystal display panel l; Fig. l8 is a longitudinal sectional view of an optical system of a liquid crystal projector according to a fourth embodiment of the present invention; Fig. l9 is a longitudinal sectional view of an optical path of light passing through beam compression lens l2 and correction lens l3 when viewed in the lateral direction, according to a fifth embodiment of the present invention; and Fig. 20 is a graph showing luminance distribution of beams emitted from the light source and incident on beam compression lens l2 and beams passing through lenses l2 and l3. An embodiment of the present invention will be described with reference to the accompanying drawings. Referring to Fig. 3, reference numeral l denotes a transmission type dot-matrix liquid crystal display panel whose display portion la has a rectangular shape. Panel l is supported by frame-like light-shielding plate 7 such that the longitudinal direction of display portion la is inclined with respect to optical axis O, as shown in Fig. 4. Panel l has primary color filter 8 on the front surface (the projection lens side) thereof, thus constituting a color liquid crystal display panel. Filter 8 comprises red (R), green (G), and blue (B) stripe filters RF, GF, and BF which are alternately arranged. Filters RF, GF and BF of color filter 8 respectively correspond to electrodes along the widthwise direction of display portion la, among the scanning and signal electrodes (not shown) of panel l. Reference numeral 2 denotes a light source, arranged behind liquid crystal display panel l, for illuminating panel l from the rear surface thereof. Light source 2 comprises lamp 3 such as a Xenon lamp and parabolic reflector 4 for reflecting light from lamp 3 in a direction parallel to optical axis O, in the same manner as in the conventional liquid crystal projector shown in Fig. l. Light source 2 emits illumination light A0 in a direction parallel to axis O toward panel l. Reference numeral 5 denotes a projection lens arranged in front of panel l. An image displayed on panel l is enlarged and projected by the projection lens onto screen S. Referring to Fig. 3, reference numeral 9 denotes a linear Fresnel lens arranged between light source 2 and liquid crystal display panel l. Lens 9 comprises parallel stripe lens portions 9a formed symmetrically about center L of the lens, as shown in Figs. 5 and 6. In this embodiment, a lens having a convex lens function is used as lens 9. Lens 9 is arranged between light source 2 and liquid crystal display panel l such that the lens surface is directed toward light source 2, that the longitudinal direction of stripe lens portions 9a is horizontal, and that center L of the lens is aligned with optical axis O of the projector. Linear Fresnel lens 9 causes lens portions 9a to focus beams of illumination light A0 emitted from light source 2 parallel to optical axis O in the widthwise direction of lens portions 9a, i.e., in the vertical direction. Only the vertical width of circular illumination light A0 from light source 2 is compressed by lens 9, as shown in Fig. 8. However, the horizontal width of light A0 is not compressed and propagates straight, as shown in Fig. 9. Therefore, illumination light A0 from light source 2 is converted into elliptical illumination light Al (Fig. 4) obtained by compressing the vertical width corresponding to the width of rectangular display portion la of liquid crystal display panel l. Light Al is then incident on panel l. Light Al is obtained by compressing light A0 in only the vertical direction. If a beam spot of illumination light A0 from light source 2 is sufficiently larger than the length (i.e., a longitudinal width) of display portion la of panel l, display portion la can be entirely illuminated. If a distance between linear Fresnel lens 9 and panel l is selected according to the focal length of lens 9 to the extent that the minor axis of elliptical light Al focused by lens 9 is not smaller than the width of rectangular display portion la, display portion la can be entirely illuminated with light Al. In the liquid crystal projector of this embodiment, since linear Fresnel lens 9 is arranged between light source 2 and liquid crystal display panel l to compress the beams of light A0 from light source in only the widthwise direction of rectangular display portion la of panel l, the beam width corresponding to the length of display portion la of panel l is not compressed (i.e,. the width is the one which allows illumination of the entire area of display portion la), and only the beam width corresponding to the width of display portion la is compressed to the extent that the minor axis of the beam spot is not smaller than the width of display portion la. The resultant elliptical light is then incident on panel l. According to the liquid crystal projector of this embodiment, as compared with the conventional liquid crystal projector wherein illumination light from light source 2 is directly incident on panel l, the amount of beams for illuminating both short sides of display portion la can be reduced. The amount of unused light illuminating portions, excluding display portion la of panel l, can be reduced, and therefore illumination light from light source 2 can be effectively projected. In addition, in the liquid crystal projector of this embodiment, the beams of illumination light A0 from light source 2 are focused by linear Fresnel lens 9, and the focused light is incident on panel l. Most of the unused beams incident on portions excluding the display portion can be incident on display portion la, thus increasing the intensity of light incident thereon. Therefore, brightness of panel l can be increased. A bright image can be projected onto screen S. In the above embodiment, stripe filters RF, GF, and BF of primary color filter 8 for displaying a full-color image on panel l are formed so as to correspond to electrodes along the compression direction of the illumination light, i.e., the widthwise direction of display portion la, among scanning and signal electrodes of panel l. Light focused by lens 9 and obliquely incident on color filter 8 through panel l is incident on filters RF, GF, and BF at an inclination along the longitudinal direction thereof. The beams do not become incident on the adjacent two of filters RF, GF, and BF, thus eliminating color interference. Therefore, a clear color image free from color misregistration and irregularity can be projected onto screen S. Figs. l0 and ll show a second embodiment of the present invention. In this embodiment, linear Fresnel lens 9 comprises an eccentric linear Fresnel lens which has the center located below optical axis O of the projector by predetermined distance d . Liquid crystal display panel l is inclined such that its center is aligned with optical axis O of the light source. Projection lens 5 and screen S are inclined to be parallel to liquid crystal display panel l such that the centers of lens 9 and screen 5 are aligned with projection side optical axis O′ passing through the center of lens 9. The liquid crystal projector according to this embodiment aims at illuminating with uniform luminance the entire area of display portion la of obliquely arranged panel l. More specifically, when panel l is obliquely arranged with respect to the optical axis, the length of an optical path from light source 2 to the upper edge of display portion la of panel l is different from the length of an optical path from light source 2 to the lower edge of display portion la. Brightness at the upper edge of the display portion la near light source 2 increases, while brightness at the lower edge of the display portion la away from light source 2 decreases. However, according to this embodiment, when linear Fresnel lens 9 comprises an eccentric linear Fresnel lens having the center located below the light source side optical axis by predetermined distance d , a difference between the length of the optical path from light source 2 to the upper edge of display portion la and the length of the optical path from light source 2 to the lower edge of display portion la can be decreased. Therefore, light with uniform luminance can be emitted onto the entire surface of display portion la of obliquely inclined panel l, and an image with uniform luminance can be projected onto screen S. In the above embodiment, the longitudinal direction of display portion la of liquid crystal display panel l is aligned with the horizontal direction. However, the longitudinal direction of display portion la of panel l may be aligned with the vertical direction, and an image having a rectangular shape having long sides aligned with the vertical direction may be projected onto the screen surface. In this case, parallel lens portions of linear Fresnel lens are aligned in the vertical direction, and the beams are focused in only the widthwise direction of display portion la of panel l. The above embodiment is a color liquid crystal projector for projecting a color image on a color liquid crystal display panel. The present invention can apply to a liquid crystal projector for projecting a black-and-white image onto a liquid crystal display panel. A third embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 12 is an arrangement of this embodiment in which second Fresnel lens 10 is added to the optical system shown in Fig. 3. Referring to Fig. 12, reference numerals 9 and 10 denote first and second linear Fresnel lenses arranged between light source 2 and liquid crystal display panel 1. Lens 9 has a convex lens function, as shown in Figs. 5 and 6. Lens 10 has a concave lens function, as shown in Figs. 13 and 14. Lenses 9 and 10 have parallel lens portions 9a (Figs. 5 and 6) and 10a symmetrical about centers L1 and L2 of lenses 9 and 10. Lens 9 is located on the side of light source 2 and its lens surface is directed toward light source 2. Lens 10 is located on the side of panel 1 and its lens surface is directed toward panel 1. The distance between lenses 9 and 10 is shorter than the focal length of lens 9. Centers L1 and L2 of lenses 9 and 10 are aligned with optical axis O such that the longitudinal directions of lens portions 9a (Figs. 5 and 6) and 10a are aligned with the horizontal direction. First linear Fresnel lens 9 aims at focusing beams of illumination light A0 parallel to optical axis O and emitted from light source 2 along the widthwise, i.e., vertical direction of lens portions 9a. Illumination light A0 having a circular cross section passes through lens 9 and the vertical width is compressed by the focusing behavior of lens 9, as shown in Fig. 15. In this case, the horizontal width is not compressed, and light A0 propagates straight, as shown in Fig. 16. Lens 10 converts the beams having the vertical width compressed by lens 10 into the beams parallel to optical axis O. The light focused by lens 9 and obliquely incident on lens 10 is diverged by the diverging behavior of lens 10, as shown in Fig. 15, thereby obtaining light parallel to axis O. Light propagating straight without being influenced by the focusing behavior of lens 9 passes through lens l0 and propagates straight, as shown in Fig. l6. Therefore, only the vertical width of illumination light A0 (i.e., only the widthwise direction of display portion la of panel l) having a circular cross section and emitted from light source 2 is compressed to obtain parallel light Al having an elliptical cross section, as shown in Fig. 4. If the beam range of light A0 from light source 2 is sufficiently larger than the length of display portion la of panel l, the entire area of display portion la along the direction of length can be illuminated. When the distance between lenses 9 and l0 is selected according to the focal length of lens 9 such that the vertical width of the beams is corrected by lens l0 into light parallel to axis O to the extent that the vertical width is not smaller than the width of display portion la, the entire area of display portion la along the vertical direction can be illuminated with light. Since first linear Fresnel lens 9 for focusing the beams of illumination light A0 from light source 2 along the widthwise direction of display portion la of panel and second linear Fresnel lens 9 for correcting the beams compressed by lens 9 along the vertical direction to beams parallel to axis O are arranged in the liquid crystal projector described above, the width (i.e., the width enough to illuminate the entire area of display portion la) of illumination light A0 from light source 2, i.e., the longitudinal direction of display portion la of panel l, is not compressed. Only the width of light along the widthwise direction of display portion la is compressed, and panel l is illuminated with the resultant light. According to the liquid crystal projector of this embodiment, as compared with the conventional liquid crystal projector wherein illumination light from light source 2 is directly incident on panel l, the amount of unused beams illuminating the both long sides of display portion la can be reduced. The amount of unused beams illuminating portions excluding display portion la of panel l can be reduced. Therefore, illumination light from light source 2 can be effectively utilized for image projection. In the liquid crystal projector of this embodiment, illumination light A0 from light source 2 is focused, and panel l is illuminated with the focused light. A large amount of unused beams emitted on the portions excluding the display portion can be incident on display portion la of panel l. The intensity of light incident on display portion la can be increased. Therefore, brightness of panel l can be increased, and a bright image can be projected onto screen S. In this liquid crystal projector, the width of the beam is narrowed or compressed by first linear Fresnel lens 9, and the resultant light is converted by second linear Fresnel lens l0 into light parallel to the optical axis O. The light from lens l0 is incident on liquid crystal display panel l. The light passing through display portion la of panel l is not subjected to a color mixture when it passes through color filter 8 formed on the front surface of panel l. More specifically, when light having a width compressed by lens 9 is directly incident on panel l, the beams are obliquely incident on display portion la in the vertical direction of panel l. In this case, the beams passing through display portion la obliquely pass through color filter 8, as indicated by the broken line in Fig. l7. A beam passing through the boundary between two adjacent filters of stripe filters RF, GF, and BF is colored with the corresponding two colors, thus resulting in a color mixture. However, as described above, the light from lens 9 is converted by lens l0 into light parallel to axis O, and the resultant light is incident on panel l. The light passing through display portion la of panel l passes through color filter 8, as indicated by the solid line in Fig. 9, thus preventing a double color mixture. As a result, a clear color image free from color misregistration and irregularity can be projected onto screen S. Fig. l8 shows a fourth embodiment of the present invention. In this embodiment, a second linear Fresnel lens for converting light having a width compressed by first linear Fresnel lens 9 into light parallel to optical axis O comprises linear Fresnel lens ll having the same function (i.e., the convex lens function) as that of first linear Fresnel lens 9. Lens ll is spaced apart from lens 9 by a distance larger than the focal length of lens 9 while the lens surface of lens ll is directed toward lens 9. The same effect as in the above embodiment can be obtained in this embodiment. In the third and fourth embodiments, the longitudinal direction of display portion la of panel l is aligned with the horizontal direction. However, panel l may be located such that the longitudinal direction of display portion la is aligned with the vertical direction. In this case, an image is displayed on display portion la having long sides aligned with the vertical direction. The lens portions of first linear Fresnel lens 9 and second linear Fresnel lens l0 or ll are aligned with the vertical direction to narrow the beam along only the widthwise direction of display portion la of panel l. The above embodiment exemplifies a color liquid crystal projector for projecting a color image on a screen using a color liquid crystal display panel. However, the present invention is also applicable to a liquid crystal projector for projecting a black-and-white image on a liquid crystal display panel. Figs. l9 and 20 show a fifth embodiment of the present invention. Reference numeral l2 denotes a beam compression lens for focusing outer beams of light emitted from light source 2 into an interior area of display area of liquid crystal display panel l. Reference numeral l3 denotes a correction lens for correcting the light focused by lens l2 into light parallel to optical axis O. As shown in Fig. l9, lenses l2 and l3 comprise circular Fresnel lenses of transparent plates such as acrylic resin plates having concentric lens portions l2a and l3a, respectively. Lens l2 has a surface area slightly larger than the cross-sectional area of light emitted from light source 2. The central portion of lens l2 corresponding to the central portion of light from light source 2, i.e., the portion having the highest intensity, comprises circular flat portion l2b having no lens function. Concentric convex lens portions l2a are contiguous with flat portion l2b. Beams with a high intensity emitted from light source 2 and incident on flat portion l2b propagate straight. Beams having a low intensity incident on lens portions l2a are focused toward the center of light emitted from light source 2, so that the intensity of the outer beams of light from lens l2 is also high. Lens l3 is located at a position immediately before the focused outer beams are mixed with the central straight beams (the beams passing through flat portion l2b of lens l2). Lens l3 comprises central circular flat portion l3b through which the beams passing through lens l2 propagate straight, and concentric concave lens portions l3a for receiving outer beams focused by lens l2. The central beams passing through lens l2, i.e., the beams incident on flat portion l3b, propagate straight. The outer beams focused by lens l2 are corrected by lens portions l3a into beams parallel to optical axis O. The parallel light is incident on panel l. Lens l3 has a surface area slightly larger than the display area of panel l. The focusing power of lens l2 is determined such that the width of light corrected as parallel light by lens l3 is not smaller than the width of display area of panel l3. Fig. 20 shows luminance distribution Ll of the cross section of light L emitted from light source 2 and incident on lens l2 and luminance distribution L2 of cross section of light corrected by lens l3 into parallel light. Reference symbol La in Fig. 20 denotes a straight propagation area of the center beams; Lb, the focusing area of outer beams; and D, the width of the display area of panel l. As is apparent from Fig. 20, distribution Ll of light reflected by reflector 4 in light source 2 has a high intensity at the central portion but an extremely low intensity at the peripheral portion. However, distribution L2 of light passing through lens l3 has a relatively high intensity in the peripheral portion as compared with distribution Ll, thus obtaining a substantially uniform intensity. When lenses l2 and l3 are arranged between light source 2 and panel l, light with a high intensity is incident on the peripheral portion of panel l, and thus a bright image can be displayed at the peripheral portion of panel l.

Claims: A liquid crystal projector comprising: a liquid crystal display panel (1) having a rectangular display portion (1a), the short side of the rectangle defining a widthwise direction, a light source (2) for illuminating said liquid crystal display panel and a projection lens (5) for projecting an image displayed on said liquid crystal display panel, said projector including optical means (9, 10, 11) arranged between said light source and said liquid crystal display panel characterized in that, said optical means narrows the light beam emitted from said light source (2) onto said panel in only said widthwise direction of said display portion (1a). A projector according to claim 1, characterized in that said optical means (9) comprises a linear Fresnel lens. A projector according to claim 1 or 2, characterized in that said optical means comprises an eccentric linear Fresnel lens (9), the center of which is deviated from the optical axis of said light source (2) by a predetermined distance (d). A projector according to any of claims 1 to 3, characterized in that said liquid crystal display panel (1) has stripe-shaped primary-colour filters (RF, GF, BF), arranged side by side, and extending in the direction in which the light beam is narrowed by said optical means (9). A projector according to any of claims 1 to 4, characterized in that the optical means (9 in Figures 12, 18) narrowing the light beam is followed by an optical means (10, 11) rendering the light beam emitted from the narrowing means parallel to the optical axis of said light source (2). A projector according to claim 5, characterized in that said optical means (9, 10, 11) comprises a first linear Fresnel lens (9) narrowing the light beam emitted from said light source (2) onto said panel in only the widthwise direction of said rectangular display portion (1a) and a second linear Fresnel lens (10, 11) rendering said light beam emitted from said first Fresnel lens (9) parallel to the optical axis of said light source (2). A projector according to claim 6, characterized in that said first linear Fresnel lens (9) has a convex lens function and said second linear Fresnel lens (10) has a concave lens function. A projector according to claim 6 or 7, characterized in that the distance between said first and second linear Fresnel lens (9, 10) is sufficiently shorter than the focal length of said first linear Fresnel lens (9). A projector according to claim 6, characterized in that said first and second linear Fresnel lenses (9, 11) respectively have convex lens functions, said second linear Fresnel lens (11) being spaced apart from said first linear Fresnel lens (9) by a distance being the sum of the focal lengths of the first and second Frensel lenses. A liquid crystal projector comprising: a light source (2) for emitting a light beam; and a liquid crystal display panel (1) having a display portion (1a) smaller than the light beam emitted from said light source (2) onto said liquid crystal display panel (1), wherein an image displayed on said liquid crystal display panel (1) is enlarged by a projection lens (5) said projector including optical means (12, 13) arranged between said light source (2) and said liquid crystal display panel (1), characterized in that said optical means comprises a first and a second circular Fresnel lens, the first Fresnel lens (12) converging the light beam emitted from said light source (2) and the second Fresnel lens (13), being arranged at a distance from the first Fresnel lens (12) which distance is sufficiently shorter than the focal length of said first Fresnel lens (12), diverging the light beam emitted from the first Fresnel lens so as to produce a parallel light beam. A projector according to claim 10, characterized in that said first and second circular Fresnel lenses (12, 13) comprise a central flat portion (12b, 13b) having no lens function, so that the portion of the light beam emitted from said light source (2) onto said central flat portions propagates straight through. A projector according to claim 11, characterized in that said second Fresnel lens (13) has a concave lens function in the annular portion (13a) surrounding the central flat portion (13b). A projector according to claim 12, characterized in that said second Fresnel lens (13) is positioned at such a distance from said first Fresnel lens that the light beam emitted from said annular portion (13a) and said light beam emitted from said central flat portion (13b) are arranged side by side.

Inventors: OGAWA, MASAHIRO C/O PATENT DEP. DEVELOPMENT DIV., SHIMAZAKI, TATSUO C/O PATENT DEP. DEVELOPMENT DIV, UMEDA, OSAMU C/O PATENT DEP. DEVELOPMENT DIV.
Applicants: CASIO COMPUTER COMPANY LIMITED
Application year: 1987
Main classification: G03B21
IPC codes: G02F1, G03B21