Optical system for rear projection picture display apparatus

Abstract: An optical system for a rear projection picture display is made in a small size keeping a screen size as it is. Two plane mirrors (13,15) are provided on a light path between a picture display unit (10) and a screen (17). The first plane mirror (13) at a projection lens side is made of a reflecting polarizer and is located on a light path between the second plane mirror (15) and the screen. The two plane mirrors are set in a relative angle so that the incident angle of the light traveling from the projection lens to the first plane mirror is large enough as compared with the incident angle of the light traveling from the second plane mirror to the first plane mirror. A polarizing plate is attached at the rear side of the screen (17).

Description: BACKGROUND OF THE INVENTION (1)Field of the Invention The present invention relates to an optical system of a rear projection picture display apparatus for displaying a large picture by projecting the picture displayed on a picture display unit using a projection lens onto a screen. (2) Description of the Prior Art A picture display apparatus having a high resolution and a large screen has been required for commercial use such as High Vision and industrial use such as a high grade computer display. According to the above needs, projection type picture display apparatus are spreading the market. Projection type picture display apparatus are classified into two types. One is a two unit type which projects a picture on the screen separated from the projector unit. Another is a single unit type which a projector unit and a screen are built in one enclosure. The latter is called rear projection type. The former is considerably small in unit itself, has less restriction in projection distance and is free in projection size but if external light comes on the screen the projected picture accompanies black buoying. Therefore, it is necessary to use it in a dark room or under an environment near that. The latter can display a good picture even under a bright environment because no external light from the projector side comes to the screen but the apparatus has to provide a space corresponding to the projection distance, that is the enclosure of the apparatus is big. It is not desirable for home use. To make an apparatus size small, two kinds of solutions have been considered in a rear projection type. One is means to use a projection lens having a short focal distance and reduce a projection distance. Another means is to miniaturize a projection tube, a projection lens, etc.. But a lot of trials have been made so far and it is difficult to expect further progress in the future although a small improvement may be devised. A trial to make the apparatus size small is proposed in EP-A-0,333,333 (Japanese Patent Laid-Open H2-140792). An optical system for a rear projection picture display apparatus in accordance with the prior art is shown in FIG.1. The apparatus includes a picture display unit (not shown in the drawing), a beam splitter 2 for partially reflecting and partially transmitting the incident light, a mirror 3 for returning the reflected light from the beam splitter 2 toward the beam splitter 2 again, a louver 4 provided at the rear surface of the beam splitter 2 so that the light in the previously designated range of the incident angle against the perpendicular line to the beam splitter 2 can transmit the beam splitter 2, and a screen 5. By the above-mentioned construction, a space for the folded light path of the mirror is reduced. However, because the louver 4 splits transmitting light and not-transmitting light according to its incident angle. The incident angle differs at the center and at the periphery of the screen, brightness unevenness occurs unless a constant amount of light is made to transmit for any incident angle. Further, because the louver 4 has plural light paths, the transmission factor may decrease, a moiré due to a mutual relation with a displayed picture may appear and as a result, picture quality deterioration will be induced. SUMMARY OF THE INVENTION The present invention provides an optical system for a rear projection picture display apparatus as defined in claim 1. In this arrangement, the S-polarized light among the light coming to the first plane mirror from the projection lens is reflected at the first plane mirror. The S-polarized light reflected at the first plane mirror is reflected at the second plane mirror toward the screen keeping its oscillation direction. The first plane mirror locates between the second plane mirror and the screen, but because the first plane mirror is made of a reflecting polarizer and the angle relation between the first plane mirror and the second plane mirror is set so that the incident angle of the light traveling to the first plane mirror from the picture display unit is an angle which the first plane mirror can sufficiently display its performance as a polarizer, as well as the incident angle of the light traveling to the first plane mirror again after being reflected at the second plane mirror is an angle which the first plane mirror can not sufficiently display its performance as a polarizer, the light reflected at the second plane mirror transmits the first plane mirror and can arrive at the screen, although the first plane mirror locates to screen the light traveling from the second plane mirror to the screen. Therefore, the space in the enclosure which is necessary to fold the light path by the first plane mirror can be made smaller than the space of a usual rear projection picture display apparatus using surface-coated mirrors or rear surface-coated mirrors as the first and second plane mirrors and the entire apparatus size can be made small. Picture quality deterioration is protected by providing a polarizing plate between the screen and the first plane mirror to absorb the P-polarized light oscillating in the parallel direction to the incidence plane among the light traveling to the first plane mirror from the projection lens and eliminate harmful light. A picture display device in which the oscillating direction of the emitted light is restricted to a specified direction like a liquid crystal display panel can be used as a picture display unit and it does not bring brightness decrease. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 illustrates an optical system for a rear projection picture display apparatus in accordance with the prior art. FIG.2 illustrates an optical system for a rear projection picture display apparatus in accordance with a first exemplary embodiment of the present invention. FIG.3 shows an incident angle dependency characteristic of a reflecting polarizer used in a rear projection picture display apparatus in accordance with the first exemplary embodiment of the present invention. FIG.4 illustrates an optical system for a rear projection picture display apparatus in accordance with a second exemplary embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION (First exemplary embodiment) An optical system for a rear projection picture display apparatus in accordance with a first exemplary embodiment of the present invention is illustrated in FIG.2. A cathode ray tube 10 displays a picture on its faceplate coated with phosphor. A video processing circuit, deflection circuits, etc. are not shown in the drawing. A projection lens 11 is provided in front of the faceplate of the cathode ray tube 10 to project and magnify the picture on the cathide ray tube faceplate to a designated size. The light transmitting the projection lens 11 travels to a first plane mirror 13 which is fixed slanting to the light axis 12. The first plane mirror 13 is made of a reflective polarizer and the S-polarized light oscillating in the perpendicular direction to the incidence plane is reflected along a light axis 14. The P-polarized light oscillating in the parallel direction to the incidence plane transmits the first plane mirror 13. The light reflected along the light axis 14 is reflected along the light axis 16 after traveling to a second plane mirror which is fixed slanting to the light axis 14 and travels toward a screen 17 and transmits the screen 17 and focalizes a magnified image on the screen 17 by the projection lens 11. The cathode ray tube 10, the projection lens 11, the first plane mirror 13 and the second plane mirror 15 are fixed inside an enclosure 18 having the screen 17 at the opening of the enclosure 18. An incident angle dependency characteristic of a reflecting polarizer used as a first plane mirror 13 in an optical system for a rear projection picture display apparatus in accordance with the first exemplary embodiment of the present invention is shown in FIG. 3. The incident angle of the light axis 12 directly travelling to the first plane mirror 13 is defined as 1 and the incident angle of the light axis 12 travelling to the first plane mirror 13 after being reflected at the second plane mirror 15 is defined as 2. The incident angle 1 is chosen to be big enough as compared to the incident angle 2 and to have the biggest quenching ratio at the incident angle dependency characteristic of the first plane mirror (reflecting polarizer) 13 shown in FIG.3, that is 1 is chosen so that the value which the transmission factor of the P-polarized light having a parallel oscillation direction to the incidence plane is divided by the transmission factor of the S-polarized light having a perpendicular oscillation direction to the incidence plane is the biggest or its near value. The angle 2 is chosen to have a high transmission factor of the S-polarized light having a perpendicular oscillation direction to the incidence plane at the incident angle dependency characteristic of the first plane mirror 13 shown in FIG.3. Therefore, in FIG.2, the light reflected at the second plane mirror 15 travels toward the first plane mirror 13 and transmits the first plane mirror 13 and arrives at the screen 17. The light coming from the projection lens 11, transmitting the first plane mirror (reflecting polarizer) 13 and oscillating parallel to the incidence plane travels to the screen 17 locating at the rear side of the first plane mirror 13. Because the incident angle of the light directly travelling from the projection lens 11 at the screen 17 is different from that of the light being reflected at the second plane mirror 15 at the screen 17 and is set to be big enough, the light transmitting the screen 17 does not travel to the viewer. Therefore, it is not harmful light. Further, by providing a polarizing plate 19 at the rear side of the screen 17 (that is, between the screen 17 and the first plane mirror 13) to absorb the P-polarized light leaking through the first plane mirror 13 and transmit the S-polarized light travelling along the light axis 16, the harmful light for the viewer can be eliminated and a good picture can be displayed. Diffuser may be painted on the surface of the screen 17 or compounded in the screen 17. According to the exemplary embodiment of the present invention, a rear projection picture display apparatus can be made small without picture quality deterioration, because the light traveling from the projection lens reflects at the plane mirrors and travels taking a zigzag course. The upper part of first plane mirror 13 can be cut away, that is a smaller reflecting polarizer having just an area to reflect the incident light from the projection lens toward the second plane mirror can be used as a first plane mirror 13. Although among the light reflected at the second plane mirror and arriving at the screen 17, for the lower part of screen 17 the light travels through the first plane mirror again, but for the upper part of the screen 17 the light travels directly to the screen and it brings a little unevenness in the picture brightness, as a reflecting polarizer is expensive, cost reduction by using a smaller reflecting polarizer is also effective. (Second exemplary embodiment) An optical system for a rear projection picture display apparatus in accordance with a second exemplary embodiment of the present invention is illustrated in FIG.4. In the second exemplary embodiment, a liquid crystal display panel is used instead of a cathode ray tube as a picture display unit. The light emitted from a light source 21 and transmitting a first polarizing plate 22 provided at a light source side is restricted with its oscillation direction to one direction. The oscillation direction of the light transmitting each element of the liquid crystal panel 20 is varied corresponding to a video signal and after transmitting a second polarizing plate 23 provided at a projection lens side and thus, a picture is obtained. A video processing circuit, a driving circuit, etc. are not shown in FIG.4. A projection lens 11 provided in front of the liquid crystal panel 20 magnifies and projects the picture to a designated size on a screen 17. The light from the projection lens 11 is reflected by a third plane mirror 25 slanting to a light axis 24 from the light axis 24 to a light axis 26 and then travels to a first plane mirror 27 slanting to the light axis 26. The first plane mirror 27 is made of a reflecting polarizer. The S-polarized light oscillating in the perpendicular direction to the incidence plane is reflected by the first plane mirror 27 along a light axis 28. While, the P-polarized light oscillating in the parallel direction to the incidence plane transmits the first plane mirror 27. The S-polarized light reflected along the light axis 28 travels to a second plane mirror 29 slanting to the light axis 28, is reflected along the light axis 30 there, transmits a screen 17 and focuses on the surface of the screen 17. Thus, a projected picture can be offered to the viewers. The components are supported in an enclosure 18 having the screen 17 at the opening of the enclosure 18 and the enclosure protects external light to come in. A third polarizing plate 31 is provided behind the screen 17 to absorb the P-polarized light leaking through the second polarizing plate 23 and transmit the S-polarized light travelling along the light axis 30, similar to the first exemplary embodiment of the present invention. The incident angle of the light axis 26 against the first plane mirror 27 is defined as 1 and the incident angle of the light axis 30 against the first plane mirror 27 is defined as 2 and the relation between 1 and 2 is chosen similar to that in the first exemplary embodiment. The difference from the first exemplary embodiment is in a picture display unit. In the second exemplary embodiment, the picture display unit is a liquid crystal type and the oscillating direction of the light emitted from the picture display unit is perpendicular to the first plane mirror 27 (reflecting polarizer). That is, the light transmitting through the second polarizing plate 23 is to be an S-polarized light. Even if it is not, it can be substituted by using means which varies the oscillation direction such as a half-wave plate between the second polarizing plate 23 and the projection lens 11 or between the projection lens 11 and the first plane mirror 27. In the second exemplary embodiment, the brightness does not decrease and it is possible to greatly decrease the light which travels along the light axis 26, transmits the first plane mirror 27 and becomes harmful light at the screen 17. Thus, a high quality picture is obtained. In the second exemplary embodiment, a transmission type is used as a liquid crystal display panel but a similar effect is, of course, obtained by using a reflecting type. A rear projection picture display apparatus in accordance with the first exemplary embodiment of the present invention includes a picture display unit for displaying a picture corresponding to a video signal, a projection lens for magnifying and projecting a picture displayed on the picture display unit, a screen for displaying a projected image on it and two plane mirrors on the light path between the projection lens and the screen. A first plane mirror of the two plane mirrors locating at the projection lens side and between the screen and another second plane mirror is made of a reflecting polarizer and have a characteristic to transmit P-polarized light and reflect S-polarized light. Further, it features that the first plane mirror is provided so that the incident angle of the light traveling from the picture display unit to the first plane mirror is big enough as compared with the incident angle of the light traveling to the first plane mirror after being reflected at the second plane mirror. According to the above-mentioned construction, a part of the light paths coming-in and reflected at the first and second plane mirrors can be lapped over the other paths and thus, the space in the enclosure can be made small, that is the entire apparatus size can be made small. In the first exemplary embodiment, a polarizing plate is attached at the rear side of the screen to absorb the light which transmitted the first plane mirror. By this means, the application field can be spread. Further, picture quality can be improved by providing a polarizing plate to absorb the polarized light which was reflected at the first plane mirror at the rear side of the screen. Furthermore, when the above-mentioned means is used, because the picture display unit uses a picture display device which the oscillating direction of the emitting light is restricted to a specified direction like a liquid crystal panel, the entire apparatus size can be made small with minimum brightness deterioration.

Claims: An optical system for a rear projection picture display apparatus comprising: a picture display unit (10) for displaying a picture according to a video signal; a projection lens (11) for magnifying and projecting the picture displayed on said picture display unit; a first plane mirror (13,27) and a second plane mirror (15,29) for sequentially reflecting a beam of light emitted from said picture display unit and projected by said projection lens; a screen (17) for displaying said projected and reflected picture thereon, said first plane mirror being disposed between said screen and said second plane mirror: wherein said first plane mirror is a reflective polarizer which has the function of transmitting P-polarized light oscillating in the parallel direction to the incident plane and reflecting S-polarized light oscillating in the perpendicular direction to the incident plane to a degree dependent upon the incident light angle and, said first plane mirror (13,27) and said second plane mirror (15,29) are positioned such that the incident angle (1) of the light beam from said picture display unit to said first mirror is set to be in a range where said first mirror functions substantially as a polarizer and the incident angle (2) of the light beam from said second mirror (15,29) to said first mirror is set to be in a range where said first mirror substantially does not function as a polarizer so that the S-polarized light coming from the second mirror can be transmitted by the first mirror and can arrive to the screen, characterized in that said screen (17) is provided with a polarizer (19,31) located between the screen and the first mirror, which absorbs said P-polarized light from said first mirror. An optical system for a rear projection picture display apparatus as recited in Claim 1: wherein, the incident angle (1) to said first mirror (13,27) is set at or adjacent to an angle where the value obtained by dividing a transmittance of said S-polarized light by a transmittance of said P-polarized light is maximised.

Inventors: YAMAGISHI SHIGEKAZU, YAMAGISHI, SHIGEKAZU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1995
Main classification: G03B21
IPC codes: G03B21, H04N5