Optical system for rear projection picture display apparatus

Abstract: An optical system for a rear projection picture display ia made in a small size keeping a screen size as it is. Two plane mirrors are provided on a light path between a picture display unit and a screen. The first plane mirror at a projection lens side is made of a reflecting polarizer and is located on a light path between the second plane mirror 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.

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. The present invention provides an optical system for a rear projection picture display system as defined in claim 1 and a rear projection picture display apparatus as defined in claim 5. In the preferred arrangement S-polarized light among the light traveling from the projection lens to the first plane mirror is reflected at the first plane mirror and then, when it is reflected at the second plane mirror the oscillating direction of the S-polarized light is rotated by 90 degrees by passing through the quarter-wave plate on the second plane mirror and the S-polarized light is transformed to a P-polarized light and travels toward the screen. Because the light traveling from the second plane mirror is P-polarized light, it transmits the first plane mirror and can arrive at the screen. Here, the first plane mirror is made of a reflecting polarizer similar to the beam splitter and no restriction is needed for a relation between the incident angle of the light traveling from the picture display unit to the first plane mirror and the incident angle of the light traveling to the first plane mirror after being reflected at the second plane mirror. Because the space in the enclosure can be made small by folding the light path by the first plane mirror, the entire apparatus size can be made small. Further, a polarizing plate is preferably disposed between the first plane mirror and the screen to absorb P-polarized light among the incident light to the first plane mirror. Thus, harmful light can be eliminated and picture quality deterioration is prevented. Providing eliminating means of the light oscillating in a parallel direction to the first plane mirror between the picture display unit and the first plane mirror also increases design freedom. 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 an embodiment now claimed in a divisional application herefrom; FIG.3 shows an incident angle dependency characteristic of a reflecting polarizer used in a rear projection picture display apparatus in accordance with FIG. 2; FIG.4 illustrates an optical system for a rear projection picture display apparatus in accordance with an alternative embodiment now claimed in the divisional application herefrom; FIG.5 illustrates an optical system for a rear projection picture display apparatus in accordance with an exemplary embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION An optical system for a rear projection picture display apparatus 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 this arrangement is shown in FIG. 3. The incident angle of the light axis 12 directly traveling to the first plane mirror 13 is defined as el and the incident angle of the light axis 12 traveling to the first plane mirror 13 after being reflected at the second plane mirror 15 is defined as 2. The incident angle el is chosen to be big enough as compared to the incident angle 02 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 el 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 traveling 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 traveling 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. 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. An alternative optical system for a rear projection picture display apparatus is illustrated in FIG.4. In this arrangement, 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 traveling along the light axis 30, similar to the first arrangement described above. The incident angle of the light axis 26 against the first plane mirror 27 is defined as el and the incident angle of the light axis 30 against the first plane mirror 27 is defined as 2 and the relation between el and 02 is chosen similar to that in the first exemplary embodiment. The difference from the first arrangement is in a picture display unit. In the second arrangement, 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 this arrangement, a transmission type is used as a liquid crystal display panel but a similar effect is, of course, obtained by using a reflecting type. An optical system for a rear projection picture display apparatus in accordance with an exemplary embodiment of the present invention is illustrated in FIG.5. A cathode ray tube 10 displays a picture on its faceplate coated with phosphor, corresponding to a video signal. A projection lens 11 provided in front of the cathode ray tube 10 projects and magnifies the picture to a designated size. The light transmitting the projection lens 11 travels to a first plane mirror 34 provided slanting to a light axis 32 after transmitting a polarizing direction restricting means 33 provided on the light axis 32. The first plane mirror 34 is made of a reflecting polarizer. The polarizing direction restricting means 33 is provided so that only the S-polarized light travels toward the first plane mirror 34 along the light axis 32. The S-polarized light is reflected at the first plane mirror 34 along a light axis 35. The second plane mirror 38 is fixed slanting to the light axis 35 and a quarter-wave plate 36 is tightly attached on the reflecting surface 37 of the second plane mirror 38. The S-polarized light reflected along the light axis 35 transmits the quarter-wave plate 36, is reflected at the surface 37 of the second plane mirror 38, transmits the quarter-wave plate 36 again, the oscillating direction is rotated by 90 degrees, and the S-polarized light changes to P-polarized light, travels along a light axis 39, transmits the first plane mirror 34 and arrives at the screen 17. The projection lens 11 focusses an image on the screen 17 and offers a picture to the viewers. An enclosure 18 provides the screen 17 at an opening of the enclosure 18, protects external light to come in and supports the components such as the plane mirrors 34 and 38, the polarizing direction restricting means 33, the projection lens 11, the cathode ray tube 10, the video processor, etc. Because the light along the light axis 32 is S-polarized light, it is reflected at the first plane mirror 34 but because the light along the light axis 39 is P-polarized light, it transmits the first plane mirror 34 made of a reflecting polarizer. Therefore, according to the exemplary embodiment of the present invention, the entire apparatus can be made small like the first and second arrangements described above and further, there is no restriction in the incident angle against the first plane mirror 32 when the light travels to the first plane mirror 32 again and the design freedom increases. Most of the incident light oscillating in a perpendicular direction to the incidence plane at the first plane mirror 34 is reflected as explained before but because the incident light is not parallel, a little bit of the incident light could transmit the first plane mirror 34 depending on a relation between the cathode ray tube 10 and the projection lens 11. The P-polarized light transmitting the first plane mirror 34 travels to the screen 17 located at the rear side of the first plane mirror 34 but similarly to the first arrangement above the incident angle is set to be large enough as compared with the incident angle of the light along the light axis 39 which is viewed by the viewers, the light does not travel to the viewers and is not harmful. Thus, the picture is not deteriorated. Further, similar to the arrangements described above to provide a polarizing plate between the screen 17 and the first plane mirror 34 to absorb the P-polarized light leaking through the polarizing direction restricting means 33 and transmit the P-polarized light traveling along the light axis 38 is effective for excluding harmful light for the viewer. This polarizing plate is not shown in FIG.5. Diffuser may be painted on the surface of the screen 17 or compounded in the screen 17, also similar to the above arrangements. In the exemplary embodiment, the polarizing direction restricting means 33 can be realized by a material to transmit only light having a certain oscillating direction and absorb light having other oscillating directions such as a polarizing plate or material to reflect only light having a certain oscillating direction and transmit light having other oscillating directions such as a reflecting polarizer. It is obvious that the combination of a cathode ray tube 10 and a polarizing direction restricting means 33 can be replaced by the combination of a liquid crystal panel 20, a light source 21, a first polarizing plate 22 and a second polarizing plate 23. A rear projection picture display apparatus in accordance with the 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 a second plane mirror is made of a reflecting polarizer and have a characteristic to transmit P-polarized light and reflect S-polarized light. It features that a quarter-wave plate is attached on the reflecting surface of another second plane mirror and rotates the polarizing direction of the incident light by 90 degrees and emits as reflected light. 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 exemplary embodiment, means eliminating P-polarized light at the first plane mirror is provided between the picture display unit and 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) arranged to display a picture on a faceplate thereof according to an input video signal, a projection lens (11) in front of the picture display unit, arranged to magnify and project said displayed picture, a polarization direction restricting means (33) in front of the projection lens, arranged to polarize light projected by said projection lens to have a first linear polarization direction, a first plane mirror (34) inclined with respect to the axis (32) of the projection lens, being a reflective polarizer arranged to reflect the polarized light with said first polarization direction; a second plane mirror (38) arranged to reflect the reflected polarized light from said first plane mirror and being provided on the reflective surface with a quarter wave plate (36) to rotate the polarizing direction of said reflected polarized light from said first plane mirror by 90 degrees upon reflection; and a screen (17) for displaying said picture thereon positioned with respect to said first and second plane mirrors such that said first plane mirror is placed between said screen and said second plane mirror and such that said rotated reflected polarized light from said second plane mirror passes through said first plane mirror to reach said screen. An optical system for a rear projection picture display apparatus as recited in claim 1: wherein, said screen (17) is provided with a polarizing plate for absorbing light having said first polarization direction to eliminate deterioration of the picture. An optical system for a rear projection picture display apparatus as recited in claim 1 or 2: wherein, said picture display unit for displaying a picture according to the video signal is a liquid crystal display panel. An optical system for a rear projection picture display apparatus as recited in claim 1 or 2: wherein, said picture display unit for displaying a picture according to the video signal is a cathode ray tube (10). Rear projection picture display apparatus comprising an optical system according to any of claims 1-4 disposed within an enclosure (18), said screen being disposed at an opening of the enclosure and the enclosure protecting against the entry of external light to the optical system.

Inventors: YAMAGISHI SHIGEKAZU, YAMAGISHI, SHIGEKAZU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1995
Main classification: G03B21
IPC codes: H04N5, G03B21, G02B27