OPTICAL ARRANGEMENT FOR A REAR PROJECTION TELEVISION RECEIVER HAVING REDUCED DEPTH

Abstract: A video projection device such as a television receiver includes a cabinet (36) having front and rear sections and a projection tube (30) for projecting a video image. A screen (32) is located in the front section of the cabinet and has a first surface onto which the video image is projected and a second surface for displaying the video image so that it is observable by a viewer. A mirror (35) is arranged in the cabinet for reflecting light to the first surface of the screen. The projection tube is rearwardly facing for projecting the video image onto the mirror.

Description: OPTICAL ARRANGEMENT FOR A REAR PROJECTION TELEVISION RECEIVER HAVING REDUCED DEPTH Field of the Invention The present invention relates generally to rear projection televisions, and more particularly to an arrangement of optical elements for reducing the cabinet thickness of a rear projection television. Background of the Invention Rear projection type television receivers are very popular due to the large display screens that are available without the need for special installations and/or large viewing areas. In such receivers, three color cathode ray tubes (red, green and blue) project an image onto a mirror, with the image being reflected (and magnified) onto a display screen that may comprise a Fresnel lens arrangement combined with a diffuser. In recent years, improvements in both the brightness and the resolution of the image on the screen have made it possible to view the projection television from a distance much closer to the screen than before. As a result, such televisions can now be installed in smaller rooms in which the size of the television cabinet becomes a primary concern. In particular, the relatively large depth of the cabinet prevents installation in many environments. The horizontal distance between the screen and the cathode ray tubes limits the depth of the cabinet. For cabinets of shorter depths, the cathode ray tubes must be located as close to ' the screen as possible. Accordingly, it would be desirable to provide an optical arrangement that reduces the distance between the cathode ray tubes and the screen in a projection television receiver. Summary of the Invention In accordance with the present invention, a video projection device includes a cabinet having front and rear sections and a projection tube for projecting a video image. A screen is located in the front section of the cabinet and has a first surface onto which the video image is projected and a second surface for displaying the video image so that it is observable by a viewer. A mirror is arranged in the cabinet for reflecting light to the first surface of the screen. The projection tube is rearwardly facing for projecting the video image onto the mirror. In accordance with one aspect of the invention, the mirror and the projection tube have optical axes coincident with one another. In accordance with another aspect of the invention, the optical axis of the mirror and an optical axis of the screen form an angle of 45 degrees with respect to one another. Alternatively, the optical axis of the mirror and an optical axis of the screen may form an angle of less than 45 degrees with respect to one another. In accordance with yet another aspect of the invention, at least a portion of the projection tube extends in front of the screen. In accordance with another aspect of the invention, a diverging lens is provided which is located in an optical path between the projection tube and the mirror. In accordance with another aspect of the invention, the projection tube is a television projection tube for receiving and projecting a television image. In accordance with yet another aspect of the invention, the mirror may be a planar mirror. Alternatively, the mirror may be a spherical mirror or even an aspherical mirror. The spherical or aspherical mirror may be a Fresnel mirror. In accordance with another aspect of the invention, a video projection device includes a cabinet having front and rear sections and a projection tube for projecting a video image. A screen is located in the front section of the cabinet and has a first surface onto which the video image is projected and a second surface for displaying the video image so that it is observable by a viewer. A first mirror is arranged in the cabinet for reflecting light to the first surface of the screen. A second mirror is also arranged in the cabinet for reflecting light to the first mirror. The projection tube is arranged so that it is forward facing for projecting the video image onto the second mirror. Brief Description of the Drawings FIG. 1 is a schematic diagram of a conventional projection television receiver. FIG. 2 shows a planar mirror with a real object and its corresponding virtual image. FIG. 3 shows a projection television receiver having a first embodiment of the optical arrangement constructed in accordance with the present invention. FIGs. 4-6 show alternative embodiments of the invention. FIG. 7 shows the affect of replacing a planar mirror with a spherical mirror on the location of a virtual image. FIG. 8 shows another alternative embodiment of the invention. Detailed Description FIG. 1 is a schematic diagram of a conventional projection television receiver. A projection cathode ray tube 10 projects an image though a plano-convex optical lens 11 located in front of the tube 10. While FIG. 1 only shows a single cathode ray tube, one of ordinary skill in the art will recognize that three cathode ray tubes are typically employed. Associated with the projection tube 10 is a chassis (not shown) that supplies operating voltages and video information to the projection tube 10 by well known means. The focal length of optical lens 11 is selected so that an image produced by the tube 10 appears as an (inverted) image on a screen 12. As shown, the optical axes of the projection tube 10, optical lens 11 and screen 12 are all coincident with one another. The screen 12 directly receives and transmits the light reflected from the lens 11. Thus the light produced by the projection tube 10 is projected onto the screen 12 via the optical lens 11, whereby images are displayed. In order to increase the size of the image appearing on the screen 12 in this conventional arrangement, the distance between the optical lens* 11 and screen 12 must be increased, thus requiring a cabinet with an increased depth. FIG. 2 demonstrates well-known operational principals of a planar mirror to facilitate an understanding of the present invention. An object 21 is located a distance x from the front of the planar mirror 20. The mirror 20 forms a virtual image 22 of the object 21. As shown, the virtual image 22 is located a distance x behind the mirror 20 and has the same dimensions as object 21. For the purposes of locating images in accordance with the principals of geometric optics, the virtual image may be treated as if were equivalent to the original object. That is, the virtual image can serve as a virtual object for a subsequent mirror or lens. FIG. 3 shows a projection television receiver having a first embodiment of the optical arrangement constructed in accordance with the present invention. A planar mirror 35 is located in the cabinet 36 and has its reflecting surface facing the screen 32. The plane of the mirror 35 is oriented at a 45 angle with respect to the plane of the screen 32. The projection tube 30 is rearwardly facing so that it projects its image onto the planar mirror 35. The optical axis of the projection tube 30 (including its associated optical lens) is parallel to the optical axis of the mirror 35. Projection tube 30 is located so that the mirror 35 forms a virtual image 34 of the picture tube 30 at the location of the picture tube depicted in FIG. 1. Accordingly, based on the analysis presented in FIG. 2, the image formed on the screen 32 by projection tube 30 and mirror 35 is the same as the image formed by a projection tube and lens arrangement that have optical axes coincident with the optical axis of the screen. That is, the screen 32 treats the image 34 of the projection tube received from the mirror 35 as a virtual object. A primary advantage of the present invention is that since the horizontal distance between the screen and the projection tube is less in FIG. 3 than in FIG. 1, the depth of the television cabinet needed for the FIG. 3 arrangement is less than the cabinet depth required in FIG. 1. Of course, as shown, virtual image 34 will be located outside of cabinet 36. A second embodiment of the invention is shown in FIG. 4. This embodiment is the same as that shown in FIG. 3 except that a plano-convex or diverging lens 43 is now located between the mirror 45 and the screen 42. The diverging lens 43 has the effect of increasing the distance between the screen 42 -A- and the virtual object 44 of the projection tube 40. Accordingly, the addition of the diverging lens 43 allows the projection tube to be situated closer to the mirror 45 along their optical axes than would otherwise be the case. The depth of the cabinet in the embodiment of the invention shown in FIG. 3 is determined by the maximum horizontal distance between the mirror 35 and screen 32. This distance can be reduced if the mirror were moved closer to the screen by reducing the angle between them. FIG. 5 shows such an embodiment of the invention in which the plane of the mirror 55 and the plane of the screen 52 form an angle of less than 45°. In order to maintain the virtual image 54 of the projection tube 50 along the optical axis of the screen 52, the projection tube 50 needs to be positioned, at least in part, in front of the screen and thus the front portion of the cabinet below the screen needs to be extended. Thus in this embodiment of the invention the depth of the top portion 56a of the cabinet 56 containing the screen can be less than in FIG. 3 at the expense of an increased depth of the bottom portion 56b of the cabinet 56 containing the picture tube 50. This configuration nevertheless can be advantageous in many cases because the primary space consideration that must often taken into account is the distance of the screen from the wall against which the cabinet 56 is placed. Since viewers typically will not be located immediately in front of the screen 52, it generally will not present a problem if this space is occupied by a portion of the cabinet containing the picture tube. FIG. 6 shows another embodiment of the invention in which the planar mirror employed in the previous embodiments of the invention is replaced with a spherical mirror 65. FIG. 7 shows the affect of replacing the planar mirror with a spherical mirror on the location of the virtual image of the projection tube and its associated optical lens. Virtual image 73 (shown in dashed lines) represents the location of the picture tube 70 when a flat mirror is employed. Virtual image 73' represents the location of picture tube 70 when the flat mirror is replaced by spherical mirror 75. As shown, the spherical mirror increases the distance between the mirror 75 and the virtual image. Thus, when the spherical mirror is used in FIG. 6, the distance between the screen 62 and the virtual object 64 of the projection tube 60 is increased. Accordingly, the use of a spherical mirror allows the projection tube 60 to be situated closer to the mirror 35 along their optical axes than would we possible with a flat mirror. While the mirror 65 shown in FIG. 7 has been described as a spherical mirror, mirror 65 more generally may be an aspherical mirror, which has a curvature, selected to minimize defocusing of the image on the screen. FIG. 8 shows another embodiment of the invention in which a second mirror 86 is employed. Second mirror 86 is a planar mirror used to reflect the image from the projection tube 80 to the first mirror 85 so that the projection tube 80 may now be forward facing rather than rearwardly facing. The second mirror 86 and projection tube 80 are arranged so that the second mirror 86 forms a virtual image 87 of the picture tube at the location of the picture tube shown in FIG. 5. Since the projection tube 80 no longer needs to extend beyond the front of the cabinet 88, the extended portion 56b of the cabinet shown in FIG. 5 can be eliminated. Accordingly, some or all of the virtual image 87 may be located outside of the cabinet. While the first mirror 85 shown in FIG. 8 is a planar mirror, first mirror 85 alternatively may be a spherical or aspherical mirror such as shown in FIG. 6. In some embodiments of the invention the spherical or aspherical mirror shown in FIG. 6 can be replaced with a Fresnel mirror. As well known to those of ordinary skill in the art, a Fresnel mirror is a flat mirror that behaves as a concave or convex mirror, thus eliminating the problems associated with relatively large spherical mirrors such as their size, weight and difficulty of manufacturing. Although various embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and are within the purview of the appended claims without departing from the spirit and intended scope of the invention. For example, the invention is also applicable to video projection devices that employ LCD displays.

Claims: Claims: 1. A video projection device comprising: a cabinet 36 having front and rear sections; a projection tube 30 for projecting a video image; a screen 32 located in the front section of the cabinet, said screen having a first surface onto which the video image is projected and a second surface for displaying the video image so that it is observable by a viewer; a mirror 35 arranged in the cabinet for reflecting light to the first surface of the screen; and wherein said projection tube is rearwardly facing for projecting the video image onto the mirror. 2. The video projection device of claim 1 wherein said mirror and said projection tube have optical axes coincident with one another. 3. The video projection device of claim 1 wherein the optical axis of the mirror and an optical axis of the screen form an angle of 45 degrees with respect to one another. 4. The video projection device of claim 1 wherein the optical axis of the mirror and an optical axis of the screen form an angle of less than 45 degrees with respect to one another. 5. The video projection device of claim 4 wherein at least a portion of the projection tube extends in front of the screen. 6. The video projection device of claim 1 further comprising a diverging lens 43 located in an optical path between the projection tube and the mirror. 7. The video projection device of claim 1 wherein said projection tube is a television projection tube for receiving and projecting a television image. 8. The video projection device of claim 1 wherein said mirror is a planar mirror. 9. The video projection device of claim 1 wherein said mirror is a spherical mirror. 10. The video projection device of claim 1 wherein said mirror is an aspherical mirror. 11. The video projection device of claim 9 wherein said spherical mirror is a Fresnel mirror. 12. The video projection device of claim 10 wherein said aspherical mirror is a Fresnel mirror. 13. A video projection device comprising: a cabinet 88 having front and rear sections; a projection tube 80 for projecting a video image; a screen 82 located in the front section of the cabinet, said screen having a first surface onto which the video image is projected and a second surface for displaying the video image so that it is observable by a viewer; a first mirror 85 arranged in the cabinet for reflecting light to the first surface of the screen; a second mirror 86 arranged in the cabinet for reflecting light to the first mirror; and wherein said projection tube is forward facing for projecting the video image onto the second mirror. 14. The video projection device of claim 13 wherein the optical axis of the first mirror and an optical axis of the screen form an angle of 45 degrees with respect to one another. 15. The video projection device of claim 13 wherein the optical axis of the first mirror and an optical axis of the screen form an angle of less than 45 degrees with respect to one another. 16. The video projection device of claim 13 further comprising a diverging lens 43 located in an optical path between the projection tube and the first mirror. 17. The video projection device of claim 13 wherein said projection tube is a television projection tube for receiving and projecting a television image. 18. The video projection device of claim 13 wherein said first mirror is a planar mirror. 19. The video projection device of claim 13 wherein said first mirror is a spherical mirror. 20. The video projection device of claim 13 wherein said first mirror is an aspherical mirror. 21. The video projection device of claim 19 wherein said spherical mirror is a Fresnel mirror. 22. The video projection device of claim 20 wherein said aspherical mirror is a Fresnel mirror. 23. A video proj ection device comprising: a cabinet 88 having front and rear sections; a projection tube 80 for projecting a video image, said projection tube having a first optical axis; a screen 82 located in the front section of the cabinet, said screen having a first surface onto which the video image is projected and a second surface for displaying the video image so that it is observable by a viewer, said screen having a second optical axis orthogonal to the first and second surfaces, said first optical axis being non-coincident with said second optical axis; means 86, located in said cabinet, for forming a virtual image of the picture tube such that a third optical axis of the virtual image is coincident with the second optical axis of the screen. 24. The method of claim 23 wherein said virtual image forming means includes a mirror arranged in the cabinet for reflecting light to the first surface of the screen and said projection tube is rearwardly facing for projecting the video image onto the mirror. 25. The video projection device of claim 24 wherein said mirror has a fourth optical axis and said first optical axis of projection tube and the fourth optical axis are coincident with one another. 26. The video projection device of claim 24 wherein the fourth optical axis of the mirror and the second optical axis of the screen form an angle of 45 degrees with respect to one another. 27. The video projection device of claim 24 wherein the fourth optical axis of the mirror and the second optical axis of the screen form an angle of less than 45 degrees with respect to one another. 28. The video projection device of claim 24 wherein at least a portion of the projection tube extends in front of the screen. 29. The video projection device of claim 24 further comprising a diverging lens located in an optical path between the projection tube and the mirror. 30. The video projection device of claim 24 wherein said projection tube is a television projection tube for receiving and projecting a television image. 31. The video projection device of claim 24 wherein said mirror is a planar mirror. 32. The video projection device of claim 24 wherein said mirror is a spherical mirror. 33. The video projection device of claim 24 wherein said mirror is an aspherical mirror. 34. The video projection device of claim 32 wherein said spherical mirror is a Fresnel mirror. 35. The video projection device of claim 33 wherein said aspherical mirror is a Fresnel mirror. 36. The video projection device of claim 23 wherein said virtual image is located at least in part at a location external to the cabinet. 37. The video projection device of claim 23 wherein said virtual image forming means includes a first mirror arranged in the cabinet for reflecting light to the first surface of the screen and a second mirror arranged in the cabinet for reflecting light to the first mirror, and wherein said projection tube is forward facing for projecting the video image onto the second mirror.

Inventors: ISHII FUSAO, ISHII, FUSAO
Applicants: SONY ELECTRONICS INC, SONY ELECTRONICS INC.
Application year: 2001
Main classification: G02F1
IPC codes: G03B21, H04N5