WO2002021205A1

WO2002021205A1 - Footprint reduction in a rear projection television system

Info

Publication number: WO2002021205A1
Application number: PCT/US2001/026100
Authority: WO
Inventors: Shinjiro Umeya
Original assignee: Sony Electronics, Inc.
Priority date: 2000-09-08
Filing date: 2001-08-20
Publication date: 2002-03-14
Also published as: AU2001285147A1

Abstract

The invention includes an apparatus to produce and transmit a particular form of polarized light so as to be able to reduce the footprint depth of a rear image projection system. The apparatus includes a laser projector (310) that projects a first form of polarized light along an optical path. The apparatus also includes a conversion mirror (312), a screen (314) having a rear facing the conversion mirror (312), and a functional mirror (319) adjacent to the rear of screen (314) and in the optical path between the conversion mirror (312) and the screen (314).

Description

FOOTPRINT REDUCTION IN A REAR PROJECTION TELEVISION SYSTEM

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention includes a rear projection television system employing a laser projector and having a reduced footprint depth as a result of employing laser light in a particular polarized light form.

2. Background Information

People have enjoyed going to movie theaters for decades because of the experience of watching movies on the "big screen." As a display medium, large screens provide a look and feel that is not available through any other medium. The big sights and sounds of the theater big screen makes the movie experience almost lifelike. Because people receive so much enjoyment from attending movies, there has always been the desire to duplicate the big screen environment in the privacy and comfort of the home. While the cathode ray tube (CRT) televisions were able to bring movies and television programs into the home, the small screen of most CRT based televisions does not deliver the big screen experience of the theater.

The small screen of most CRT based televisions does not deliver the big screen experience of the theater primarily due to the technological limitations inherent in the cathode ray tube. The glass display screen of a conventional CRT television generally is limited to less than a forty inch diagonal since the cost and weight of a CRT television increases significantly as the size of the display increases. As a consequence, rear projection television systems were developed to provide viewing screens larger than those provided by CRT based televisions at a cost and weight that is less than even the largest CRT televisions.

Although rear projection television systems provide a large, greater than 40-inch diagonal screen display for viewing, these systems require a significant amount of floor space. For example, most rear projection television systems have a depth of approximately twenty inches so that the system must extend at least twenty inches from a room wall. This footprint dimension generally is too large for many homes.

The size of the rear projection television footprint is due mostly to the focus length of the projector lens. That is to say, the light image that leaves the projector lens must travel a specific length or distance before impacting the rear of the display screen so as to be in focus on impacting the display screen. This focus length dictates the length of the optical path. Thus, the optical path length dictates the footprint depth dimension of the rear projection television system.

Prior art rear projection systems have attempted to reduce the footprint depth dimension by using mirrors or by using a focus lens having a shorter focus length. For example, the teachings of U.S.4,969,732, U.S. 5,184,238, U.S., 5,557,343, U.S. 5,573,324, and U.S. 5,796,446 claim inventions that use mirrors to reduce the footprint depth dimension. These have met with limited success. Under the alternate technique, a shorter focus lens is difficult to fabricate, is heavier, and is more expensive that the standard focus lens.

SUMMARY OF THE INVENTION The invention includes an apparatus to produce and transmit a particular form of polarized light so as to be able to reduce the footprint depth of a rear image projection system. The apparatus includes a laser projector that projects a first form of polarized light along an optical path. The apparatus also includes a conversion mirror, a screen having a rear facing the conversion mirror, and a functional mirror located adjacent to the rear of screen and in the optical path between the conversion mirror and the screen.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a front view of rear projection system 100; Figure 2 is a side view of a prior art rear projection system 200; Figure 3 is a side view of system 300 of the invention; and Figure 4 illustrates a plot of a dimension simulation result comparing prior art system 200 against system 300 of the invention.

DETAILED DESCRIPTION OF THE INVENTION As noted above, prior art rear projection systems have attempted to reduce the footprint depth dimension by using mirrors or by using a focus lens having a shorter focus length. However, modifying or replacing the light used to transmit the moving image is a relatively unexplored area. By employing a given focus lens, special mirrors, and laser light in a particular polarized light form, the invention may reduce the footprint depth dimension by approximately one half over conventional rear projection systems.

Figure I is a front view of rear projection system 100. Figure 2 is a side view of a prior art rear projection system 200. Figure 2 is provided for comparison to, Figure 3. Figure 3 is a side view of system 300 of the invention.

Rear projection system 100 of Figure 1 may display the footprint width and height dimensions of either prior art rear projection system 200 or rear projection system 300 of the invention. Conventionally, rear projection system 100 may define an overall width of 100.0 centimeters (cm) (39.4 inches) and an overall height of 115.0 cm (45.3 inches). The projection screen may define a width and height of 64.0 X 85.3 cm, so as to give a screen diagonal of 106.6 cm (42 inches). The invention is not restricted to these width and height dimensions and may be defined by any industry standard or nonstandard width and height.

Prior art rear projection system 200 may define depth 202. Depth 202 may be thought of as a footprint depth dimension of system 200. Here, depth 202 is 50.0 cm (19.7 inches). Prior art rear projection system 200 includes projector 210, mirror 212, and screen 214 housed in cabinet 216. Projector 210 puts forth light waves 218 so that light waves 218 impact mirror 212. Light waves 218 then are reflected from mirror 212 onto the rear of screen 214. The moving images may then be viewed from the front of screen 214.

Conventionally, the focus length of the lens within projector 210 determines the optical path length that is required in order to project a focused image onto screen 214. As seen in Figure 2, the optical path length of one portion of the light wave 218 is equal to the distance traversed by wave segment 220 plus the distance traversed by wave segment 222. Projector 210 could be located directly behind screen 214 at a distance that is represented by wave segment 220 plus wave segment 222. However, this would result in a maximum footprint depth dimension 202. Footprint depth dimension 202 is shortened by locating projector 210 at the bottom of cabinet 216 and reflecting light waves 218 against pre-positioned mirror 212 and onto the rear of screen 214. Here, the optical path length of one portion of the light wave 218 remains equal to the distance traversed by wave segment 220 plus the distance traversed by wave segment 222, but depth 202 may be reduced to 50.0 cm.

As note above, Figure 3 is a side view of system 300 of the invention. System 300 may be any rear projection system, such as a rear projection television, oscilloscope, or radar. System 300 may define depth 302. Depth 302 may be thought of as a footprint depth dimension of system 300. Here, depth 302 is less than 50.0 cm (19.7 inches). In one embodiment, depth 302 is almost one half of depth 202. In another embodiment, depth 302 is 30.0 cm (11.8 inches).

Included with system 300 may be laser projector 310, conversion mirror 312, and screen 314 housed in cabinet 316. i addition, system 300 may include functional mirror 319. Functional mirror 319 may be positioned adjacent to the rear of screen 314 in the optical path between conversion mirror 312 and screen 314.

Ordinary light may be thought of as being produced by vibrations transverse or perpendicular to the direction of the ray, and distributed so as to show no distinction as to any particular direction. Polarized light may be thought of as light that vibrates in one plane (plane-polarized light), light with a rotary vibration (circular polarized light), or light that vibrates elliptically (elliptically polarized light). For example, when the vibrations of the light are made to take place in one plane, the light is said to be plane polarized. If only a portion of the vibrations lie in one plane the ray is said to be partially polarized. Although moonlight and skylight are polarized, as is much reflected light, naturally polarized light is, on the whole, rather imperfectly polarized.

Laser projector 310 may be any device that converts incident electromagnetic radiation of mixed frequencies to one or more discrete frequencies of highly amplified and coherent ultraviolet, visible, or infrared radiation and projects this radiation. Preferably, laser projector 310 may put forth one of two discrete forms of light: p- polarized light and s-polarized light. The letters "p" and "s" are used in this patent to distinguish each polarization of light and represents no independent meaning.

For a given focus lens, p-polarized light (or s-polarized light) has a short optical path length than ordinary light passing through that same lens. With a short optical path length than ordinary light passing through the same lens, depth 302 of system 300 may be range over a shorter distance than depth 202 of system 200. Thus, the footprint depth dimension of system 300 may be reduced over a conventional rear projection system, such as system 200 of Figure 2.

To transmit the polarized form of light emanating from laser projector 310 to the rear of screen 314, a series of special mirrors may be used. Functional mirror 319 may include a surface capable of reflecting a first form of polarized light while transmitting a second form of polarized light from laser projector 310. Accordingly, when light waves

318 impact functional mirror 319, the first form of polarized light is reflected towards conversion mirror 312. The expected 80%+ efficiency of this reflection may be viewed as high since the polarization of functional mirror 319 is set to match the polarization of light waves 318 incident thereon. In one embodiment, functional mirror 319 reflects p- polarized light and transmits s-polarized light. In another embodiment, functional mirror

319 transmits p-polarized light and reflects s-polarized light.

Conversion mirror 312 may include quarter- wave plate 320 disposed in front of

reflection mirror 322. Quarter-wave plate 320 (sometimes written λ/4 plate) may be a plate or series of plates that, along with reflection mirror 322, may be adapted to convert the first form of polarized light to the second form of polarized light. Reflection mirror 322 may include a surface capable of reflecting the second form of polarized light back towards functional mirror 319. For example, quarter-wave plate 320 may convert an incoming plane-polarized light (p-polarized light) to clockwise circular polarized light. Reflection mirror 322 may convert the clockwise circular polarized light to counterclockwise circular polarized light. On the counterclockwise circular polarized light being reflected through quarter-wave plate 320 by reflection mirror 322, quarter- wave plate 320 may convert the counterclockwise circular polarized light into s-polarized light. This second form of polarized light may then become incident on functional mirror 319.

Recall that functional mirror 319 reflects the first form of polarized light but permits the second form of polarized to pass directly through functional mirror 319. With the second form of polarized light now incident on functional mirror 319, the second form of polarized light passes through functional mirror 319 and onto the rear of screen 314. As the second form of polarized light impacts the rear of screen 314, the moving image represented by this polarized light is conveyed to the viewer of the front of screen 314.

Figure 4 illustrates a plot of a dimension simulation result comparing prior art system 200 against system 300 of the invention. As shown in Figure 4, rear projection system set depth is plotted as a function of set height. In both prior art system 200 and system 300 of the invention, as set height increases, set depth decreases. However, depth 302 of system 300 may be much less (approximately 3/5 less) than depth 202 of system 200 for a set height of 112 cm (44 inches). As the set height increases, the footprint depth dimension difference becomes more pronounce. For example, at a set height of 122 cm (48 inches), depth 302 of system 300 may be as much as 1/2 of depth 202 of system 200.

The exemplary embodiments described herein are provided merely to illustrate the principles of the invention and should not be construed as limiting the scope of the subject matter of the terms of the claimed invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. Moreover, the principles of the invention may be applied to achieve the advantages described herein and to achieve other advantages or to satisfy other objectives, as well.

Claims

CLAIMSWhat is claimed is:

1. An apparatus, comprising a means for projecting a first form of polarized light along an optical path; a conversion mirror (312); a screen (314) having a rear facing the conversion mirror (312); and a functional mirror (319) disposed adjacent to the rear of screen (314) and in the optical path between the conversion mirror (312) and the screen (314).

2. The apparatus of claim 1, wherein the means for projecting is a laser projector.

3. The apparatus of claim 1, wherein the conversion mirror includes a reflection mirror and at least one plate adapted to convert the first form of polarized light to the second form of polarized light.

4. The apparatus of claim 3, wherein the at least one plate is a quarter-wave plate.

5. The apparatus of claim 4, wherein the quarter-wave plate is disposed between the reflection mirror and the functional mirror.

6. The apparatus of claim 5, wherein the quarter-wave plate is a series of plates.

7. The apparatus of claim 1, wherein the functional mirror includes a surface that is adapted to reflect the first form of polarized light while transmitting a second form of polarized light.

8. The apparatus of claim 7, wherein the first form of polarized light is p- polarized light and the second form of polarized light is s-polarized light.

9. A method, comprising: projecting a first form of polarized light along an optical path and onto a functional mirror (319); reflecting the first form of polarized light from the functional mirror (319) towards a conversion mirror (312); converting the first form of polarized light into a second form of polarized light; reflecting the second form of polarized light towards the functional mirror (319); and passing the second form of polarized light through the functional mirror (319) and onto a rear of a screen (314).

10. The method of claim 9, further comprising: presenting a conversion mirror, a screen having a rear facing the conversion mirror, and a functional mirror disposed adjacent to the rear of screen.

11. The method of claim 9, wherein projecting a first form of polarized light along an optical path includes employing a laser projector.

12. The method of claim 9, wherein converting the first form of polarized light into a second form of polarized light includes employing at least one plate adapted to convert the first form of polarized light to the second form of polarized light.

13. The method of claim 9, wherein converting the first form of polarized light into a second form of polarized light includes employing a quarter-wave plate.

14. The method of claim 9, wherein reflecting the first form of polarized light from the functional mirror towards a conversion mirror includes transmitting a second form of polarized light through the functional mirror.

15. The method of claim 14, wherein reflecting the first form of polarized light includes reflecting p-polarized light and wherein transmitting the second form of polarized light includes transmitting s-polarized light.