WO1988001824A1

WO1988001824A1 - Cathode ray tube with integral mirror optics for three-tube projection television systems having increased light output

Info

Publication number: WO1988001824A1
Application number: PCT/US1987/002070
Authority: WO
Inventors: Marvin P. Hodges
Original assignee: Tds Patent Management, Inc.
Priority date: 1986-08-26
Filing date: 1987-08-25
Publication date: 1988-03-10

Abstract

A cathode ray tube (8, 9, 10) for use in a three tube projection television system. The cathode ray tube (8, 9, 10) employed includes a concave faceplate (14), curved away from the electron source (12) for energizing a phosphor surface (19). The concave faceplate (14) includes a phosphor image generating surface (15) which is bounded by an electron-permeable metallic layer (21). The electron-permeable metallic layer (21) is preferably bonded to an intermediate lacquer layer (20) to the phosphor surface (19). By forming a spectral surface, light emitted from the phosphor (19) is further concentrated along the useful radiation angle of the CRT (8, 9, 10). Light normally backscattered away from the useful viewing angle is directed through the faceplate (14) along the useful viewing angle.

Description

CATHODE RAY TUBE WITH INTEGRAL MIRROR OPTICS FOR THREE-TUBE PROJECTION TELEVISION SYSTEMS HAVING INCREASED LIGHT OUTPUT

Background of the Invention The present invention relates to three-tube television projection systems. Specifically, a cathode ray tube having an increased light output for use in such projection systems is described.

Three-tube projection television systems have been developed in order to increase the total light output of a projected image. As television images are magnified through conventional optics, total image^' brightness decreases. The reduced image brightness has, in some instances, necessitated the reduction of lighting in the room in which such images are generated. The reduced light output from projection television systems has been a serious performance obstacle in the widespread acceptance of color television projection systems by the consuming public.

Although the three-tube projection system generally provides greater image brightness than a single tube projection system, there is a limit to the light intensity each of the three tubes may generate. The higher voltages and electron currents which are employed in the three-tube projection systems increase light output. A limit is reached, however, beyond which the tube life is seriously reduced.

U.S. Patent No. 4,249,205 describes one type of three-tube projection system. Combining separate red, green and blue images from a three tube projection system is described using an immersed optical combining technique. The aforesaid patent has described a technique utilizing concave image surfaces in the CRT. As the CRT used in a three-tube projection system is not directly viewable, a concave image surface has been found useful in reducing the complexity of the projection lens.

The interfaces between a CRT and the mirrors and other optical components necessary to combine and project the individual images from each tube necessarily impose losses on the total light output.

Additional losses in brightness are incurred in generating images in a cathode ray tube by virtue of a portion of the total light generated from a phosphor picture element being backscattered rearward of the phosphor surface instead of through the viewing projection angle. The phosphor layers on conventional CRT tubes are, in themselves, very thin, and convey visible light rearward where it does not contribute to the overall image brightness of the projected image.

The present invention makes improvement to cathode ray tubes which employ curved phosphor surfaces. Useful light output is increased by decreasing the amount of light which is backscattered from the rear of an excited phosphor surface.

Summary of the Invention It is a primary object of this invention to improve the total light output of a projection television system.

It is a more specific object of this invention to increase the light output of each CRT in a multiple CRT projection television system.

These and other objects are carried out by apparatus and methods in accordance with the present invention. The invention includes a new cathode ray tube structure which increases the total amount of light in the useful light radiation angle of a CRT.

The invention is implemented in cathode ray tubes which have curved phosphorescent image surfaces which produce light images when excited by an electron beam. The invention provides for a metallic, electron-permeable, optically-reflecting surface between the exciting electrons and the phosphor surface. Light emitted by the phosphor surface which would normally be scattered rearward of the CRT faceplate is directed forward along the optical axis of the curved phosphorescent surface.

In carrying out the invention, three cathode ray tubes, each emitting one of the three primary colors, are arranged with an image combiner. Separate images generated by each of the cathode ray tubes are combined to form a composite color image.

Each CRT has a phosphor surface which is concave with respect to the direction of projection. A fine grain phosphor is solidified over the curved faceplate. A lacquer layer is interposed on the back side of the phosphor surface. The lacquer layer is thereafter metallized with a metallic surface which is typically aluminum. The metallized surface is permeable to incident electrons from the CRT electron gun, but substantially opaque to visible light. The metallized surface is formed to provide a mirrored surface of high spectral integrity.

The surface smoothness of the metallic layer is controlled so as to obtain a mirror optic surface. Light from an excited phosphor element which is normally backscattered to the rear of each CRT, is forward directed along a desired radiation angle. The metallized layer, although permeable to electrons, in cooperation with the curvature imposed on the phosphor surface and the metallized layer, tends to focus that portion of the emitted light which would normally backlight the cathode ray tube.

Description of the Figures

Figure 1 is a schematic illustration of a color projection system constituting a preferred embodiment of the invention.

Figure 2 is a section view of the CRT faceplate and image generating surface of the cathode ray tubes of Figure 1.

Figure 3A illustrates the approximate light radiation from a concave phosphor surface without metallizing.

Figure 3B illustrates the effect on light radiation from metallizing the rearward side of the phosphor surface of a cathode ray tube.

Description of the Preferred Embodiment Figure 1 illustrates a preferred embodiment of a three- tube projection television system incorporating a high brightness cathode ray tube. Each of the tubes has a phosphor generating light images of one of the primary colors. The three-tube projection system includes a blue 8, red 9 and green 10 cathode ray tube. Additionally, the faceplate of each of the cathode ray tubes is curved concave from the direction of the projection optics 18. From the direction of the electron gun 12, the faceplate is seen to be curved away from the gun. The concave faceplate 14 and associated image plane of the excited phosphor surface is coupled with a beam splitter 17 in an immersed system such as is described in U.S. Patent No. 4,249,204. Optical coupling between the immersed system 16 and individual cathode ray tubes 8, 9 and 10 provides for a minimum of losses due to interface reflection between individual faceplates of the cathode ray tubes and the immersed optical coupler 16.

The use of curved images as described in the aforesaid patent permitted the design of the objective lens to be greatly simplified. The present invention provides for additional advantages not realized in the 4,249,205 patent which can result from a curved faceplate on each of the cathode ray tubes 8, 9 and 10.

The curved image surfaces provided for each of the cathode ray tubes not only simplifies the design of the objective lens used in the projection system, but total light output from each of the tubes is enhanced. The curved image surfaces, formed of a phosphor layer 15 on the faceplate 14, assumes the concave geometry of the faceplate. Utilizing a 5" diagonal measured cathode ray tube, having a spheric phosphor surface with a radius of curvature within a range of 6 to 10 inches, images may be generated on a screen ranging from a 52" diagonal to a 72" diagonal measurement. Using the parameters set forth in the 4,249,205 patent, a simplified lens structure may be provided, as well as increasing the useful light output.

Additional light output is realized by the selection of the concavity for the image surface 15. Referring in particular to Figure 2, a section view of the faceplate 14 of each of the cathode ray tubes is shown. The section view includes the faceplate 14 shaped to have the desired concavity or aspheric radius of curvature. The image generating surface 15 includes three (3) subsidiary layers which are formed to enhance the total light output through the faceplate 14 with a minimum of light being backscattered in the direction of the scanning electron gun 12 and the tube cap 13. The phosphor layer 19 is applied by a conventional suspension process. The phosphor 19 of a single color-producing phosphor grain is selected to have a fine granularity, such as in a P-53 phosphor which is a brighter phosphor or high beam currents. The phosphor layer 19 has a thickness which is the same as that of any conventional single phosphor CRT. Generally, a phosphor layer is utilized of a density of 7.61 milligrams/cm². Additionally, a lacquer layer 20 is formed over the phosphor 19 using a conventional flotation process. The lacquer layer 20 provides for a smoother surface for applying a metallized layer 21 to the image surface 15.

Tube manufacturers are capable of producing premium lacquer layers which will reduce the r s surface smoothness to any desired amount sufficient to provide a surface which, when metallized, has acceptable specularity properties.

The phosphor layer 19, because of its granularity, provides for an RMS surface area at the interface with lacquer 20. In order to form a specular mirror optic surface with metallic layer 21, the lacquer 20 will reduce the total RMS surface granularity of the phosphor layer 19.

Once the lacquer has been applied through the flotation process and the water removed, as is conventional, from the phosphor layer 19, the metallized reflecting layer 21 may be added to the total image display surface 15. This may be accomplished through vapor deposition, or other precision controlled metallizing techniques. Using vapor deposition, a layer of approximately 1500 to 2500 A

(Angstroms) is formed over the lacquer 20. The metallized layer is, for example, an aluminum layer of at least 1500

A, which is totally opaque to visible light. For cathode ray tubes operating at potentials of 26 kv, the film thickness can be 2000 to 2500 A and still be penetrable by the electron beam. The thickness will be selected to be permeable to incident electrons, but of sufficient thickness to optimze the specular properties of the resulting concave mirror formed on the lacquer surface. The layer is deposited to provide for a highly specular metal layer after deposition. With a minimum thickness of 1500 A, a 100% efficient reflector system is derived, conforming to the phosphor layer curvature. It should be noted that such a metal layer in a conventional flat face CRT would be disadvantageous as various bright spots would appear along the surface due to the reflecting nature of the metallic layer. However, with the present invention, utilizing a curved image surface, the total light output for the cathode ray tube increases without a corresponding loss in image quality.

Referring to Figures 3A and 3B, the effect of the metallization layer 21 and its curved image generating phosphor layer 19 can be shown. Figure 3A demonstrates a gausian distribution of light from a small elemental area of the phosphor surface 19, in the absence of a metallic layer 21. Approximately 30% of the total light energy is backwardly scattered toward the electron gun 12.

Figure 3B illustrates the inclusion of the aluminized metallic layer 21 where a highly specular metal layer is formed. The 30% of the visible light which would normally be rearwardly directed in the cathode ray tube, forms a portion of the main lobe, extending out the front face of the faceplate 14 in a direction of the main optical axis for the projection system. Directing the 30% of light energy into the useful desired forward radiation angle as controlled by the concave mirror will increase the light available to the projection lens 18 by as much as 50%. Thus, an improvement of total light output for the tube of upwards of 50% may be realized utilizing this technique. As is known in three-tube projection systems, the tube energy is combined in a ratio of approximately 59, 30 and 11% to generate a conventional color image. Thus, by combining three tubes each having an enhanced light output, further improvements in total light output for three-tube projection television systems may be realized.

It is to be understood that the particular metallizing technique or the material selected for metallizing is by example only. Those skilled in the art will recognize yet other techniques for implementing the invention described more fully by the claims which follow.

Claims

What is claimed is:

1. A cathode ray tube for concentrating light output from a phosphor surface in a preferred radiation pattern comprising: ■

(a) a faceplate having a surface which is curved away from a source of exciting electrons, said . faceplate supporting a phosphor surface which emits light when excited by electrons;

(b) a metal layer disposed between the phosphorescent surface and said source of electrons, said metal layer having a thickness which is permeable to electrons but opaque to visible light, said metal layer forming an optical surface; and

(c) a scanning electron source for selectively exciting said phosphor surface through said metal layer to produce an image for viewing from the opposite side of said phosphor surface, said phosphor surface emitting visible light energy which is radiated at least in part under control of said metal layer.

2. The cathode ray tube of claim 1 wherein said faceplate phosphor surface is spherical.

3. The cathode ray tube of claim 1 wherein said faceplate phosphor surface is aspherical.

4. The cathode ray tube of claim 1 wherein said metal layer is aluminum exceeding 1500 Angstroms in thickness.

5. A method of enhancing the light output of a CRT which generates images from a scanning electron beam comprising:

forming a faceplate of said CRT in a curved shape curving away from an electron source which generates said electron beam;

depositing a phosphor layer on said curved faceplate which produces light in response to an incident electron beam; and

depositing a metallic surface on a surface of said phosphor layer in the path of said incident electron beam, having a thickness which permits passage of said electron but is opaque to light, said metallic surface forming an optical reflective surface for lignt emitted by said phosphor layer.

6. The method of claim 5 wherein said metallic layer is aluminum having a thickness greater than 1500 Angstroms.

7. In an image projection system for generating a multicolor image by combining the light output from multiple cathode ray tubes, a method for increasing the light output of said system comprising:

curving each faceplate supporting a phosphor surface of each cathode ray tube in a concave direction away from an illuminating electron gun; and

depositing a metal layer on the rear of each curved phosphor surface which receives electron illumination from said electron gun, said metal layer being a thickness which is permeable to electrons and opaque to visible light, said metal layer at least in part controlling the light radiation from said phosphor surface.