Projection television system

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PROJECTION TELEVISION SYSTEM

CROSS-REFERENCE TO RELATED PATENT

APPLICATION 5

This application is related to but in no way dependent upon copending application of common ownership herewith, Ser. No. 154,197 filed May 29, 1980.

BACKGROUND OF THE INVENTION AND io PRIOR ART DISCLOSURES

This invention is concerned with television systems, and is particularly directed to projection television systems in which discrete images are projected on a projection screen to provide a composite color picture. 15

FIG. 1 is a schematic plan view of the essentials of a representative prior art projection television system 6 in which a bank 8 of light projection devices 10,12 and 14 project through the indicated associated lens means blue, green and red images, respectively, on projection 20 screen 16. The projected images are intended to form the composite color image. Projection screen 16, which may be either of the front-projection or rear-projection type, typically has an aspect ratio of 3:4 in consonance with the aspect ratio of the standard television picture. 25

The centrally located device 12 typically has its projection optical axis 18 in congruence with screen axis 20, and as a result projects an undistorted light image on projection screen 16. The light images projected by devices 10 and 14, however, whose projection optical 30 axis 22 and 24 respectively are located "off-axis" with respect to projection screen axis 20, inherently project light images which are distorted because of their offaxis location.

Two types of optical distortion are inherent in the 35 system which can degrade through misconvergence the composite projected image to the point of unacceptability. The types are trapezoidal distortion and horizontal non-linearity, and they can best be described by the single term "non-linear magnification". As noted, the 40 light image projected by the centrally located device 12 is not subject to non-linear magnification distortion because its projection optical axis 18 is congruent with the projection screen axis 20. As a result, the light image projected on light image screen 16 will be symmetrical 45 and undistorted. The light image projected by device 12 is typically that shown in FIG. 2 by light image 26, indicated as being rectilinear.

This is not so with the light images as projected by devices 10 and 14. The inherent distortion of the light 50 images due to the off-axis location is depicted by FIG. 2, wherein the light image projected by device 10 is indicated by configuration 28 as being trapezoidal. Similarly, the image projected by device 14 is indicated by configuration 30 as being trapezoidal. In a typical prior 55 art projection television system, the non-linear magnification distortion may be of the order of five percent, an amount sufficient to so misregister the images as to render the composite picture unacceptable to the viewer. 60

It is to be noted that if the projection devices 10, 12 and 14 are vertically stacked, a similar distortion will be realized in the case of devices 10 and 14. The non-linear magnification distortion realized is described, in the context of this disclosure, as "keystoning" distortion, 65 rather than "trapezoidal" distortion.

The second form of distortion—horizontal nonlinearity—is also in consequence of the location of de

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vices 10 and 14 off the projection screen axis 20. The effect of this type of distortion is depicted in FIG. 3, using as an example the light image 28 projected by light projection device 10, indicated as being trapezoidal due to the aforedescribed non-linear magnification distortion (the trapezoidal shape is exaggerated for the purpose of illustration). The lines 32A-G represent the vertical lines of a television screen cross-hatch generator, as projected. The effect of horizontal non-linearity distortion is apparent in the progressive increase from left to right in the distance between lines 32A-G. The vertical lines of the light image 30 projected by device 14 would be similarly distorted, but in the opposite direction.

One approach to the correction of trapezoidal distortion is by electronic means. For example, the image projected by the off-axis cathode ray tubes of projection means 10 and 14 can be made compensatorily trapezoidal. This can be done by synthesizing a correction wave form for application to a high-speed writing-type yoke which is placed in tandem with the main deflection yoke. The end result is a trapezoidally shaped raster inverse in orientation to the normal distortion of the image projected by the off-axis cathode ray tubes of projection means 10 and 14. Correction by such electronic means is plagued by the complications introduced in the television circuit, with a consequent increase in cost. The complexity and added cost is even greater in consequence of the fact that the correction circuitry for the two off-axis CRT's must be designed to exert an opposite effect on their projected images. The economic burden imposed by the electronic approach is further underscored in view of the fact that while it may be effective against trapezoidal distortion, it is largely ineffective in terms of correction for horizontal nonlinearity distortion, wherein additional and very complex electronic correction circuitry must be employed.

Optical systems for reducing or otherwise ameliorating distortion include Oland—U.S. Pat. No. 4,004,093, which discloses a truncated Schmidt optical system wherein a plurality of Schmidt systems is clustered closely together by truncating the mutually adjacent edges of mirrors and correcting lenses which comprise individual Schmidt systems. Such clustering is said to provide a reduction in trapezoidal distortion by virtue of the fact that the cathode ray tubes for each primary color project images which arrive almost orthogonally at the screen.

Hergenrother et al—U.S. Pat. No. 4,024,579—discloses a projection television system in which the composite image is projected onto a curved screen by three discrete cathode ray tubes arranged as a triad, with each projecting a different primary color. The tube optics are folded into a catadioptric configuration and the three images are caused to converge into a composite image by an elaborate optical system that includes a Schmidt correction lens mounted externally to the envelope of each tube. Although the system has achieved a measure of consumer acceptance, the need for an extensive alignment procedure to achieve a satisfactory composite image, and the general lack of brilliance of the projected image, has limited its acceptance.

It is known in the art that if the axis of an electron gun is at an angle with respect to the axis of a cathode ray tube, the visible image on the face panel will exhibit non-linear magnification distortion. This type of distortion was common to certain early image iconoscope tubes of Vladimir Zworykin and was considered a performance liability. An example of a cathode ray tube configuration having an electron gun at an angle with respect to the tube axis is shown in U.S. Pat. No. 2,777,084 to Lafferty. Pat. No. 28 37 249 (German) 5 discloses a system for optically correcting trapezoidal distortion of the image projected by cathode ray tubes located off a central axis. The projection system includes cathode ray tube color picture sources, each of which projects its image through a projection lens. 10 Trapezoidal distortion is stated as being corrected by tilting the picture sources away from the central axis relative to the light axis of the associated projection lens. As a result, the image on the faceplate of the cathode ray tube is no longer parallel with the screen. When '5 the image on the cathode ray tube faceplate is projected, the image on the screen is reputed to be parallel.

Examples of circuit means intended to provide convergence of multiple color image projectors are to be found in the following U.S. Pat. Nos.: Seright 2,654,854, Mengle 2,989,584, Austefjord 3,943,279.

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OBJECTS OF THE INVENTION It is a general object of the invention to provide for

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improved performance in certain projection television systems.

It is another object of the invention to provide for improved performance in projection television systems having off-the-axis image projection devices. ^

It is yet another object of the invention to provide for a reduction in the cost of projection television systems in terms of enhanced simplicity in design, easier set-up, and minimized need for electronic distortion-correction circuitry. 35

It is a more specific object of the invention to provide for the elimination of trapezoidal distortion in off-optical axis image projectors in projection television systems.

It is a specific object of the invention to provide for the simultaneous correction of trapezoidal distortion and horizontal non-linearity distortion in projection television systems by purely mechanical means.

BRIEF DESCRIPTION OF THE DRAWINGS

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The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in con- 50 junction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:

FIGS. 1-3 show diagramatically the cause and effects of two types of distortion experienced in certain prior 55 art projection television systems;

FIG. 4 is a highly simplified schematic view of a projection television system having off-axis light projection means;

FIG. 5 shows diagramatically and in greater detail, 60 one such off-axis projection means according to an embodiment of the invention;

FIG. 6 is a schematic view in perspective showing the beneficial effect of the FIG. 5 embodiment of the invention; 65

FIG. 7 is a simplified view in perspective of a projection television system having three light projection means according to the invention;

FIG. 8 is a view in elevation of an array of nine imageprojection devices according to the invention;

FIGS. 9 and 10 are schematic views of further embodiments according to the invention;

FIGS. 11 and 12 indicate diagramatically the relative positions and orientations of an array of light-projection components according to the invention; and

FIGS. 13A-C are views in elevation of a cathode ray tube showing steps in the process of forming the cathode ray tube according to the invention disclosed in referent copending application Ser. No. (D4097).

DESCRIPTION OF THE PREFERRED
EMBODIMENT

In FIG. 4 there is depicted schematically a projection television system 70. The system 70 has a projection screen 72 for displaying a light image cast thereon. Screen 72 is spaced apart from a plurality of light projection means 74. At least one light projection means, designated as being projection means 76, has a projection optical axis 78 at a non-zero, acute-angle A with respect to the projection screen axis 80.

With reference also to FIG. 5 wherein projection means 76 is depicted in greater detail, projection means 76 is indicated as including a cathode ray tube means 82 having a cathodoluminescent screen 84 on the inside surface of the face panel 86 whose axis is substantially parallel to the projection optical axis 78. The seal land 85 indicates the junction of the facepanel 86 and the funnel 87 of cathode ray tube 82; the significance of the seal band and its orientation will be described infra. The screen is made cathodoluminescent by a deposit of a monochrome phosphor which may comprise, for example, one of a number of phosphors emitting red, green or blue light upon excitation by an electron beam. The electron beam generating means 88, which is typically an electron gun, is disposed on the electron-optical axis 90 of cathode ray tube 82. Electron beam generating means 88 is indicated as emitting a scanning electron beam 92 which forms an electron image on the cathodoluminescent screen 84 in response to television signal information. The electron image is converted to a visible image by cathodoluminescent screen 84 as screen 84 is excited by beam 92.

Lens means 94 on projection optical axis 78 provides for projecting on projection screen 72 the light image of the electron-formed formed visible image on cathodoluminescent screen 84. The light image inherently has a non-linear magnification distortion attributable to the location of projection means 76 off the projection screen axis 80.

The non-linear magnification distortion of the light image as projected on projection screen 72 is shown in FIG. 4 as being trapezoidal, as indicated by the dashline image 98. Horizontal non-linearity distortion is also present as described heretofore.

The projection television system according to the invention is characterized by the electron-optical axis 90 of cathode ray tube means 82 defining a non-zero, acute-angle B with respect to the axis of cathodoluminescent screen 84. The value of angle B and the orientation of the electron-optical axis is selected to cause the electron-formed visible image to have an orientation and non-linear magnification distortion effective to substantially compensate for the off-axis-induced non-linear distortion of the projected light image.

The remedial effect is depicted in FIG. 6, which is view looking over the screen 72 and toward the face