US 2234227 A
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March 11,1941. F. BELOW ETAL PROJECTION RECEIVER Filed Dec Patented Mar. 11, 1941 v UNITED STATES PATENT OFFICE PROJECTION RECEIVER Application December 15, 1937, Serial No. 180,018 In Germany December 16, 1936 3 Claims.
Certain difliculties arise in the construction of television projection receivers if the screen is positioned oblique to the optical axis of the optical system for reasons of convenient viewing or is in 5 any manner attached to the receiver, as may be desired for reasons of construction and operation.
If an arrangement is made as illustrated in Fig. 1, where the screen 2 of a cathode ray tube I is mounted in the upper part of a receiver cabinet facing the viewer and where the received image on the fluorescent screen is projected upon the projection screen 5 by means of a projection system 3 and a mirror 4, it is not possible to position the projection screen under an angle on of a value 15 exceeding between 60 and '70 deg. considering a certain minimum length of the tube of between 40 and 50 centimeters and the distance determined by the magnification of the image. This angle is determined by the fact that the screen 5 is always disposed in a plane perpendicular to the central optical axis.
A screen in such a position, however, is inconvenient for viewing. It is often undesirable to pcsition the cathode ray tube outside of the receiver cabinet proper because this means separation of the receiver into two parts and such a measure alone does not permit of positioning the screen at a greater angle.
This invention makes it possible to use the projection screen in a vertical position by compensating the distortion or lack of sharpness, respectively, caused by the acute angle between the optical aXis and the plane of the screen. This compensation is accomplished by providing for distortion of the image in the opposite sense in the cathode ray tube as well as by positioning the fluorescent screen in a correspondingly oblique position in the path of the light rays. A tube is preferably used where the fluorescent screen is in an oblique position and where the light emitted from the bombarded side of the fluorescent screen is used for projection. I
The object of the invention will be described with the aid of the Figs. '2 to 5, which schematically show arrangements in the receiver cabinet.
In the arrangement shown in Fig. 2, the screen 5 is shown in almost a vertical position. The axis of the light path is oblique to the plane of the screen. In order to balance the lack of sharpness caused thereby in the upper and lower portions of the image, screen 2 of the cathode ray tube is also not disposed in a plane perpendicular to the optical axis, but in an oblique plane. The angle p between the optical axis and the normal in the center of the screen 2 is, however, substantially smaller than the angle between the optical axis and a normal in the center of the screen because the magnification factor of the optical system 3 must be taken into consideration when determining the ratio of the two angles. A trapezoidal image is obtained uponthe screen 5 with such an arrangement if the image on the screen 2 has the usual rectangular shape. In order to eliminate this keystone effect, the invention provides for placing the axis of the electron gun oblique to the plane of the screen 2 so thata trapezoidal fluorescent image is obtained when using normal deflecting voltages or currents. Simultaneously, with a trapezoidal or lateral distortion, a vertical distortion of the fluorescent image occurs which isnoticeable in a variation of the distance between lines over the height of the image as is indicated in Fig. 3 and which is compensated for optically so that equal distances between lines appear upon the projection screen. If the trapezoidal shape were corrected by electrical means, the vertical distortion would not be eliminated and the quality of the image would be inferior to that with optical correction. The deviation of the image on screen 2 from a rectangular shape is made to be such that it compensates for the distortion in both directions caused by the optical projection. The cathode ray tube is so positioned that the narrower side of the trapezoidal picture area is on that side of the screen 2 which is closest to the optical projection system.
. Fig. 3 shows the trapezoidal shape of the fluorescent image on screen 2, while a rectangular image is produced on the screen 5. This type of projection can be carried out with or without an intermediate mirror in the light path. In Fig. 2 a mirror 4 is provided which is also in a substantially vertical position. The optical system 3 and the screen 2 are in fairly close proximity to the mirror 4 so as not to make the required mirror size unnecessarily large. The optical system must, of course, lie outside of the path of the light reflected by the mirror.
While Fig. 2 shows an arrangement in which the light generated on the side of the screen 2 remote from the electron gun is used, it may seem preferable to use the light generated on the side of the screen facing the electron gun in order to increase the efliciency. Such arrangements are shown in Figs. 4 and 5. The vacuum receptacle of Fig. 4 contains a plane-parallel plate of glass 9, a preferably plane screen In in a position oblique to the optical light path and also oblique to the axis of the electron gun. This screen may be de- 34o is desirable, which may compensate for this disposited upon a plane portion of the wall of the vacuum receptacle or also upon a carrier plate in the interior of the vacuum receptacle. The carrier plate may also consist of a metallic reflective material so that the light radiated to the back is also utilized. I T
Fig. 5 shows a further construction of a vacuum receptacle II which contains a mirror l2. The luminescent screen [3 is so positioned that it lies oblique to the axis of the electron gun. This shape of the vacuum receptacle has the'advantage that only one plane plate must be fused into the receptacle and thatthe enlarged portion I4 of the receptacle may possess a cylindrical form which can readily be produced. If the fluorescent screen is arranged in a plane oblique to the axis of the electron gun, a field is obtained, when deflecting the unmodulated beam in two directions, whereby the lines are not equally sharp over the entire area because the electrical focusing usually does not possess sufiicient depth of focus. It is, therefore, preferable to vary the focal length of the electrostatic'or magnetic lens with the angle of deflection of the electron beam.
It is, thereby, advantageous to adjust the deflecting system in such a manner that the lines lie parallel to the two parallel sides of the trapezo-idal field, as is indicated in Fig. 3. If magnetic focusing, for instance, is applied in this case, the magnetic focusing field is periodically varied with a vertical or frame deflection by either sending a periodically undulating direct current through the focusing coil proper or by providing an auxiliary coil supplied with alternating current. A corresponding variation of the focal length can be obtained by use of varying focusing voltages in the case of electrostatic focusing.
vThe oblique position of the fluorescent screen causes the fluorescent spot to have an elliptical shape, so thata non-symmetrical focusing action tortion. This may be accomplished, for instance, by the use of elliptical coils for magnetic focusing or elliptical apertures for electrostatic focusing.
The invention is not limited to home receivers, where the screen is attached to the lid of the cabinet, but is also applicable in such cases where a White wall of the room serves as a screen or where a projection screen is attached to the wall and where it is not possible to let the axis of the projection system coincide with a normal in the center of the screen. In such apparatus where the components are in no fixed position with respect to each other, it ispreferable not to mount the tube in a fixed position in the receiver, but to mount it rotatably so that the receiver can be made to match various positions of the screen.
Such an adjustment, however, allows only focusing of the image but not a compensation for the trapezoidal distortion for random angles, so that it may seem preferable in such cases to provide a possibility of varying the trapezoidal shape of the image on the screen 2. To this end, the screen may be made rotatable inside of the cathode ray tube also, so that it may be rotated in the interior of the vacuum receptacle with the aid of external means. Such a rotation of the screen through a limited angle may, for instance, be achieved by means of a bimetal strip in the inside of the vacuum receptacle, which strip can be .heated from the outside, or also by magnetic means. In case such mechanical adjusting devices are too complicated to in corporate, electrical variation of the trapezoidal form can be used.
1. In a projecting television receiver, a cathode ray receiving tube having a luminescent screen positioned obliquely therein with respect to the undeflected path of the rays, whereby the deflection of said rays will produce a trapezoidal pattern on said screen, an optical Viewing screen, and an optical projecting system for producing an image of said pattern on said viewing screen, the optical am's of said system being oblique to said screens in such degree as to distort said trapezoidal pattern into a rectangular one.
2. In a television receiver the combination of a substantially vertical viewing screen, a cathode ray receiving tube mounted beneath said screen, said cathode ray tube having a luminescent screen mounted obliquely to the undeflected path of the cathode rays whereby deflection of said rays will produce a trapezoidal pattern thereon, and an optical projecting system having its axis oblique to said screens for projecting an image of said pattern on said viewing screen, said system including an oblique mirror and the angle of obliquity of said axis and mirror being such as to distort said trapezoidal pattern into a rectangular one. 1
3. In a television receiver, the combination of a cabinet having a. cover hinged at the rear edge thereof, a viewing screen mounted on the inner surface of said cover, said cover being adapted to be raised into a substantially vertical position when said screen is in use, a cathode ray receiving tube mounted within said cabinet and having a luminescent screen positioned obliquely with respect to the undeflected path of the cathode rays, whereby deflection thereof'will produce a trapezoidal pattern on said luminescent screen, a mirror mounted in the front of said cabinet facing both of said screens, and an optical system including said mirror mounted Within the cabinet and having an axis oblique to said viewing screens in a vertical plane, the angle of obliquity of said axis being such as to project a rectangularly distorted image of said trapezoidal pattern onto said viewing screen.