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1948-49 Theatre Catalog, 7th Edition, Page 544 (529)

1948-49 Theatre Catalog, 7th Edition
1948-49 Theatre Catalog
1948-49 Theatre Catalog, 7th Edition, Page 544
Page 544

1948-49 Theatre Catalog, 7th Edition, Page 544

flective optics developed for television delivers thirty percent of the light output of the kinescope to the screen.

Reflective mirror and aspheric correcting plate are an adaptation of Schmidt optics. The principal feature involved is in a method developed to correct the spherical aberration of the system for finite focus. The elements of the optical system are shown, in Fig. 3, consists of the spherical mirror, the correction lens, and the projection kinescope tube face. The form of the correction lens design can be seen from the figure and by tracing several rays we can see how the "S2 curve is evolved. The shape of the correcting lens must be such that all rays emanating from an object point 20" on the tube face and reflected by the mirror, shall meet at the image point 21,, located at a distance 282 from the correcting lens. The figure 3 shows three rays emanating from "O" and striking the mirror at different apertures. Without the presence of the correcting lens, rays 1, 2, 3, would intersect the axis at distances Q1, Q2, and Q3 from the center of curvature. The slopes on the correcting lens have to be such that all three rays intersect at I; hence the lens has hat zone where ray

'2 passes, a negative slope where ray 1 paSSes and a positive slope where ray 3 passes. The shape of the lens can thus be determined by tracing a number of rays to evolve the final curve. Such a curve is very difficult to generate and must be made by dividing the glass lens into a number of zones and grinding them progressively and in the final operations blending them into a smooth curve; as you may imagine this is a difficult and costly operation.

The lenses now used in large projection systems are made of plastic, formed in glass moulds by a cold setting process; the results are excellent and lenses as large as twenty (20) inches in diameter can be made by this process.

For optimum design the diameter of the mirror is three times that of the kinescope and the lens twice that of the kinescope. The radius of the mirror is approximately the same as its diameter while the radius of the kinescope tube face is approximately one half that of the mirror. These general criteria are for optimum efficiency of the system, therefore the size of the elements of the system depends on the size of the kinescope, and the characteristic of these designs are one of short focal length giving short projection throw distances.

Reflective optics have been designed for large screen projection for pictures up to 18 x 24 feet; systems for a 7% x 10 foot screen use a 21 inch mirror, 14 inch lens and a 7 inch 50 kilovolt kinescope. The largest system ever built consisted of a 42 inch mirror, a 26 inch lens, and 12 and 415 inch projection kinescopes operating at 80 kilovolts; the throw was fixed at 40 feet and by changing the kinescope either a 15 x 20 or an 18 x 24 foot picture was shown, the magnification being fixed by the mirror radius. Limited production facilities and the high cost of the 42 inch mirror system indicated the advisability of concentrating on smaller optics and increasing the voltage capabilities of the seven inch



FIGURE NUMBER 2. Functional diagram of typical. projection kinescope.

projection kinescope in order to make a compromise system which might be successful commercially.

Viewing Screen

The viewing screen from the third and final element of direct projection television. Standard motion picture screens have a diffuse surface which distributes the light in all directions. A great deal of light is then lost to the ceiling and fioor. Directivity therefore in the vertical plane could concentrate the light where it can be most useful and effect an important gain. Beaded screens have been made to increase the light from the screen, but the directivity pattern while showing a gain of two does not restrict the vertical reflective pattern and in addition tends to refiect a great deal of illumination back to the optical system where it reduces the contrast of the images. Developments in directional screens are underway and promise gains as high as three. A lenticular screen of this type was successfully used in the Philadelphia Fox Theatre where an 15 x 20 foot picture was shown featuring the Louis-Walcott fight; this screen is embossed on an aluminized surface, with small convexed lens element to control the directivity pattern. The observed results were excellent and a gain of two and one half times was measured.

Rear Screen

The rear or transmission screen should be reviewed because it may seem to be ideal for large screen applications. Such is not the case with a normal translucent screen because, the light being from a small source, is a diverging cone of light at the screen. The screen will be receiving direct rays normal to the screen in the center and diverging as you go toward the edges; the result a hot spot in the center of the screen.

A field lens can be used on the rear of the screen to bring the rays in a parallel pattern and hence give even illumination over the entire screen or by a modification the pattern may be made to suit the application. The field lens can only be accomplished in small screen such as the home type projection sets where a moulded plastic screen is used. A compromise screen of high density translucent material can be made, but the gain will be low and the directivity pattern becomes very sharp.

Intermediate Film System

The alternate system of large screen television projection, which was referred to earlier as the intermediate film system also consists/of three major units. The first is the television recording unit with a high quality television monitor and a special 35mm motion picture camera, the second consists of a high speed

FIGURE NUMBER 3. Layout diagram for reflective lens system.
1948-49 Theatre Catalog, 7th Edition, Page 544