Journal of the Society of Motion Picture and Television Engineers (1950-1954)

If we put the expression from (1) for , and using m = \/A'/A we get: From (4) we can compute the necessary brightness B of the Eidophor picture, if we know the brightness of the screen. If the screen area A' = 75 sq m = 7.5 X 10 sq m and the Eidophor picture area A = 7.5 X 10 sq cm = 75 sq cm, we get m = 100. In cinema projection the area A has a fixed value and is much smaller namely 3.5 sq cm, which makes m = 463. Table II presents the values of B for some values of D/f for both cinema and television. Table II m = 100, m = 462, D/f B B 1:1 175 3,740 1:2 700 15,000 1:5.5 5,250 114,000 ' B in candles /sq cm. It follows that the area A plays a decisive part, and if we compare with cinema projection we see that in television we have the advantage of having this area 20 to 30 times larger. In consequence, we can use apertures (of about 1 :5.5) which do not present great optical difficulties even in more complicated systems, and which allow quite reasonable lens diameters. However, the projection cannot be effected without losses, as we have assumed so far. The losses that we have to take into account are very considerable and may reach the factor of 10 (and more) against about three for the cinema. The brightness of the Eidophor picture actually required for the production of 10,000 1m is, therefore, in our example for an //number of 5.5 about 50,000 to 70,000 c/sq cm. But modern arc lamps, such as the Ventarc lamp developed by the firm Dr. E. Gretener, A.G., can produce average brilliancies in the crater up to and above 120,000 c/sq cm, so that the conditions in this respect are fairly favourable. I should like to point out that the figures mentioned here refer to one single example. But according to the given practical requirements, all sorts of variations can be made. The only purpose of the figures given above is to show that the production of light fluxes as required in the largest theatres can be effected by existing means. 3. Description of the Optical System The basis of our projection system is a kind of light valve and we shall presently see the elegant principles that govern the light control. The most important element is the so-called schlieren-optical system. Figure 2 shows the diagram of such a system. The image of a system of bars and slits, lying in the plane S and illuminated from the left, is projected by the schlieren-optical system Os on to an analogous system of bars and slits lying in S'. As shown in our diagram, the images of the slits 7 and 2 fall on the bars T and 2', and the images of the bars, a, b, c, on the slits a', br, c' . Under these circumstances no light is allowed to pass into the space to the right of S' '. We now expand the system by joining a new lens 0P immediately behind .S" (Fig. 3) and choose its focal length so as to project the image of a plane P near the schlieren lens Os, on to the screen P'. A small glass prism N is now placed on a point of the plane P. Consequently the light-pencil, passing N is deflected from its original direction according to the deflection angle of N. The rays of this pencil can therefore partly pass the bars T and 2' and are focused by the lens Op on the screen in N't where they appear as a luminous point. The brightness of the point Nf depends on how much of the light-pencil is allowed to pass bars 7', 2', and this varies according to the deflection angle of the prism. We E. Baumann: Fischer System — A Reprint 347