British Kinematography (1948)

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86 OPTICAL APPARATUS FOR PROJECTION A. Howard Anstis, M.B.K.S., A.lnst.P.* Read to the B.K.S. Theatre Division on March 16, 1947, to the Manchester Section on February 3, 1948, and to the Newcastle-on-Tyne Section on March 2, 1948. EVERY projection system must consist of a light source and collecting system, the object to be projected, the projection lens and a screen. In the design of the optical system the initial considerations are :— (a) The required magnification of the object being projected. (b) The brightness required in the image. (c) The quality of reproduction, which is bound up with the resolving power of the projection lens. Focal Length of Projection Lens The dimensions of the picture frame are standard, so that if the screen size is fixed this governs the magnification immediately, the magnification being the width of the screen, say, divided by the width of the projection aperture. If then the throw is fixed, the focal length of the projection lens follows at once. The focal length of a projection lens is given by the following formula :— \ Jl^L / s — r~J^ //\ y\ \^r\ V' \ \fl /\< y\ \r\/ \ 'A""'\ / / /X s\ /\ \^-"\^ \ \ -f-J / / v^ J< \ ^^T *\ \ \ l^r^ffl*^^5teltectoo< 1 to boo i i5dop ll\\VX. nj ^i^v / * / 7 — — -JL^_^ i ■W\ \ /C ^~~>C /^^-/^ III J \7S — V / r*N V% \^^^\ / \/ x\ — \^V-<^V<^>^ / / / DxH; where (Co«r<esy of Ship Carbon Co., Ltd.) Fig. I . Horizontal Distribution of Candlepower from 8mm. H.I. Positive Carbon, burning at 60 amps. 38 arc volts Total Candlepower Crater Candlepower f — focal length of the projection lens D = throw from lens to centre of screen W= width of projection aperture (0.825 in. L — width of screen Light Source and Collecting System If the source of light were a point source, the collecting system would converge the light to a point, and the gate would be situated within the apex of the solid cone of convergent light. If we assume that a D.C. arc is being used, the hot positive crater has a definite size, so the light is not converged to a point, but to a disc which is the image of the crater. This magnified image of the crater is located at approximately the plane of the gate, and it will be seen that the degree of magnification produced is an important factor. The brightness of the spot of light on the gate will be governed by the effective angular , pick-up of the collecting system and the intrinsic brightness of the source, or that is, the grasp of lumen output of the source made by the collecting system. In deciding upon the pick-up angle of the collecting system, the polar distribution curve of the light source has to be considered, as it is no use making the collecting system with a very large pick-up angle, if there is very little light coming from the source at the extremities of the pick-up angle. ♦Ross, Ltd.