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41'
CINEMATOGRAPHIC ANNUAL
profitable. This article will be limited, therefore, to a consideration of some aspects of the illumination problem arid to some interesting phases in the history of the development of photographic lenses.
The most interesting optical unit in illuminating equipment is the parabolic mirror. Why is a mirror used at all and why is it para
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Fig. la — (Left) Diffuse reflection from
magnesium carbonate block. Fig. 1 b (Right) Specular reflection from a plane mirror.
bolic? The first may seem like a silly question to which the answer is obvious but the following discussion of its performance may present some new points of view, at least, to some of the readers of this article. If we assume any small source of light such, for example, as a 6 V automobile headlight bulb placed in the middle of a room it radiates light in all directions. The distribution will not be absolutely uniform because parts of the filament intercept light from other parts and the base of the lamp will cast a shadow but except for the region under the base of the lamp all portions of the room will receive direct light. If we now place a blackened screen on one side of the lamp it will intercept part of the radiation and cast a shadow. The intercepted radiation is lost. It is absorbed by the black pigment and its energy used up in raising the temperature of the screen; the light has been transformed into heat. If we substitute a white screen, the illumination of the area of the room in front of the screen will be increased while the region back of the screen is dark. If the white screen is a perfect reflector no light will be lost; it is simply directed to a different part of the room.
A block of magnesium carbonate is a nearly perfect reflector, so is also a silvered plane glass mirror. They are greatly different in their behavior, however, for the first is practically a perfect diffuse reflector and the second a nearly perfect specular reflector. The difference in their behavior is indicated in Fig. la and lb. In Fig. la a number of rays of light are shown emanating from the light source and fall