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168 HANDBOOK OF PROJECTION FOR
symmetrically to their proper place, the refraction will be uneven, the resultant illuminaton of the film will also be uneven, and since the projection lens receives the image of the film exactly as it is illuminated by the condenser, it follows that the screen itself will be unevenly illuminated.
We learn from this the importance of having a condenser with a ground surface of perfectly true curvature.
The condenser lens which has poor polish will be a source of continual loss, because the poorly polished surface will
Figure 38.
reflect a greater percentage of the light than will one with high polish. Optic Projection, page 602, paragraph 841, says :
The polished surface of a lens reflects some light about 4 per cent, or 5 per cent, at each surface between glass and air; 8 per cent, to 10 per cent, for each lens or plate of glass. If the surface of the glass be not perfectly clean or perfectly polished the light losses may amount to much more, sometimes 15 per cent, at each surface.
We thus see the great importance of having not only perfectly clean lenses but lenses with highly polished surfaces.
TYPES OF CONDENSER.— For motion picture projection, where the electric arc is the light source, there are three types of condenser (other than the curved mirror used with the reflector type arc lamp, which really is a condenser, though we do not call it that) in use in the United States of America and Canadian America.
They are (A) the plano-convex, consisting of two planoconvex lenses, which in practice should be spaced with the crests of their curved surfaces not to exceed one-sixteenth of an inch apart, (B) the Cinephor parabolic, which consists of a plano-convex collector lens of suitable focal length for the work in hand, and a parabolic converging lens of fixed, unvarying focal length and (C) the miniscus bi-convex condenser, which is very little used since the Cinephor parabolic was put out by the Bausch and Lomb Optical Company. Its only advantage over the plano-convex condenser is that it gives a longer distance from the optical center of the condenser to the film ("Distance Y") for any given arc distance, than did the planoconvex, hence it possessed advantages when the projection lens working distance was long. It consists of a meniscus