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160 HANDBOOK OF PROJECTION FOR
Practical Projection Optics
IMPORTANT: The following has principally to do with the plano-convex condenser, though in some points it will apply more or less equally to the Cinephor parabolic condenser, and the reflector type of arc lamp light source, both of which will be dealt with separately.
THE optical system or optical train of the motion picture projector is made up of two entirely separate elements, or lens systems, which are joined together in such a way that one system (the projection lens) receives at one of its conjugate foci points the magnified and more or less out of focus image of the light source projected by the other system, the condenser.
The optical train of the projector consists of the condenser, (A) the office of which is to pick up as great a number as possible of the diverging rays from the light source, refract them into converging rays and concentrate them into what is known as the "spot" at the aperture of the projector, and (B) the projection lens, the office of which is to pick up the film image illuminated by the concentrated rays from the condenser and project it to a focused image upon the screen.
Joining these two optical systems is easy if we disregard the item efficiency, but to join them in such way that they will work together with the greatest possible degree of efficiency is somewhat intricate, and in some conditions met with, an almost impossible problem. Yet, unless the two lens systems are made to work together with the least possible loss, the waste in light, which means waste in electric power, may be a very serious matter, indeed.
We do not believe the optical system of the modern projector can be judged by ordinary optical standards, because of the fact that the conditions under which the lenses must work are themselves, in almost every particular, highly abnormal. In the case of the condenser the heat is excessive under any condition, and when the amperage mounts to 80, 90, 100, and even in some extreme cases higher, a condition is set up which calls for special treatment. In the case of the projection lens, it may, under some conditions, receive along with the film image it is to project to the screen, a more or less in focus image of the floor of the electric crater, or Mazda filament projected by the condenser system, and unless the projectionist very thoroughly understands his