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128 RICHARDSON'S BLUEBOOK OF PROJECTION
7 inch working distance, the zone readings average: center 13 c. p., zone two 10.1, while zone 3 holds up to 5.6, which is almost its value under the other condition.
Failure to get the entire light beam into the projection lens means very real loss of light. Long focal length lenses generally provide more even screen illumination than the shorter focal length ones.
In Fig. 40 we see a normal light beam from a 2-lens plano-convex condenser. Note the circle of light on the back of the aperture plate. It is caused by light reflected from surfaces of various elements of the projection lens, each of which adds its quota. When the picture was made the lenses were very clean. Had they been fogged with dust or dirt, reflection loss would have been increased in proportion to the amount of fogging.
Projector Optical Train
(114) The projector optical train consists of two entirely separate elements joined together more or less efficiently. Efficiency of the jointure depends upon the amount of skill with which it is made.
(115) First, the condenser or mirror which collects light from the light source, bends or reflects its diverging rays and concentrates it upon the cooling plate and aperture of the projector mechanism. Second, (116) the projection lens, which receives the rays passing through the film photograph and focuses all their millions of numbers upon their appointed spot in the screen image.
Light leaves the light source in diverging rays (Figs. 30 and 38), which move forward until the face of the collector lens or curved mirror is reached. (117) The light source and collector should be as close together as possible.
(118) Examining Fig. 42, the reason for minimum distance becomes self-evident. We see that it would require a collector 7.25 inches in diamter located 4.5 inches from the light source to collect the same amount of light available to collector 4.25 inches in diamter only 2.5 inches from the light source.