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142 MOTION PICTURE PROJECTION
o-lite lens the former of these two methods was adopted and skilfully executed. The "back focus" of the ordinary type of 6% -inch lens is about 6 inches while in the case of the Keeno-lite it is only a little more than one-third as much, and the clear aperture of the back lens is only slightly smaller than that of the other. The entering beam in the case of the "Keeno-lite" is therefore very considerably larger, and with it the total brightness of the picture as was verified by test.
There are, however, two further advantages of the Keen-oHte lens which are even more important than the foregoing. They are: a more uniformly illuminated picture, from center to edge or corner, and better definition.
The rays which the ordinary lens does not utilize are to a much greater exTent from the edges and corners of the film than from the central part and therefore would increase the illumination of these parts of the picture relatively to the center, thereby producing a more uniform illumination over the entire surface of the screen.
The definition given by a lens can in general be made more perfect by the addition of more elements and curved surfaces. The addition of the extra element of the Keen-o-lite has produced a lens of very decidedly better definition than any other.
The figures hereinafter given were obtained with a set-up like that of the average machines using 6^ -in., 7^-in. condensers. The source was brightly illuminated opal glass bounded by a ^-in. circular aperture in imitation of the positive crater. The position of source condensers and frameplate were absolutely the same during all tests and the brightness of the source invariable. The conditions were hence the same as in the projection machine itself but their invariability made the test much more dependable than if an arc had been used. The results of the test were as follows:
Keen-o-lite gave 12.5% more light at center of screen. Keen-o-lite gave 82.5% more light half way out to corner. Keen-o-lite gave 63.0% more light at corners.
Integrating these results we find that the total illumination of the screen is about 32.8% greater in the case of the Keeno-lite lens over the lens of another make.
The definition given by the Keen-o-lite ' is also decidedly superior.