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Condensers
By C. Francis Jenkins
In optical projection, the lantern slide, or the single frame of the motion picture film, is simply a stencil to let directed light therethrough to a receiving screen upon which the enlarged stencil is imaged. This stencil may be opaque with transparent portions, in definite arrangement, as in titles, or it may also contain half tones as in pictures. If light filtered by this stencil were projected from a point source, a projection lens would be unnecessary.
Sharpness of image on the screen depends, therefore, on those rays of light which meet most nearly in a point at or near the shutter position, and in the exact axis of the projection system. All other rays tend to blur the image on the screen though adding to the illumination.
For proof of the first proposition one has but to shift the position of the light source considerably to one side of the axis of projection, while still keeping the aperture fully illuminated, a position which gets no picture at all on the screen; and in proof of the second proposition, to observe that a light source of large area will not, even with the best lens, put as sharp a motion picture on the screen as a point source.
If one had a large stencil to work with, as a lantern slide, a fairly good screen picture could be had with a rather large light source, but as the motion picture frame has less than one-eighth the area of the slide, the most concentrated and intense source of light is required and the highest possible corrected lens.
Given, therefore, a well-corrected lens which will make a picture in spite of defects in the light, further improvement in the system must be in an effort to gather the greatest amount of light and direct it through the stencil of the frame, in the most right lines to a common point, and with the least possible loss.
Designers of early motion picture projectors had the misfortune to begin by using the two thick, light-wasteful, glass condensers employed in magic lanterns. In the lantern the condensers must have a diameter greater than the diagonal of the slide, hence the selection of the 4>^-inch diameter in lanterns.
But as the diagonal of the motion picture frame is less than 1% inches, a thin, 2-inch condenser would have been quite ample, and we would have avoided the great light loss in the use of 4>^-inch condensers, and which has been variously estimated by different authorities to be 75 to 80 percent. Which simply means that if we could effectively employ it, 10 amperes of electric current would do our work where 40 amperes is now required.
Any conservation of light lost between the source and the aperture frame is, therefore, well worth our best efforts, and to determine how we shall proceed, we should analyze the losses and seek the sources. Certainly one of the most glaring is the light reflection and absorption loss and chromatic fringing of the glass of the present system of condensers.
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