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THE FUNCTION OF THE CONDENSER IN THE PROJECTION APPARATUS
By Hermann Kellner
Light radiates from every point of a source in all directions. If we had a very intense, and at the same time cold, point-shaped source, projection would be possible without any additional optical apparatus. Supposing we have such a source s separated by the distance d from a stencil / and at a distance D from the projection screen, the size
of the image on the screen would be j times the actual size of the Stencil or, differently expressed, the magnification M+M^-5. The
magnification may be changed in two ways:
(a) By keeping the projection distance D constant and varying the distance between source and stencil.
(b) By keeping the distance d the same and varying the projection distance.
The illumination on the screen is more favorable with the shorter projection distance, because a greater amount of the radiation from the source is utilized. The illumination in the center of the field is inversely proportional to the square of the distance D of the screen from the source. For points outside of the center of the projection field the illumination is less than in the center. If a be the angle from the source to a point of the field distant the length b from the center, the illumination / at the point will be proportional to the cosine and the square of the sine of that angle and inversely proportional to the square of the distance from the center of the field to the point.
cos,a s'm^a
To utilize more of the radiation from the source, we apply a condensing system, which takes in as large as possible a solid angle of the radiation from the source, and forms an image of this source at which the rays forming this image meet under a smaller angle or, more generally speaking, under an angle which is more suitable to the particular conditions of the case than the large angle of radiation taken in by the condenser.
By combining an optically perfect condenser, i. <?., a condenser which forms a point-shaped object, with a point-shaped source of great intensity, a very simple and perfect projection apparatus could be realized, which would need no projection lens and which would utilize a greater percentage of the energy radiating from the source than the condenserless arrangement. For example: In figure 2, s may be a point-shaped source and the distance between s and the condenser c may be shorter than the distance from the image of the source s' to r, so that the aperture of the condenser appears under a greater solid angle from the light source than from its image. If we assume
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