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56
W. B. RAYTON
[J. S. M. P. E.
area in the plane of the film is not large enough. The size of the illuminated area can be increased, however, by reducing the distance from arc to condenser. An adjustment can be found in which the spot at the film gate will be large enough to circumscribe the 23 X 46 mm. rectangle. Such a condition is represented in Fig. 4. It is obvious that much light will be intercepted by the film gate, but still the illumination will be greater than we might expect as a result of comparison of screen image sizes. In reducing the distance from arc to condenser we have increased the amount of light picked up by the condenser and we are not limited entirely to the light flux which
FIG. 4. Ordinary round spot projected on the plane of the film showing great loss of light for pictures of 1:2 ratio of height to width.
passed through the aperture in the film gate while we were projecting ordinary 35 mm. film.
The old 4*/2 in. diameter condensers with the high intensity arc, however, did not exhaust the possibilities of the projection lens in respect to its angular aperture. One obvious means of increasing illumination, therefore, lay in employing condensers of larger converging angle. Since the approach of the arc to the condenser cannot be carried on indefinitely this led at once to larger condensers. We found it possible to obtain a marked increase in angle with condensers of 6 in. diameter with aspheric surfaces, of course. A substantial increase in illumination resulted.