Preliminary Measurements of Illumination in Motion Picture Projection

W. E. Story

First efforts to compare accurately the total illumination in motion picture projection with different optical arrangements, led to such improbable conclusions as to point to errors of observation so large as to render the required comparison unsatisfactory. The problem is to compare the quantities of light falling on the objective lens through a standard motion-picture aperture, with different sizes of source and objective; with different condenser systems, and with different distances between the various components. The reason for considering the light falling on the objective rather than on the screen is that the efficiency of the objective depends on its design, and with the large number of designs now at hand, the addition of this complication seemed inadvisable—especially as this problem belongs to the lens designer alone, and has nothing to do with the relation of the different parts of the projection apparatus. Accordingly, for the objective lens there was substituted a dummy objective, that is, a circular hole in a brass plate, this hole being of the size of the assumed effective diameter of the lens and the plate being at a distance from the aperture equal to the assumed focal length of the lens. Immediately behind this dummy objective was put a piece of flashed opal glass considerably larger than it, and 4 inches from this glass another similar one—a connecting brass tube blocking out stray light between. The illumination of the center of this second opal glass by the first is very even, and is proportional to the total light flux delivered by the rest of the optical system to the objective. The even illumination of this second opal glass was measured by a photometer mounted directly behind it, and built in as one assembly with the opal glasses, the dummy objective, and the aperture plate. The aperture plate was 5½ inches in front of the dummy objective; the 5½ inches corresponding to the focal length of an objective frequently met in practice. An arrangement was made for changing the size of the objective opening.

Print ISSN
Published
1919-10
Content type
Original Research
DOI
10.5594/J04159