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The problem of measuring and specifying the characteristics of diffusing surfaces has been treated by Dr. Nutting^ and results on several characteristic surfaces are given. The data given, however, relate almost entirely to miscellaneous surfaces, only two commercial screens being included among the materials examined by him. Recently a paper by Charles W. Gamble^ has appeared in which he deals with the subject in a more or less general way. Here again the data given relate largely to surfaces which are not extensively used in the motion picture theatre. His results are of a qualitative character and the highest efficiency obtained is that of a mirror ground on the front surface.
Method and Apparatus
The method adopted in this investigation is quite similar to that used by other investigators and consists in measuring the brightness of the surface under examination when viewed at various angles of observation, the illumination being incident normally upon the screen surface. The incident illumination in this case was approximately parallel light, such conditions being considered as more closely approaching those existing in practice than the method used by Dr. Nutting in which case a completely diffused source subtending 1/10 of a steradian at the surface was used. In order to expedite the experimental work which involved the examination of a large number of samples, a special goniophotometer was constructed. A diagram showing the essential parts of this instrument is shown in Fig. 1.
A heavy cast-iron base, A, supports the arm B, at the end of wdiich is carried the photometric apparatus. In order that the observer and the photometric equipment might not interfere with the illumination of the sample at angles approaching closely to the normal, the axis of observation was' bent at right angles by use of the total reflecting prism C. A portable photometer of the illuminometer type was mounted at D, and the small lens E placed immediately in front of this photometer permitted the formation of an image of the surface under examination in the plane of the photometer cube. In case the texture of the surface was such as to interfere with precise photometric settings, this lens could be displaced by amounts sufficient to eliminate the disagreeable surface texture. A rigid bearing E supported by the base casting carries a movable arm G on one end of which is mounted the lamp house H, while at the other end a counterpoise weight / is placed. A holder is provided for the sample at /, this is so mounted that it holds the surface of the samples being examined in the plane passing through the axis of rotation of the arm G. The circular scale plate K is mounted in a fixed position relative to base A. A pointer attached to the sample
^Nutting, P. G., Trans. 111. Eng. Soc, Vol. II : 1916, p. 92. ^Gamble, Charles, W. B., B. J., Vol. LXVII, 1920, p. 553.
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