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holder indicates the angle on the divided circle. By means of a pin M, the sample holder can be connected rigidly with the moving arm G, so that the plane of the sample will remain perpendicular to the incident illumination for all positions, and as the arm G is rotated the angle of observation alone varies. This provides for the measurement of the surface brightness at various angles of observation and fixed direction of illumination. By removing the pin M and clamping the sample holder to the base, A, the arm G moves independently of the sample, and observations of brightness at a fixed angle of observation, but with a variable angle of incidence can be made. In the front of the lamp house is mounted a lens O of such focal length that the source 5^ falls at its focus. Under such conditions the light incident on the sample is approximately parallel. The dimensions of importance are as indicated in the figure. It is also so arranged that the lens O can be removed and in its place substituted a disk of diffusing material such as ground pot opal glass.
The dimensions are so adjusted that the effective area of this diffusing material is just sufficient to subtend an angle of 0.1 steradians at the surface of the sample under examination. A 500-watt Mazda C lamp of the concentrated filament type was used as a source for illuminating the samples. A photograph of the apparatus is shown in Fig. 2. The voltage was precisely controlled in order to eliminate variations in brightness due to fluctuations in the line voltage. Brightness measurements were made at angles of observation from 0 to 70°. Three complete sets of readings were made by two observers on each screen, and the final result obtained by taking the average of all readings. In practically all cases excellent agreement between the individual sets of data was obtained.
The results are expressed in terms of the reflecting power of magnesium carbonate with normal illumination and observation as 100%. The absolute value of the reflecting power of magnesium carbonate is at the present time subject to some question and in view of that fact it was not deemed advisable to attempt to reduce the measurements of screen reflecting powers to absolute terms. The values given are therefore relative to that of magnesium carbonate. AMien a satisfactory value for this material is determined that data can then be reduced to absolute terms, if such seems c'esirable.
Materials Esa mined
Orders for samples of the commercial projection screens were placed with practically every maker whose advertisement could be located in the trade journals. Response was not obtained from all makers, but a fairly representative group of samples was received. The standard samples were prepared by mounting a piece of the projection screen 8 inches square on a thin metal plate, thus insvu-ing flatness of the sample under examination. In addition to the commercial screens mentioned, several miscellaneous surfaces of interest were examined.
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