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MANAGERS AND PROJECTIONISTS 487
In table 2 are given all of the data relative to the surfaces, both experimental and commercial. The number in the first column is merely the identification number of the surface. The values in each of the following columns is that of reflection factor as measured at the angle) indicated at the top of the column. In making these measurements a beam of collimated light was projected normally onto the surface being examined, that is the axis of the projection system was perpendicular to the screen surface. A brightness measurement was then made with the observing photometer set at the various angles indicated at the top of the columns in table 2, that is, the angle between the optical ax's of the photometer and a line perpendicular to the screen surface as was indicated by the angular values.
For further details relative to methods of measuring distribution of reflected light the reader is referred to the original paper on the subject (Proceedings of Society of Motion Picture Engineers No. 11, page 59, and Handbook of Projection, Fourth Edition, page 260). It will be noted that the value given for surface No. 1, magnesium carbonate, at 0° is 100%. This value of course is taken arbitrarily as the unit in which to express the values for other surfaces and at other angles. It should also be noted that for this same surface the reflection factor at 5° and 10° is also 100% decreasing to 97% at 50°. it is evident therefore that the reflection factor of this surface is practically independent of the angle and hence the brightness of such a surface is independent of the angle of observation. It will be noted that many of the surfaces at 0° have reflection factors greater than 100% This means of course that when observed normally, that is along the axis of the incident light beam, they will appear brighter than the magnesium carbonate surface. For instance material No. 8, which has a value of 414% at 0° will when viewed along the axis of the illuminating beam appear over four times as bright as the carbonate surface.
The reflection factor for this surface at 30° however has fallen to 73% which is appreciably less than that of magnesium carbonate, and at 50° its reflection factor is only 17% showing that a picture projected upon such a surface and observed at an angle of 50° from the axis of projection will have a brightness approximately 1/25 (17/414) as great as along the axis of projection and approximately 1/6 as bright as the same picture if projected onto a screen having reflection characteristics the same as those of magnesium carbonate. It is perfectly obvious from a consideration of these facts that the