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220 HANDBOOK OF PROJECTION FOR
screen surfaces on looking at the performance of screens in different theatres.
This is an utterly unreliable test, because it almost never happens that there are two screens in neighboring theatres where the various factors which may affect the result are of equal value, and the working conditions precisely alike.
In any two theatres the brilliancy of projection light may, and in all human probability will, have different value, because of differences (a) in amperage at the arc, (b) in crater angle, (c) in the carbons themselves, (d) in the condensers or their spacing or discoloring, (e) in the general adjustment of the optical train, (f) in the projection lens diameter or working distance, and (g) in the revolving shutter.
The result may also be very much altered by the decorations of the theatre, by its lighting, by the number and arrangement and power of the orchestra lights, by the screen surroundings, the screen border, the shape and height of the auditorium, size of the picture, angle of projection, etc., etc., through a long list.
In fact the things affecting apparent screen brilliancy in any given theatre are so very many that the judging of relative screen values by observing the picture in various theatres is an utterly futile endeavor.
It is even impractical to judge closely of values by substituting one screen surface for another while a picture is running, because of possible differences in light values. Suppose we run half a picture on one screen and then drop another down to receive it, but the crater angle has, unknown to even the projectionist, changed, or the supply voltage has dropped, thus altering the light brilliancy considerably.
The only way such a test can be made with any assurance of reliabie results is to cover half the screen with the surface it is desired to test, and then project a picture, observing results from all parts of the theatre.
This is a test which is in every way fair ; also it is not a difficult one to make, but exhibitors and projectionists should remember that it is not to be expected that the surface of a screen which has been in use for a considerable time can enter into successful competition with a new surface.
We would most emphatically warn exhibitors, projectionists and theatre managers of the danger of judging hastily as between various screen surfaces. We would also caution