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FUNDAMENTALS OF ILLUMINATION IN MOTION PICTURE PROJECTION
By R. P. Burrows
A paper presented before this Society entitled "Light Intensities for Motion Picture Projection" called attention to the many factors which enter into the determination of a suitable light intensity for the motion picture screen. Since this paper was presented, experience in the motion picture industry has shown the necessity of bringing to the attention of those interested in the development of the industry, through motion picture journals, conventions such as this, and through personal contact, a conception of the fundamentals of illumination measurement.
In reviewing the Transactions of this Society and the handbooks on projection, it is found that serious attention has not been given to light measurement. It seems desirable at this time to present material in a simple form which can be used as a basis for educational work which it seems to me this Society should do to help eliminate the guesswork from motion picture projection.
Comparisons of the candlepower of arc lamps when employing right angle carbons, inclined carbons, various sizes of carbons, various ampere strengths, etc., etc., are interesting and very instructive from certain standpoints, but the one result in which the projectionist is chiefly interested is the brightness of the picture to the eye of the observer. It is readily apparent that because of differences in the optical systems, differences in the reflection characteristics of various screens, differences in the fineness of adjustment, etc., two sources of very different candlepower might give equal results so far as the spectators are concerned.
To obtain a true comparison of the results obtained with different systems, it is necessary to use units of measurement which are not familiar to the motion picture industry. Just as we once had to learn that there is a unit called the yard which is used to measure length, that the gallon is a unit used to measure quantity and contains 231 cubic inches, so in dealing with screen intensities certain fundamental units must be studied before measurements can be made and before definite relations of cause and effect can be comprehended or expressed. Basic definitions have a very academic and sometimes a very technical sound, although the units themselves, once their definitions have been assimilated, and not merely learned by rote, are comparatively simple. The definitions need not be committed to memory but should be thoroughly digested so that the distinction between the different units will be grasped and a working knowledge of what each stands for and the quantity it represents will be obtained. Very few electrical engineers or electricians could offhand give a basic definition of the ampere*, although they might all know that it is a measure of
* The ampere is defined technically as the practical equivalent of the unvarying current which, when passed through a solution of nitrate of silver in water, in accordance with standard specifications, deposits silver at the rate of 0.001118 gram per second.
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