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CHARACTERISTICS OF HIGH INTENSITY ARCS
D. B. JOY AND A. C. DOWNES*
INTRODUCTION
The amount of light used at the aperture plate of the motion picture projector has steadily increased until at the present time only the high intensity arc can furnish the light concentration necessary to satisfy the demands of the larger theaters. Recent changes which have been made and rumors of others about to occur in the motion picture industry have again emphasized the constant demand for more light on the screen of the theater. It therefore seems desirable to call attention to certain characteristics of high intensity arcs which may help in the solution of the ever present problem of increasing the useful light.
The light from a high intensity arc emanates from two distinct sources, the crater and the tail flame. The tail flame produces about thirty per cent of the total light from this type of arc but is of no value for projection because it cannot be focused on account of its large size, shape, and position. Therefore, in a consideration of the characteristics of the high intensity arc only the crater light should be studied. The characteristics should include candle power both directly in front of the arc and at various angles, the area of crater opening, intrinsic brilliancy, and spectral energy distribution for the various carbon sizes and operating currents.
The literature1 to 8> mc' contains angular distributions of candle power, spectral energy curves, and values of intrinsic brilliancy, but in many cases the conditions under which they were obtained and the identification of the carbons are not clear. The spectral energy distribution curves for high intensity arcs given in the Bureau of Standards Scientific Paper No. 539 were obtained at the given currents and voltages but include the tail flame light. These curves are therefore only of value as a means of comparison, for practically none of the tail flame is picked up by the optical system of the high intensity equipment used for the projecting or taking of motion pictures.
* Research Laboratories of National Carbon Co., Inc., Cleveland, Ohio.
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