International projectionist (Jan-Dec 1935)

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Gorgeous effect work in Radio City Music Hall (N. Y.) through application of ultraviolet light process Projection ! Effect Work With Ultra-Violet Light By A. STROBL and ROBERT L. ZAHOUR MORE than three hundred years ago, the Italian scientist Balmain discovered the photo-chemical property of certain substances to absorb energy from visible light sources, and produce in themselves a luminous glow when observed in the dark. Further study of light sources, capable of energizing these substances, revealed that the longer wave-lengths of the ultra-violet invisible radiations (3,200 A to 4,200 A) are transformed by certain substances into longer waves of a length found in the visible spectrum. This phenomenal transformation varies in different materials, making available a variety of luminous colors. Pigments extracted from these substances are commonly called "ultra-violet paints," and because of their ability to glow they are said to be "luminescent" when exposed to the ultra-violet radiations. Two kinds of paints have been discovered for producing practical luminous effects — namely, fluorescent and phosphorescent. Fluorescent pigments are those which become luminous when subjected to ultra-violet radiations. These paints are available in nearly all colors of the rainbow. Phosphorescent paints will emit a self-luminous glow for some period after the ultra-violet has been withdrawn. This latter kind of paint is available in several colors. The phenomenal glowing characteristics of each of these two types of paints permit one to obtain two distinctly different luminous effects when used jointly in a single composite design. Since most of the early uses of ultraviolet paints were confined to stage effects, the arc spotlight, equipped with From I. P., September, 1933 suitable glass filters to screen out practically all of the visible light, was used satisfactorily to project invisible ultra-violet radiations onto the specially treated scenes and costumes. Because of the abundance of ultra-violet generated, the arc projector still is employed for this purpose and particularly where coverage of large production scenes at long beam throws is necessary. An outgrowth of the stage effects brought about uses for ultra-violet paints in advertising posters, small dual-scene paintings, and in miniature design novelties. For these applications, light-tight projectors equipped with dense blue heatresisting glass filters and ordinary highwattage Mazda lamps, were found more convenient than arc units. However, due to the proportionately small amounts of ultra-violet liberated by Mazda lamps compared with visible light, only medium-density filters can be used for screening out the visible radiations. For some applications, the small percentage of visible light which escapes from these modified equipments is objectionable, which in other cases it is an asset, "Black Bulb" Mercury Lamps Continued growth in the application of ultra-violet effects has led to the development of dense blue grass mercury vapor tubes, and finally, the "black bulb" mercury vapor lamps. Since the visible light from these sources consists of, chiefly, blue-green mercury spectrum lines, the thickness, of the dense blue filter glass in the tube of "black bulb" is not as great as in the screens employed for masking carbon arcs or highwattage Mazda lamps. Consequently, more ultra-violet is transmitted for useful purposes. [8] The mercury vapor tube and its accessories for operation is essentially the same as the standard Cooper-Hewitt lamp, except that the tube is made of a dense blue ultra-violet filter glass. The "black bulb" ultra-violet lamp is similar to the ultra-violet health lamp in construction details and operating characteristics, but is provided with a special dense purple glass bulb which absorbs practically all of the visible light, yet transmits a high percentage of the ultraviolet. Ziegfeld's Follies of 1922 brought forth a gasp of admiration from the audience as the stage lights were dimmed to black out, and the pale lace gowns turned luminously brilliant and many-hued. The gowns were treated with vari-colored luminous painted designs which were practically invisible to the audience under the stage illumination, but beautifully luminous in the presence of ultraviolet furnished from concealed footlights containing quartz-mercury lamps with special glass filters. Many theatrical productions since have enhanced the beauty of their settings and in some instances, effected dual-scenes on the same curtain drop, through the use of luminous paints. At the Century of Progress, Chicago, several exhibitors feature posters showing two and three different scenes through the combined use of non-luminous, fluorescent, and phosphorescent paints. In one setting, the wall paper of a room exhibits a simple conventional design of cross-section lines when viewed in ordinary light. Under the ultra-violet radiation, this simple design is transformed into a beautiful, luminously colored sea garden of various fish and flowering plants. The ceiling changes