International projectionist (Jan 1961-Dec 1962)

heat filter in the arc beam will reduce the light to a value of about 80%, and the aperture heat to about 60% of its original value. Is this a 40% reduction in the heat? Not really. In order to maintain the former level of screen illumination, the total amount of radiation must be increased by raising the arc amperage. Lumen for lumen of light, therefore, the absorption filter reduces aperture heat to about 60/80 = 75% of its original value — a heat reduction of only 25%! Interference-type heat filters fare somewhat better: Light — 85% of original value, heat = 55% of original value, lumen-for-lumen heat value = 55/80 = 65%, a 35% heat reflection. Strong TufCold reflectors reduce the heat without reducing the light at all, and hence enjoy a tremendous advantage over heat filters of all types. As a matter of fact, tests reveal that TufCold mirrors are actually better reflectors than silvered mirrors, resulting in relative lighting efficiencies of 102% -104%. On the basis of 103% for the illumination efficiency of a TufCold reflector relative to a silvered mirror, aperture heat is approximately 56% of what it would be with a silvered mirror under the same conditions of carbon trim and current. How much does the TufCold reflector reduce heat on a lumen-forlumen basis? 56/102 = 54%, a heat reduction of 46%. The heat on the film and aperture plate of the projector is approximately halved by the TufCold, and without loss of light! "Interference" the Secret of TufCold The remarkably high efficiency of Strong TufCold reflectors is due to the scientific application of the familiar phenomenon of light-wave interference. A microscopically thin transparent film of relatively high refractive index reflects waves of radiation having a length twice the thickness of the film, and transmits waves whose length is four times the thickness of the film. For most "quarter-wave" and "halfwave" films, the reflection or transmission, as the case may be, is total. The beautiful colors exhibited by a soap bubble and by an oil-slick on water are due to interference. A cold mirror is made by depositing a large number of thin layers alternately of high and low refractive index upon a highly polished glass mirror blank by thermo-evaporation in a vacuum. The substances used are selected for transparency, refractive index, high melting point, hardness, and chemical stability; and the thickness and uniformity of the layers are meticulously controlled. The completed cold mirror accordingly consists of a glass blank upon ^ow index index FIG. 3 — A cold reflector is made by depositing a series of thin alternate layers of high and low refractive index upon the surface of the glass mirror blank by a special vacuum process. As many as 9, 11, or 13 layers, both of quarter and halfwave thickness for specified wavelengths, may be used. Dirt particle dsurface mirror FIG. 4 — How a single particle of dirt on the glass surface of a rear-surface mirror produces the light-obscuring effect of two particles. Strong TufCold reflectors have the reflecting coating on the concave front surface, and hence are free from this "doubling" effect. one surface of which is coated a "dielectric stack" comprising a large number of quarter-wave and half-wave interference films. Such an arc-lamp mirror, scientifically designed and manufactured under exacting laboratory conditions, reflects to the projector aperture only the picture-forming visible wavelengths of light while allowing the invisible ultraviolet and infrared rays to pass through harmlessly to the rear of the lamphouse. No visible radiation passes through a cold mirror except an insignificant residual of pinkish light consisting mostly of extreme violet and red rays. Front-Surface Mirrors Superior Cold mirrors sometimes have the interference coating on the rear surface of the glass. These, like ordinary silvered mirrors, require the light to pass through the glass twice — once on its way from the arc to the reflecting surface, and again as it leaves the reflecting surface on its way to the film aperture of the projector. This twofold passage occasions a small loss of light and also heats the mirror. Rear-surface cold reflectors are equally as subject to pitting and scumming by materials ejected from the arc as silvered mirrors are; and they have other disadvantages. Unlike the cold mirrors available heretofore, Strong TufCold reflectors are exclusively of the improved frontsurface type. The special interference coating on the concave front surface of the glass is remarkably stable and long-lived and resists the formation of white scum and the "burning-in" of copper splashes and carbon particles. The light does not enter the glass of a TufCold reflector — the mirror remains cooler at all times, and special heat-resistant glasses which may be faintly colored can be used for extra insurance against accidental breakage. TufCold front-surface reflectors are also superior from an optical point of view. They reflect several percent more light, and the presence of dirt particles (Continued on Page 18) International Projectionist April 1961