International projectionist (Jan 1963-June 1965)

H INTERNATIONAL PROJECTIONIST *'-■* — « * ■ . . —-_—___ — „... -., ■-^— Volume 39 September, 1964 Number 9 A Resume . . . Xenon Projection Lamps By Don V. Kloepfel (Reprinted from the Journal of SMPTE, June, 1964) The motion-picture industry has depended primarily upon two sources of light for projection purposes — the incandescent lamp and the carbon arc, each of which has certain disadvantages. The incandescent lamp has an adequate life span, consistent with its cost. It is easily replaced and aligned, and it uses alternating current, which is universally available. A major disadvantage, other than limited output, is the color of the light it emits. Across the visible spectrum, incandescent light is deficient in the short or "blue" wavelengths, and the amplitude of the longer or "red" wavelengths is high. The result is a yellow light, rather than pure white light, which is desirable because it more closely resembles average daylight (Fig. 1). The carbon arc overcomes these difficulties. The light emitted is blue-white, approaching the color of daylight (Fig. 1). Also, it is the most powerful source of artificial light known, therefore the size of motion-picture screens under carbon arc illumination can be determined more by such factors as heat dissipation, optical design, and motion-picture film properties than by the limits of carbon-arc brightness. A disadvantage is that carbon arcs must be vented because the carbon and core materials decompose, forming gas and ashes. Also, they require elaborate feed mechanisms to keep the positive crater properly positioned in relation to the optical system to prevent screen illumination changs in color, brightness and distribution. Current changes also affect the color of the light. The xenon high-pressure projection lamp emits light that is essentially flat across the visible spectrum (Fig. 1) . Other distinctive characteristics under typical operation in laboratory and studio review rooms are described below. The warranted life of a typical xenon high-pressure lamp is 1500 hr. This warranty can be given only if auxiliary equipment meets the requirements established by the manufacturer. The rated life is based on an offcycle of 20 min, such as would be the case in most theaters. Film roll sizes in studio and laboratory review rooms are quite likely to be small. If the lamps were extinguished at the end of each roll, the on-off cycle would be increased. Damage to the lamps would result, due to the increased number of ignitions. One report stated that, when on-off operation was abandoned and the lamps left burning continuouosly for a 13-hr. period each day, lamp life increased from an average of 750 hr. to over 1800 hr. Xenon lamps require a direct-current power supplv with a no-load voltage of at least 72v and an a-c ripple content of less than 17%. Practical operation has indicated that useful life will be further increased with ripple lower than that figure. In fact, most well-regulated power supplies have far lass ripple; in some cases less than 1%. Several manufacturers have designed power supplies for xenon lamps that meet these requirements. They have included relays to provide 220-v d-c for the igniter and also some means of increasing the d-c current while the lamp blackens due to decomposition of the tungsten electrodes. Sixty percent of the installations surveyed included rectifiers designed for xenon service. Forty per cent had modified existing rectifiers and generators by adding ballast resistors and, in some cases, increased filtering. The xenon arc appears to be inherantly unstable during the first few minutes of operation after ignition. Flicker was reported during that period in every installation reached by the survey. After a short time, the International Projectionist September, 1964