International projectionist (Jan 1961-Dec 1962)

ture, and the projected picture will be dim and brownish. And if the end of the positive carbon be too far from the mirror, the picture will be dim and bluish because the arc stream, not the white gas-ball in the crater, will be in focu;. The maintenance of the correct arc focus at all times for a constant, snow-white light of maximum brilliance is the job of the arc-control and cabon-feeding mechanism of the arc lamp. Efficiency of Modern Mirrors The optical efficiencies, or "speeds", of modern arc lamps have been substantially increased for the projection of CinemaScope and other wide-screen films by the use of reflectors larger than those formerly employed. The speed of any lamp can be expressed by in "f-number" obtained by dividing the working distance minus the depth of the mirror curvature by the diameter of the mirror, all in the same units of linear measurement, such as inches. Whereas the older lamps had speeds of from f.2.5 to f.2.3, modern lamps employing 16%", 18", and 21" mirrors have speeds ranging from f/2.0 or f/1.9 to f/1.7 or f/1.6. The resulting wider angles involved in the light beams obtained from the newer lamps naturally require "faster", large-dh meter projection lenses for maximum light utilization and brighter pictures on today's wide screens. Faster optical speeds are largely responsible for the tremendously gratifying success of the new large-diameter mirrors in meeting the greater light requirements of projection on giant panoramic screens. Still another chapter of this "success story" is to be found in the greater light-collecting power of large-diameter mirrors. These reflectors intercept "solid angles" of arc radiation ranging from 120° to 152°, and even more, while the new Strong Jetarc lamp with its 21-inch main mirror and special auxiliary mirror has an effective light-collection angle of more than 260°! This is a far cry indeed from the 45° collection angle of old-style condenser lamps. The shorter the arc-to-mirror distance, the greater the collection angle and the magnification of the image of the positive crater upon the aperture plate of the projector. A magnification of from about 5 to 7l/2 diameters is used, and is necessary because the actual luminous crater is much smaller than the 35-mm film aperture. This is as it should be. The remarkably high optical efficiency of the carbon arc as a projection illuminant is due to the concentrated nature of the light source. Magnification of the crater image (very nearly 5.4 diameters in modern large-mirror lamps) is the ratio of the image-to-mirror and crater-to-mirror distances. The amount of magnification is established by the manufacturer of the lamps when he designs them. He provides enough magnification to insure adequate corner-to-corner coverage of the film aperture with smooth, brilliant illumination. Needless to add, the lamp designer's intentions can be realized in practice only when the utmost optical precision is built into the reflector as an integral ingredient. Mirror Maintenance Essential The preservation of arc -lamp mirrors in a state of good condition for as long as possible is an important part of the projectionist's duties. It is equally his job to urge the immediate replacement of damaged and deteriorated reflectors. Although the life of mirrors can be extended by exercising care in striking the arcs and keeping the glass surfaces scrupulously clean at all times, a gradual loss of reflective power with long periods of use is inevitable. To repeat: the waste of carbons and power caused by pitted and "burned" reflectors is very serious, but the loss of patronage by projecting badly lighted pictures is a risk no theatre can afford. The two carbons should be separated quickly when an arc is struck. Only in this way can the danger of severe "sooting" of the mirror be avoided Black soot absorbs and releases heat so rapidly thit inequalities in the heating of the glass, with the development of mirror-cracking strains, are inevitable. A sooted mirror may break during projection or while it cools off after the reel has been run. Cracked and chipped mirrors should be replaced without delay in order to restore full light to the screen and to avoid the possibility of complete breakdown and loss of light during a show. Severe pitting of the reflectors in simplified high-intensity lamps by molten copper can be minimized by placing a wad of steel wool or a copper-net cleaning pad in the ashtray under the arc to catch the copper drippings and prevent them from splattering upon the mirror. Mirror-Cleaning Procedures Avoid wetting the backing compound when cleaning arc-lamp reflectors with water. Arc dust should be wiped from the glass surface with a soft, dry cloth; burned-in copper splashes may be removed with a razor blade; and the white scum washed off with water and soap or Bon Ami scouring powder. Stubborn deposits may require the cautious use of steel wool. (Continued on Page 18) FIG. 5 — The optical action of arclamp reflectors is based upon the geometry of the ellipse. The "major axis" and the two "foci" are important elements in the alignment and optical adjustment of arc lamps for maximum illumination efficiency. The "working distance" is the distance between the film aperture and the center of the mirror, and should be established as closely to the lamp manufacturer's specifications as possible to insure the correct size of "spot" on the aperture plate. |-*-Reflector focal plane K-3taj| SCREEN LIGHT Vjf t_4^B BRIGHT & WHITE White gas-ball in focus i-*-Focal plane Yellow shell in focus SCREEN LIGHT DIM & BROWNISH i"*"Focal plane SCREEN LIGHT DIM & BLUISH Blue arc stream in focus FIG. 6 — The intensely luminous gasball in the crater of the high-intensity positive carbon should be positioned at the "geometric focus" of the reflector for the brightest, whitest illumination of the projected pictures. International Projectionist March 1961 9