International projectionist (Jan 1963-June 1965)

Reasonably uniform illumination on drive-in screens is desirable. Side -to -center light distributions of less than 60% results in a center -screen "hot spot" they should be of the finest quality flat-ground and polished colorless optical glass. Anti-reflection coating of projector-port glasses is desirable, but not absolutely necessary. All available methods of reducing heating of the film and projector gate should be utilized in drive-in projection, otherwise it will be impossible to get sharply focused pictures on the screen. Air cooling of the film is optional, however, as there is some doubt as to its efficacy in reducing film flutter. Water cooling of the gate and aperture is a necessary adjunct to successful drive-in projection: and the use of either heat filters or cold arc-lamp mirrors must be regarded as an absolute necessity. Interference-type heat filters ( not the older absorption type! ) should be used with Suprex-type lamps burning 8-mm positive carbons at 70 amps, and with Suprex lamps burning 9-mm carbons at 75 or 80 amps. The more powerful rotating-positive high-intensity mirror lamps require either heat filters or cold mirrors, but the latter being preferred because they reduce heat as effectively as the best filters without wasting light as filters do. Light, it must always be remembered, is at a premium in drive-in projection. Filters and Cold Mirrors Compared Old-fashioned infrared-absorbing filters reduce heating of the film by about 40% with an accompanying light loss of 20% . Interference, or "dichroic,"* heat filters do not absorb infrared radiation, but reflect it tack toward the lamphouse. These reduce the heat by approximately 45% with a 15% light loss. Cold mirrors, on the other hand, allow the invisible heat-producing infrared rays to pass through into the rear of the lamphouse, and thus reduce heating of the film by about 46% with no loss of light relative to the efficiency of ordinary silvered mirrors. In order to reduce focus-ruining heat on the film without the light losses occasioned by heat filters, we urgently recommend that interference (dichroic*) cold mirrors be installed in all rotating-positive reflector lamps, and that heat filters be eliminated. As a matter of fact, the successful operation of all the more powerful high-intensity arc lamps, including those of the blown-arc type, absolutely requires the use of cold mirrors. Because of their prodigal consumption of power and relatively low luminous efficiency, condenser-type arc lamps are not recommended for drive-in use. A few of the larger indoor theatres retain these old powereaters because they are relatively insensitive to small errors in arc focus, making their operation very simple, and because they provide exceptionally uniform illumination on the screen. Frequent replacement of chipped and cracked condensing lenses adds to the cost of their operation. 60% — 80% Light Distribution All high-powered mirror lamps of American manufacture provide a reasonably uniform distribution of light without a conspicuous "hot spot" in the middle of the screen. When the arc is focused for maximum screen light, Suprex-type lamps having 14-inch mirrors give a side-to-center light distribution of 60% with 7and 8-mm positive carbons, and 65% with 9-mm carbons. Rotating-positive lamps having 16 or 18-inch mirrors provide a distribution of 55% with 9-mm carbons, 60% with 10-mm carbons, 65% with 10-mm Hitex and 11-mm regular carbons, and 70% to 80% with 13.6-mm carbons, the more uniform light distribution being obtained at currents close to the rated maximum for this size. A new lamp on the market gives a side-to-center distribution in excess of 75% by shifting the "spot," or arc-image focus, ahead of the aperture. The Strong Jetarc blown-arc lamp, which burns 10-mm Hitex carbons in the 130-138 amp. range, and 10-mm Ultrex carbons in the 155-160 amp. range, provides a light distribution of 80% and higher on the screen. This appears perfectly uniform to the eye. It was previously stated that the drive-in minimum light-level standard of 41/>> footlamberts can be obtained on matte white screens 65 feet wide with mirror lamps burning 13.6-mm carbons at 160 amps., or 75 feet wide with blown-arc lamps. The maximum widths for the same light level with the same types of lamps on aluminum screens of 150% center reflectivity are 90 and 105 feet. If the screen width is substantially smaller, the attainment of the indoor minimum standard of 10 footlamberts is entirely feasible, providing a brilliance of projection which will prove an eye-opener to most drive-in patrons, and gain considerable favor for the theatre. Audiences appreciate seeing motion pictures at their best! "Indoor" Brightness for Drive-Ins? How wide can the screen be to have not less than 10 FL of center-screen brightness when the more powerful arc lamps are used? If the screen is matte white (85^ refl. ), it may be up to 45 feet wide for a 10 FL level when 13.6-mm carbons are burned at approximately 160 amps, in mirror lamps, or up to 50 feet wide when blown-arc lamp equipment is used. If the screen is of the 150% aluminum-painted type (properly *The word "dichroic" actually means "two-colored," and should be restricetd to mean only interference filters and mirrors which transmit one color, or portion of the visible spectrum, and reflect another. The term "color" does not properly apply to infrared or any other invisible radiation. All colors are perforce visible: the terms "invisible colors" and "invisible light" are absurd. (Please turn to page 18) International Projectionist April 1963