International projectionist (Jan 1959-Dec 1960)

EYE actual positionA>V*' // ' OF STICK *v>.— u WATER Classic representation of light refraction, mentioned in the accompanying article, is the displaced image of a stick in water. a pattern of holes for the film. No matter how long the image of the plate is projected to the screen, no matter how hot the lens becomes, there is no perceptible alteration of focus. Heat on Film the Culprit The causes of focus drift are thus to be sought in the film and its reaction toward heat. Three distinct heating processes take place as film passes through the gate of a projector. (1) Direct Irradiation. The individual frame areas of motion-picture film are each subjected to two quick flashes of intensely hot arc radiation within the space of 1/24 of a second in all rear-shutter projectors employing two light-cutoffs per frame (48 "lightflashes" per second). When the film is threaded in the normal manner for 35-mm prints (emulsion side toward the arclamp) , the frame immediately assumes a "pincushion" shape, bulging toward the lamp, upon exposure to the heat-generating radiation. Each frame accordingly expands lampward during the first flashing (1/96 sec), contracts during the balancing cutoff of equal duration, and expands again during the second flashing. If the fluttery film buckling does not exceed the depth of focus of the lens, well and good: the focus is not impaired. Ordinarily, however, the flutter effect blurs the screen image when the arc current exceeds 60 amps. without heat filters, or 80 amps, with heat filters or "cold" mirrors, and when the lens E.F. is less than 5 inches. This 48-cycle flutter of the film is caused by the expansion of the heatabsorbing emulsion relative to the heattransmitting clear base, and is a normal occurrence. It is considerably less severe for color film than for blackand-white because the dye images of color film transmit the invisible infra red radiation which monochrome silver images absorb and turn into heat. Water-Cooling a "Must" (2) Conduction by Contact. Most of the heat absorbed by film usually comes from the hot gate film runners, not from radiation absorbed by the frame areas. Unless water-cooling is employed, the film rails get very hot after the projector has been running for several minutes, and the perforation margins are held against them rather firmly by the gate-door tension pads. Dye-image prints thus get almost as hot as silver-image prints when run through a projector having an uncooled gate no matter whether overall heating is reduced by heat filters or not. Because conduction heats the film base as well as the emulsion, the film retains considerable warmth even after having been wound up on the lower reel. (3) Secondary "Black-Heat" Irradiation. The entire picture and soundtrack area of the film begins to heat up when it enters an uncooled projector gate because the metal of the gate radiates long wavelengths of infrared (heat) which the film, sepa rated from the metal only by the thickness of the runners (about 3/32 inch), absorbs. It can be appreciated that film would be heated by passage through an uncooled projector gate even if there were no aperture to let light pass through! And the amount of heat taken up by the film is enough to soften it so that the lampward buckling induced by irradiation at the aperture is more severe than it would be if the film were cold. Acetate Film Acts Better It is worth remembering that the acetate film base used today softens more readily, and is on the whole less rigid, than the dangerously inflammable nitrate base used up until about 12 years ago. Nevertheless, even the more sturdy and brittle nitrate base is also "preconditioned" to buckle more severely by passage over the "trough" of hot metal from the top of the gate down to the aperture. Some of us projectionists who can remember were justifiably alarmed whenever a power failure allowed the explosive nitrate film to stop dead in (Continued on page 25) 'Zipper' Changeover Modified for Century 35/70 Head The appended notes were obtained from Essannay Electric Mfg. Co., Chicago. Century Projector Corp. had asked us to modify the design of an Essannay Zipper changeover to fit their new 35/70mm projector head. The following changes were made: The base of the regular Century casting was milled off flat with the back. The coil mountingscrew holes were MACHINE NO. 1 changed and countersunk for flush mounting. The pull rod was made longer. The brass and steel armature sections were made longer to give greater stroke. The coil had been made shorter to give a minimum 2x/2-inch stroke. The coupling on the end of the flexible shaft was turned down to prevent interference with the tube bushing. The 3 1/3 x 3 aluminum shutter is furnished by Century. MACHINE NO. 2 9 9 T f p ? 9 < -J ID ul Z o a. to < _i CO UJ ■3. 5 Z S o a CD T»A.C.or D.C.Lmt r <■ > ■*) 1 \ 0 6 6 6 Foot Switch NM Foot Switch N* 2 Two-machine wiring diagram for Essannay Zipper changeovers and footswirches. INTERNATIONAL PROJECTIONIST NOVEMBER 1959