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232 Transactions of S.M.P.E., Fehruanj 1927
D. Portahility. A complete outfit with a carrying case holding a dozen rolls of film weighs 5 to 6 pounds. The projector and film may be placed in a brief case or in a projector case especially built for carrjdng purposes.
The projector is attached to the ordinary lamp socket. The whole operation of attaching to the socket and starting the picture can be performed in less than 50 seconds.
E. Durability. Especially when the film is wound on spools, the life of each print is long. If the film is dropped it does not break as does the ordinary lantern slide. Even with rough handling a print may be projected from fifty to one hundred times, or more.
3. What are its limitations?
1. The amount of light which may be projected through the aperture is limited by the size of the film frame, f of an inch by one inch. Because standard movie film comes in this size, we find here one of the inherent limitations in the use of slide film.
By comparison, a stereopticon has an effective light opening several times as large, so the stereopticon admits and throws more light, and, of course, has greater screen illumination and may have a longer effective throw.
Each frame must remain in front of the light as long as the operator desires. There must be no burning, warping nor excessive drying out of the film. In other words the amount of heat which reaches the film must be reduced to a minimum. Of course, due to the fact that cellulose acetate or "non-flam" film is used, there is no fire hazard. Still, an excessive amount of heat would ruin the film by causing warping or drying out. The size of this aperture thus reduces the amount of available light. So the amount of illumination on the screen or screen brilliancy is definitely limited, since we have not developed a light source without heat. Depending upon the suitability of the heat resisting method and the optical system, more or less heat may reach the film. For practical purposes in the better designed projectors, heat does little or no damage. The tendency to buckle has been greatly reduced; but the amount of light coming through the aperture has the definite limitation.
The ordinary portable hand slide fihn projector uses the usual house current of 110 volts. The light is supplied by a lamp usually burning 100 or 200 watts. One make of projector uses the smaller automobile light of less amperages and with a low wattage.