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13
THE TECHNICAL DESIGN OF THE PROJECTOR
R. Robertson, B.Sc, M.I.Mech.E., M.B.K.S.*
Read to the B.K.S. Theatre Division on January 19, 1947. The paper which followed, on " The Design of the Projection Room" by Major C. H. Bell, O.B.E., M.B.K.S., is being printed in an amplified form in a later issue.
WHEN one analyses the operating conditions which the kinematograph projector must satisfy, one may well be surprised that a projector can be made to function at all satisfactorily, let alone to give ten years or more of good service.
Kinematograph film, a highly inflammable material, easily torn or scratched, subject to shrinkage and warping, must be fed intermittently through the picture gate, being stopped dead 24 times a second ; then snatched forward and again stopped dead with the next frame in positron ; while a few frames ahead — one second in time — it must be running at absolutely constant speed through the sound gate.
The whole of the light necessary to illuminate the screen must pass through each frame in turn as it is stationary in the gate. This represents a concentration of something like one horse-power on an area of celluloid 0.825 x 0.620 in.
It is not sufficient that the film stop dead momentarily in this inferno of light and heat, but it must be stopped in absolute register and focus, since any error is advertised upon the screen, magnified perhaps by 300 or 400 times.
In addition to the fundamental requirement of a sharp, steady, well-lit picture, and reliable, silent, operation, the projector must conform to certain more or less closely defined standard requirements.
Kinematograph film is probably the most widely accepted standard thing in the world. This is all to the good, but of course, necessarily imposes some limitation on the freedom of the designer. For example, the distance of 24 frames between picture and sound gate does not allow much latitude in the layout of the film path.
Soundhead, Lamp and Lens
Maintenance of adaptability to any 'soundhead is a troublesome matter, involving numerous cramping and not very clearly defined restrictions. Since the projector is usually mounted upon and driven by the soundhead, it is not only the drive arrangement which must be watched, but the base plan, holding-down bolts and available driving power — also any gears, flywheels, or the like, projecting above the top face of any soundhead. It is also necessary to maintain correct presentation of the film to the soundhead and correct synchronisation between picture and sound.
Modern American projectors and soundheads are built around a common standard, which does simplify matters, but this appears to have arisen fortuitously, and its strict observance imposes severe and inconvenient restrictions on the design of the projector — in particular, the holding-down arrangement, and the small 17-toothed driving pinion involved are by no means what a designer would choose if given a free hand.
In contrast, the requirements of the lamp designer are quite simple : he wants to get his lamp as close up to the gate as possible ; the closer he can get, the more light he can give with an optical system of given diameter. He expects also conformity with generally accepted optical height.
* A. Kershaw & Sons, Ltd.