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The lens designer, too, wants to get close up to the gate with a big diameter lens, to collect all the light available.
Finally, differing legal enactments and underwriters' requirements, also differing operating conditions and prejudices throughout the world, must be watched, and some generally acceptable compromise formed.
As an example, a requirement in certain markets, such as the U.S.A. and Canada, is that the operating side and the light beam must be enclosed. In certain other markets there is a definite preference for an open projector. Both types have their advocates, but in practice, it appears that while an operator prefers the type to which he is used, the trend is towards the enclosed machine with its advantages of cleanliness and more silent running resulting from reduction of film noise.
It is within the limitations thus briefly outlined, that the projector designer must fit the " works," and we pass on to consideration of these.
Maltese Cross Mechanism
All modern 35 mm. projectors employ as an intermittent unit the well-known 4-station Maltese cross mechanism, which was first applied
to projectors in the infancy of the art by the late Robert Paul.
The accelerations and consequent stress involved during the transport
period are quite imposing. The film is moved J in. in 1/96 of a second.
It is subjected to an acceleration reaching a maximum rate of about 4,850
ft./sec2 or 152 g's, and attains a maximum speed of 865 ft./min. — nearly 10
times its average speed through the projector.
Needless to emphasise, it is desirable to keep the cross and sprocket as light as possible consistent with adequate accuracy and rigidity, in order to reduce to a minimum the inertia forces involved, also the mechanism must be as direct acting as possible — the intermittent sprocket must be mounted directly and as solidly as possible upon the cross shaft, and the flywheel should be mounted directly upon the cam shaft and not geared thereto.
Numerous attempts have been made to supplant or improve the simple 4-station cross, but nothing has been found which so well meets the exacting conditions. Much attention has been given also to develop " continuous projectors " employing some form of optical compensation for the film movement, thus avoiding the necessity for intermittent film movement, but these efforts have been equally fruitless.
Continuous projectors are particularly delusive, since it is not difficult to show a fairly promising picture with a quite simple arrangement, but all the systems proposed involve inherent defects, and the step from " fair " to " good " requires the introduction of intolerable complications.
The massive parallel opening gate of the G.K.2I projector.