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Closely associated with the intermittent mechanism, is the shutter. It is preferable that the flicker obturation should exactly balance the cover obturation. If it is cut down one does certainly gain some light, but at cost of flicker.
Since the film movement takes place during one quarter of the picture cycle, the light should, in theory, be cut off for two periods per picture each of 90 deg., i.e., for half the cycle, corresponding to a light efficiency of 50%. In practice, however, since the shutter has to cut across a light beam of sensible width, transition from full light to complete dark cannot be instantaneous, resulting in an intervening " twilight " period of reduced illumination and a corresponding reduction of efficiency below the theoretical ideal of 50%. In part compensation, however, the "twilight" period can be allowed to encroach to a limited extent into the period at the beginning and end of the film movement, before the film has got into its stride, and again, as it is coming to rest. This encroachment, cannot be overdone, however, or film movement will be evident as ghost, but it is practicable to attain or even somewhat exceed the theoretic ideal 50% efficiency.
An obvious design point is to cut the light beam wrhere it is narrow with a relatively large diameter fast moving blade. It is usual to employ a 2bladed shutter making one revolution per picture, i.e., 1,440 r.p.m. Earlier it was usual to mount the shutter in front of the lens at the narrowest part of the light beam.
This is an efficient arrangement, but results in the gate and film being continuously exposed to the heat of the arc. In contrast, placing the shutter behind the gate effects a 50% reduction in the heat concentrated on the gate ; further, a shutter in this position can be used also as a fan to maintain a cooling flow of air around the gate.
Scissors Type and Twin Shutters
The current trend in 35 mm. practice is towards the use of a scissors type shutter, halving the twilight period by cutting the beam simultaneously from opposite sides.
An obvious scheme employed in several modern projectors is to use a pair of shutters running in opposite directions. This does involve some extra mechanical complication, but passes from 10-20% more light than is possible with a single shutter of equivalent dimensions.
An interesting variant of this arrangement is the use of twin shutters running in the same direction, but on opposite sides of the projector lens, a
The gearing of the G.K.2I projector head. The sliding spiral gears serving for shutter phasing compensation can be seen. The oiling system feeds filtered oil to all components.