Figure 1.8

The Double-Bladed Shutter

When sound filming became a reality, the speed of motion-picture cameras and projectors was standardized at 24 frames per second (fps). From that point on, in other words, the film had to travel through both machines at precisely that speed. But a film projected at 24 fps falls below the critical flicker frequency (cff) of 30 to 50 fps necessary to eliminate the viewer’s perception of flicker. The solution is the double-bladed shutter like the one shown in this drawing. Although frames on the filmstrip travel through the projector at a rate of 24 fps, the shutter rotates at a rate fast enough to expose each frame to three intervals of darkness per second, resulting in a flicker rate of 72 fps—well above the cff threshold.

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