Polaroid 636 Repair Manual page 38

39. For Fill Flash pictures, the shutter blades open as shown here. The blades are set so only
visible light passes through the first photopic aperture to the photocell. Thus, for Fill Flash,
exposure shutdown is based on the measurement of visible light only.
40. In a low light NEAR shot, the blades travel farther. During blade travel a very small
amount of visible light passes through the first photopic aperture. The blades move to the
position shown, the strobe fires and a larger amount of infrared light passes through the
infrared aperture to the photocell. Exposure shutdown is dependent upon the combination
of an extremely limited amount of visible light added to the larger amount of infrared light
(or on the clock time-out).
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