Polaroid 636 Repair Manual page 36

35. On the other hand, dark clothing would fool the photocell into timing out a long exposure,
since relatively little light was reflected back. This would result in a washed-out face.
36. To overcome this problem, the Model 636 uses a photocell with a two-part filter in front of
it, called a Dynamic Infrared Photometer system. The top half of the filter passes visible
light which the human eye can see. This light is used to time exposures for non-flash,
fill-flash, and low-light, distant scene flash pictures. The top filter is a photopic filter.
The bottom half of the filter passes infrared light which is invisible to the human eye, but
which the film can see. This light is used to time exposures for flash pictures in low light
where the subject is near the camera. The bottom filter is called the infrared filter.
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