Polaroid 636 Repair Manual page 37

37. Now you may be wondering why infrared measurements greatly reduce over- or under-
exposures in close-up flash pictures.
38. This chart shows the light spectrum. In the infrared region of the spectrum, a strange
phenomenon occurs. The light reflected off just about all materials is equal. Therefore, the
light passing through the infrared filter is balanced, preventing the photocell from being
fooled by contrasting levels of light or dark subject matter.
31