Polaroid 636 Repair Manual page 31

25. The question is answered by measuring the amount of light seen by the photocell two
milliseconds after flash fire.
26. If the measurement is equal to or more than 50% of the light needed for a proper exposure,
the camera senses subject is NEAR. If the measurement is less than 50% of the light needed
for a proper exposure, the camera senses that the subject is FAR.
25