Polaroid 636 Repair Manual page 28

19. Now, let's go back to the three conditions in which a flash picture can be taken. The first is
in high ambient light. In this condition, there is so much scene light available that the
photocell always wins the race to fire the flash. This is the Fill Flash mode of operation.
20. In the high ambient or Fill Flash mode, the photocell measures scene light. When it sees
75% of the total light required for a proper exposure, the photocell fires the flash.
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