Meaning of preexposure

pre•ex•po•sure

Pronunciation: (prē"ik-spō'zhur), [key]
— n.
  1. exposure beforehand.
  2. the exposing of a film to even light, before photographing a subject, to increase its sensitivity.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.