Fluorescent images that illustrate acid-catalyzed tert-butoxycarbonyl (tBoc) deprotection patterns in polymer films were obtained using fluorescent sensors based on 7-hydroxycoumarin dyes. Three commercial 7-hydroxycoumarins, which are highly fluorescent, become practically nonemissive upon protection of the 7-hydroxyl position with tBoc. In thin polymer films, the protected "prefluorescent" probes can return to their deprotected, fluorescent states by reaction with catalytic amounts of photogenerated acid and mild heating.