silver bromide

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A pale yellow crystalline compound, AgBr, that turns black on exposure to light and is used as the light-sensitive component on ordinary photographic film and plates.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a bromide that darkens when exposed to light; used in making photographic emulsions