I suppose there is some sort of lux-hour formula. Typical modern electronic flash units fire at 1/1000 to 1/5000 second. Granted, they are somewhat intense, but it takes 5000 flash photos to equal one second of exposure to such light. Surely the *hours* of lower level ambient light are more detrimental to the artifacts. Flash photography would seem a drop in the ocean. Christopher Bobbitt Curator of Collections, Monroe Co. Historical Museum Bloomington, IN [log in to unmask]