The mechanics of the photochemical process of deterioration of fibers in organic materials caused by visible light has been underemphasized in the literature on museum lighting. Deterioration by ultraviolet and infrared radiation is reviewed. The greenhouse effect resulting from the on-off fluctuation in glass-covered exhibition cases set off by lights within the case or external to it also accelerates damage. Rotation of specimens and reduction of light levels are the only means of control. Detection methods are described and eight ultraviolet filtering materials are cited and appraised.