If 100 footcandles hit a semitransparent surface, and 90 footcandles emerge from it, the material said to have a 90 percent transmittance. Visible light is only a small segment of the eletromagnetic spectrum. At the short-wavelength end of its spectrum and hues where ultraviolet radiation occurs, light is particularly active, quickly fading colors and breaking down materials such as fabrics, paper, leather, and wood. UV is even used to kill germs in hospitals. All daylight contains a large UV component.
Longer wavelengths are less destructive, but still very active.
All visible light, including that from electric sources, can destroy materials. Because of its lesser intensity and spectral distribution, damage from electric light is much less destructive than daylight, but can be destructive none the less. A bright pink pair of the First Lady's footwear was acquired by the Museum of the City of New York in 1951. The shoes were placed on permanent diplay in a location bathed in strong daylight. After a decade, they were bleached to a ghostly cream color and the fabric was decaying.