One footcandle represents the illumination falling on a surface 1 foot square from a standard candle located 1 foot away. So if 100 footcandles of light strike a wall and 30 footcandles are reflected by the wall, the wall's finish has a reflectance of 30 percent. Contrast is measured by dividing the lowest light level in a space by the highest level. Some light is also absorbed when it travels through transparent or semitransparent medium. 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.