nanometer
A measurement of length equal to one-billionth of a meter. Abbreviated as nm. Wavelengths of light are measured in nanometers. Eyeglass and sunglass lens manufacturers use nanometers to describe the different types of light that may pass through or be blocked by a lens, whether it be visible light, ultraviolet light, blue light, etc. For example, visible light has a wavelength range of 400 to 700 nm. The amount of light transmission itself is described as a percentage. For example, a dark sunglass lens might allow only 12 percent of visible light to pass through to the eye.