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This article is about a measure of optical absorbance. Sometimes the term 'optical density' is also used to describe a material's refractive index, see refractive index. Optical density is the absorbance of an optical element for a given wavelength λ per unit distance: ight ) Where: Although absorbance doesn't have true units, it's quite often reported in "Absorbance Units" or AU. Accordingly, optical density is measured in ODU, which are equivalent to AU cm−1. The higher the optical density, the lower the transmittance. Optical density times 10 is equal to a transmission loss rate expressed in decibels per cm, e.g., an optical density of 0.3 corresponds to a transmission loss of 3 dB per cm. Optical density is sometimes defined without regard to the length of the sample; in this case it is a synonym for absorbance. Neutral density filters are typically quantified this way.
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