Optical density

Optical density (OD): For a given wavelength, an expression of the transmittance of an optical element.

Note 1: Optical density is expressed by log₁₀(1/T ) where T is transmittance.

Note 2: The higher the optical density, the lower the transmittance.

Note 3: Optical density times 10 is equal to transmission loss expressed in decibels, e.g. , an optical density of 0.3 corresponds to a transmission loss of 3 dB.

Source: from Federal Standard 1037C and from MIL-STD-188