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.