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.