Optical density is the absorbance of an optical element for a given wavelength λ per unit distance:

where

l is the distance that light travels through the sample (i.e., the sample thickness), measured in cm;

A_λ is the absorbance at wavelength λ;

T is the per-unit transmittance;

I₀ is the intensity of the incident light beam;

I is the intensity of the transmited light beam.

Although absorbance doen'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 equals 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.

References

• Federal Standard 1037C
• MIL-STD-188