The term optical density is normally, but not always, understood to be the absolute value of the logarithm with base 10 of the power transmission factor of an optical attenuator:
If several attenuators are used in series, their optical densities can simply be added.
The optical density is often said to be identical with the absorbance. However, optical attenuation e.g. of a neutral density filter may not be entirely resulting from absorption, but at least partially from reflection. The optical density then specifies the overall power transmission, no matter to which extent it results from absorption.
Optical Density and Refractive Index
A transparent medium is sometimes said to have a high optical density (or to be relatively dense) if it has a high refractive index. For example, one may say that total internal reflection is possible only if the beam comes from the optically more dense medium. This use of the term has nothing to do with attenuation.
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