The terms “optical density” and “absorbance” are used to characterize the transmission of light through objects. The two terms are used interchangeably to define the same quantity, and there is no difference between them. One should not confuse absorbance with absorption, as these represent different underlying physical processes. Absorption refers to absorption of light as it passes through material, while absorbance includes effects of dispersion and reflection as well. Absorbance or optical density are mathematical quantities, which are calculated on a logarithmic scale.