In
physics,
optical depth or
optical thickness, is the
natural logarithm of the ratio of incident to
transmitted radiant power through a material, and
spectral optical depth or
spectral optical thickness is the natural logarithm of the ratio of incident to
transmitted spectral radiant power through a material. Optical depth is
dimensionless, and in particular is not a length, though it is a monotonically increasing function of path length, and approaches zero as the path length approaches zero. The use of the term "optical density" for optical depth is discouraged.