What is optical thickness?
Optical thickness or optical depth describes how much light passes through a material. Since the amount of light that passes through a material can be quite small (less than a fraction of 1%) or very large (nearly 100%), optical thickness is based on a logarithmic scale. There are various definitions. When the optical thickness of the atmosphere is measured, the usual definition is that the optical thickness is -ln T, where T is the transmission of the atmosphere. If T is 0.95, then the optical thickness is 0.05. The optical thickness of the atmosphere is higher than in space because of Rayleigh scattering and MIE SCATTERING.
