Fr.: profondeur optique
1) A measure of how much radiation is absorbed when traveling through a medium
(such as the atmosphere of a star or the interstellar medium) from the source to a
given point. It depends on the type of medium and the frequency of radiation.
It is defined as a dimensionless quantity
dτλ = κλ ρ dx,
where κλ is the
extinction coefficient, ρ the density, and dx the path length.
2) The ratio of the intensity of radiation (light or radio waves) incident on
a ring to that emerging from the opposite face of the ring, expressed as a natural
logarithm. If the reduction in intensity is by a factors of e (= 2.718), the ring is said to
have an optical depth of 1. Normal (or normalized) optical depth is optical depth
corrected for oblique (non-vertical) viewing. In ring studies, the terms optical depth
and optical thickness are generally used interchangeably. These terms generally refer to
a particular wavelength
(Ellis et al., 2007, Planetary Ring Systems, Springer).
→ optical; → depth.