projected on a plane at right angles to the line of sight from the observer); or it is the power arriving per unit area at the observer from unit solid angle (subtended at the observer) of the extende...projected on a plane at right angles to the line of sight from the observer); or it is the power arriving per unit area at the observer from unit solid angle (subtended at the observer) of the extended source. Here \(\frac{d L}{d s}\) is the rate of change of radiance with, and in the direction of, position \(s\) in a given medium, r is the density of the medium (kg m -3 ) and \(κ\) is the mass attenuation coefficient (m 2 kg -1 ).
