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 dLds 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 ).