Wide generality for optical radiometry can be achieved by treating the basic radiometric quantities as field quantities. The treatment is that of classical ray optics, with emphasis on the geometrical relations involved. It is shown that radiance, defined as N ≡ ∂2P∂Ω cosθ∂A [W·cm−2·sr−1], the radiant flux or power per unit solid-angle-in-the-direction-of-a-ray per unit projected-area-perpendicular-to-the-ray, has the same value at any point along this ray within an isotropic medium, in the absence of losses by absorption, scattering, or reflection. More generally, the quantity N/n2 (where n is the index of refraction of the medium) in the direction of a ray is shown to be invariant along that ray, even across a smooth boundary between different lossless media. The usefulness of this invariant property of radiance is illustrated by examples of practical applications.