Radiance

Since the eye is an optical system, radiance and its cousin luminance are good indicators of how bright an object will appear.

The nonstandard usage of "brightness" for "radiance" persists in some fields, notably laser physics.

The radiance divided by the index of refraction squared is invariant in geometric optics.

For real, passive, optical systems, the output radiance is at most equal to the input, unless the index of refraction changes.

The light at the image plane, however, fills a larger solid angle so the radiance comes out to be the same assuming there is no loss at the lens.

Its surface is Lambertian, so that its radiance is uniform with respect to angle of view, and is simply the Stefan–Boltzmann integral divided by π.

For the special case of a Lambertian surface, ∂2Φe/(∂Ω ∂A) is proportional to cos θ, and Le,Ω is isotropic (independent of viewing direction).