Radiance is the fundamental radiometric quantity for rendering because it is constant along rays in empty space

Radiance is flux density per unit area per unit solid angle. It is the most fundamental radiometric quantity for rendering: if radiance is known, all other radiometric quantities (flux, irradiance, intensity) can be computed as integrals of radiance over areas and directions. Its key property is that radiance remains constant along rays through empty space, so the radiance arriving at the camera equals the radiance leaving the surface toward it — no distance correction needed. This makes it the natural quantity to compute with in a ray tracer.

Examples

Radiance does not decrease with distance from a source in empty space. Irradiance is the integral of incoming radiance over the hemisphere weighted by cosine. The rendering equation integrates incoming radiance to compute reflected radiance.

Assessment

Explain why a ray tracer stores and propagates radiance rather than intensity or flux. What would go wrong if you tracked flux instead?