Radiance is constant along a ray through empty space, making it the natural quantity for ray tracing

Radiance is defined as flux per unit area per unit solid angle. Unlike irradiance (which spreads over distance) or intensity (only meaningful for point sources), radiance remains constant as a ray travels through vacuum — moving further from a surface decreases the area it illuminates but equally decreases the solid angle, exactly cancelling. This makes radiance the ideal quantity for ray tracing algorithms: the value carried by a ray doesn’t need to be scaled by distance, simplifying implementation. Every ray-tracer computes and accumulates radiance values. In media with participating effects (fog, smoke), radiance is attenuated — handled by transmittance calculations.

Examples

If you trace a ray from a camera to a red diffuse surface lit by a white area light, the radiance returned by that ray equals the surface’s BRDF-weighted integral of incoming radiance — the same value regardless of how far the camera is from the surface.

Assessment

Explain why a moving camera doesn’t change the brightness of objects in a ray-traced image (unlike a real camera sensor). Then state what does change radiance along a ray and which PBR chapter addresses it.