The four core radiometric quantities for rendering are energy, flux, irradiance, and radiance

Physically based rendering requires precise measurement of light. The four essential radiometric quantities are: (1) Radiant energy (joules) — the total energy carried by photons; (2) Radiant flux / power (watts) — energy per unit time, the quantity emitted by light sources; (3) Irradiance (W/m²) — flux density arriving at a surface; explains Lambert’s law (brightness varies with cosine of incident angle); (4) Radiance (W/m²/sr) — flux per unit area per unit solid angle, the quantity constant along rays through vacuum and the primary quantity ray tracers compute. The photometric equivalents (lumens, lux, candela, nits/cd/m²) relate these to human perception via the V(λ) spectral response function.

Examples

Lambert’s law: a surface tilted 60° from the light receives cos(60°) = 0.5× the irradiance of a perpendicularly lit surface. Luminance (nits) is the perceptual equivalent of radiance, used in display engineering and HDR standards.

Assessment

Match each situation to its radiometric quantity: (a) specifying a light bulb’s output, (b) measuring brightness of a patch of sky through a camera, (c) computing how much light arrives at a tilted surface. Justify each answer.