reflectance

Reflectance is the ratio of the radiant flux reflected by a surface to the radiant flux incident upon it. It is a property that depends on wavelength, angle of incidence, surface roughness, and material composition.

In radiometry, reflectance is often described as a function R(λ, θi, φi, θr, φr) that can vary with both the direction of illumination and the direction of observation. The bidirectional reflectance distribution function, BRDF, describes how light is reflected at an interface: f_r(θi, φi; θr, φr) relates the incoming radiance to the outgoing radiance. The hemispherical (or total) reflectance, or albedo α, is the ratio of total reflected flux integrated over all exitant directions to the total incident flux, and is given by α = (∬ L_r cos θr dΩr) / (⊂∬ L_i cos θi dΩi). In practice, one often uses simpler models such as Lambertian reflectance, where radiance is independent of the viewing direction and the reflectance equals the albedo.

Spectral reflectance R(λ) describes reflectance as a function of wavelength. It is essential in color science and remote sensing. Measurements are usually performed with spectrophotometry, using a reference standard such as Spectralon.

Surface reflectance can be specular (mirror-like), diffuse (scattered in many directions), or a combination. Specular reflection is directional and angle-dependent; diffuse (Lambertian) reflection is approximately isotropic.

Applications include remote sensing, computer graphics, photography, material science, and astronomy.