reflectivityreflectance

Reflectivity and reflectance refer to the fraction of incident electromagnetic energy that a surface or material reflects. They are dimensionless quantities, usually expressed as a ratio between 0 and 1 or as a percentage. In many contexts the terms are used interchangeably, but subtle distinctions exist: reflectivity often denotes an intrinsic property of the material at a given wavelength and polarization, while reflectance emphasizes the observed, geometry- and wavelength-dependent quantity.

Spectral, directional, and angular aspects are important. Spectral reflectance R(λ) describes how much light is reflected as a function of wavelength, while hemispherical or directional reflectance describes how reflection varies with the direction of illumination and viewing. Surfaces may be specular (mirror-like) or diffuse (scattered); many real surfaces exhibit a combination describable by concepts such as the Bidirectional Reflectance Distribution Function (BRDF).

The angular dependence of reflectance is governed in simple cases by the Fresnel equations for smooth interfaces. For rough or complex surfaces, radiative transfer theories and albedo concepts are used to relate reflectance to the total reflected energy relative to the incident energy.

Measurement of reflectance is performed with instruments such as spectrophotometers and integrating spheres to obtain spectral reflectance curves, and goniometers or BRDF devices to characterize angular distribution. Reflectance data underpin applications in remote sensing, astronomy, computer graphics, material science, and energy balance calculations.

See also: albedo, BRDF, Fresnel equations, spectral reflectance.