Reflectans

Reflectans, often referred to in English as reflectance or reflectivity, is the fraction of incident electromagnetic radiation that a surface or material reflects. It is a dimensionless quantity typically denoted by R and can be expressed as a fraction (0 to 1) or as a percentage. Reflectance can be specified for a single wavelength (spectral reflectance) or averaged over wavelengths (leading to concepts such as albedo).

There are different aspects of reflectans. Specular reflectance describes mirror-like reflection from smooth surfaces, while diffuse reflectance describes scattered reflection from rough surfaces. In many applications, the complete angular distribution of reflected light is needed, which is described by the bidirectional reflectance distribution function (BRDF). The measured reflectans depends on the angle of incidence, the viewing geometry, the wavelength, and the surface’s texture and coatings.

Measurement of reflectans relies on radiometric techniques. Common methods include using integrating spheres to capture total reflected flux or goniometers to measure angular dependence. Standards and references, such as calibration against white or gray materials, ensure comparability of measurements.

Reflectans is a fundamental parameter in fields such as remote sensing, astronomy, photography, material science, and architecture. It determines how colors are perceived, how materials are identified from spectra, and how surfaces interact with light in devices and environments. Typical values vary widely: fresh snow exhibits high reflectance in the visible range, while charcoal has very low reflectance. The concept is closely related to albedo and to the optical models that describe how light interacts with matter, including the Fresnel equations for interfaces.