reflektans

Reflektans, or reflectance, is a property of a material that describes the fraction of incident electromagnetic radiation that is reflected by its surface. It is typically expressed as a ratio or percentage and depends on wavelength, angle of incidence, and the physical state of the surface. Reflectance is related to, but distinct from, transmittance and absorption; for opaque surfaces, reflectance and absorption sum to about unity.

There are several notions of reflectance used in practice. Spectral reflectance describes reflectance as a function of wavelength. Diffuse (hemispherical) reflectance describes energy reflected into all directions, while the bidirectional reflectance distribution function (BRDF) characterizes how reflectance varies with viewing and illumination directions. Specular reflectance refers to mirror-like reflection, concentrated in a specific direction.

Reflectance is influenced by material composition, surface roughness, oxide or coating layers, and the presence of contaminants. It also varies with wavelength and angle of incidence, as described by Fresnel equations for smooth surfaces and by microstructural effects for rough ones. In measurements, reflectance is often determined with a spectrophotometer or integrating sphere, using a reference standard such as a diffuse white plate or a highly polished mirror.

Applications span color science, computer graphics, remote sensing, solar energy, and material science. In opaque materials, roughly, reflectance complements absorption; in non-opaque materials, it must be considered together with transmittance. Typical values vary widely: metals can reflect most visible light (roughly 0.85–0.95), white pigments 0.8–0.95, while very dark materials may reflect less than 0.1. Albedo is a related concept describing hemispherical reflectance of planetary surfaces.