absorptance

Absorptance is the fraction of incident radiant energy that is absorbed by a material or surface. It is defined for a specific wavelength, incidence angle, and polarization, and is typically expressed as a dimensionless quantity between 0 and 1. Absorptance is complementary to the fractions that are reflected and transmitted.

In general, for a planar surface or homogeneous medium, absorptance A, reflectance R, and transmittance T satisfy A = 1 − R − T. For opaque, non-scattering materials, T is approximately zero and A ≈ 1 − R. In a homogeneous slab with absorption coefficient α and thickness d, A = 1 − e^(−α d). For thin films, neglecting reflections, A ≈ α d when α d is small.

Spectral absorptance a(λ) expresses dependence on wavelength. A can also depend on incidence angle θ and polarization. Kirchhoff's law states that, at thermal equilibrium, the emissivity ε(λ, θ) equals the absorptivity for the same wavelength and direction in opaque bodies, linking absorptance to radiative emission.

Measurement and applications: Absorptance is measured with integrating spheres and spectrometers to determine A(λ); it is central to the design of optical coatings, solar cells, detectors, and radiative heat transfer control. It is common to distinguish this optical absorptance from absorbance used in chemistry, where absorbance A is defined by A = log10(I0/I) in Beer-Lambert applications, a different concept from optical absorptance.