Reflectance
The reflectance is the ratio of a surface of the light reflected luminance in the measuring direction for the luminance of a surface in the reference white .
As a quotient, the degree of remission has values between zero (black) and one (white) and is usually given in percent . An ideally white and ideally matt surface is used as the reference white.
If the reflecting surface is fluorescent or phosphorescent , as is the case with daytime fluorescent colors , depending on the lighting conditions, emitted light can also be measured; the degree of remission is then greater than 100%. For rhodamine G , up to 283% can be achieved in this way (with a high UV component in the lighting).
The remission over the entire range of visible light naturally depends on the type of light , more precisely on the wavelength . The wavelength-dependent quantity β (λ) must therefore be set for the color measurement . The measurement values required for the color location can thus be calculated by summing (more precisely by integration ) .
As a reference white material are calibrated surfaces of barium sulfate or specially prepared, filled PTFE - standards . The previously used magnesium oxide has a decrease towards the UV compared to the ideal white, which can influence the measurement results. Ideally, the real, material white standard (the reference white) has a self- absorption that remains constant over the measuring range .
Black standards (black velvet , black-lined hollow body) are also used to achieve good linearity of the measuring range . Calibrated standards, including calibrated gray standards, are used for precise measurement tasks. The special measuring device to exclude reflection ( gloss ) is the integrating sphere , in which the radiation is diffusely distributed. The inner surface is lined with barium sulfate, so that uniformity is achieved over the range of visible radiation. The lighting can be diffuse, or the diffuse amount of light is measured.
The degree of remission is often confused with the degree of reflection . The reflectance always relates to an ideally matt reference surface.
In transmitted light measurement for filters or cuvettes , the transmittance τ (λ) is used as a comparable variable , which according to Lambert-Beer’s law is also dependent on the wavelength. Analogous to the reflectance, the colored light-filtering effect is measured against a non-filtering effect ( colorimetry ).