In spectroscopy, the absorbance (also called optical density) of a material is a logarithmic ratio of the radiation falling upon a material, to the radiation transmitted through a material. Absorbance measurements are often carried out in analytical chemistry.
In physics, the term spectral absorbance is used interchangeably with spectral absorptance or absorptivity. In this case it has a slightly different meaning: the fraction of radiation absorbed at a specific wavelengths.
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