Absorbance

This article is about a quantitative expression. For the process itself, see Absorption (electromagnetic radiation).
"Optical density" redirects here. For other uses, see Refractive index, Nucleic acid quantitation, and Neutral-density filter.

In spectroscopy, absorbance (abbreviated as A) is a logarithmic value which describes the portion of a beam of light which does not pass through a sample. Whilst the name refers to the absorption of light, other interactions of light with a sample (reflection, scattering) may also contribute to attenuation of the beam passing through the sample. The term "internal absorbance" is sometimes used to describe beam attenuation caused by absorption, while "attenuance" or "experimental absorbance" can be used to emphasize that beam attenuation can be caused by other phenomena.

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