Absorbance is a measure of how much light is absorbed by a sample when light passes through it. It is a dimensionless quantity used in various fields, notably in chemistry and biology, to quantify the concentration of a substance in a solution. Absorbance (A) is defined by the Beer-Lambert Law, which states that absorbance is directly proportional to the concentration of the absorbing species in the solution and the path length of the light traveling through the sample.
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Absorbance is the dimensionless quantity , where and are reference and transmitted radiant powers at the chosen wavelength. For example, means one tenth of the reference power is transmitted. Under the Beer-Lambert law, absorbance can determine an absorbing species' concentration; attenuation by cloudiness or unrelated colored solutes needs separate controls. [ref]
Beer–Lambert law. IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry; IUPAC Gold Book: absorbance and its concentration relation