absorbance symbol: A; a measure of the ability of a substance or a solution to absorb electromagnetic radiation incident upon it. It equals the logarithm of the ratio of the radiant power of the incident radiation, Φ0, to the radiant power of the transmitted radiation, Φ. For a solution, absorbance is expressed as the logarithm of the ratio of the radiant power of light transmitted through the reference sample to that of the light transmitted through the solution, the observations being made using identical cells. (Traditionally, radiant intensity was measured instead of radiant power, which is now the accepted form.) Two quantities are defined: (decadic) absorbance (symbol: A10 or A), and napierian absorbance (symbol: Ae or B).

A10 = lg (Φ0/Φ) = lg T–1 = –lg (1 – α), and
Ae = ln (Φ0/Φ) = ln T–1 = –ln (1 – α),

where T is the (internal) transmittance and a is the absorptance. These definitions suppose that all the light incident upon the sample is either transmitted or absorbed, reflection and scattering being negligible. The more general term attenuance should be used when scattering is considerable, as when the quantity lg(Φ0/Φ) is measured to estimate the cell density of a culture. The older terms absorbancy, extinction, and optical density should no longer be used.

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