(analytical chemistry)

A logarithmic measure of the amount of light absorbed (at particular wavelength) as the light passes through a sample or substance.


The measure of the amount of light absorbed by a suspension of bacterial cells or a solution of an organic molecule with the use of a colorimeter or spectrophotometer.


Absorbance measurements are often carried out in analytical chemistry, since the absorbance of a sample is proportional to the thickness of the sample and the concentration of the absorbing species in the sample.

Absorbance may be applied in plotting the growth of bacteria in suspension cultures. It may also be used in gauging the purity and concentration of molecules (e.g. proteins) in solution.

Although absorbance does not have true units, it is quite often reported in "Absorbance Units" or AU.

Word origin: L absorbére, equiv. to ab- + sorbére to suck in, swallow + -ance.

Related forms: absorbency (noun).

Synonym: optical density.
Compare: absorption.

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