Absorbance Definition

Measuring How a Sample Interacts With Light

Spectrophotometers are instruments that may measure absorbance.
Spectrophotometers are instruments that may measure absorbance. Eugenio Marongiu / Getty Images

Absorbance is a measure of the quantity of light absorbed by a sample. It is also known as optical density, extinction, or decadic absorbance. The property is measured using spectroscopy, particularly for quantitative analysis. Typical units of absorbance are called "absorbance units," which have the abbreviation AU and are dimensionless.

Absorbance is calculated based on either the amount of light reflected or scattered by a sample or by the amount transmitted through a sample.

If all light passes through a sample, none was absorbed, so the absorbance would be zero and the transmission would be 100%. On the other hand, if no light passes through a sample, the absorbance is infinite and the percent transmission is zero.

The Beer-Lambert law is used to calculate absorbance:

A = ebc

Where A is absorbance (no units, A = log10 P0 / P)
e is the molar absorptivity with units of L mol-1 cm-1
b is the path length of the sample, usually the length of a cuvette in centimeters
is the concentration of a solute in solution, expressed in mol/L