Absorbance - Detailed Explanation

Detailed Explanation

Absorbance is a quantitative measure expressed as a logarithmic ratio between the radiation falling upon a material and the radiation transmitted through a material.

,

where is the absorbance at a certain wavelength of light, is the intensity of the radiation (light) that has passed through the material (transmitted radiation), and is the intensity of the radiation before it passes through the material (incident radiation).

Outside the field of analytical chemistry, e.g. when used with the Tunable Diode Laser Absorption Spectroscopy (TDLAS) technique, the absorbance is often defined using the natural logarithm instead of the common logarithm, i.e. as

where is the intensity of light at a specified wavelength λ that has passed through a sample (transmitted light intensity) and is the intensity of the light before it enters the sample or incident light intensity (or power).

The term absorption refers to the physical process of absorbing light, while absorbance refers to the mathematical quantity. Also, absorbance does not always measure absorption: if a given sample is, for example, a dispersion, part of the incident light will in fact be scattered by the dispersed particles, and not really absorbed. However, in such cases, it is recommended that the term "attenuance" (formerly called "extinction") be used, which accounts for losses due to scattering and luminescence.

See the Beer-Lambert law for a more complete discussion.

Although absorbance is properly unitless, it is often reported in "Absorbance Units" or AU.