In spectroscopy, absorbance or decadic absorbance it is the amount of light absorbed by a body; is equal to the common logarithm of the ratio of incident to transmitted radiant power through a material, and spectral absorbance or decadic spectral absorbance is the common logarithm of the ratio of incident to transmitted spectral radiant power through a material.
Absorbance is dimensionless, and in particular is not a length, though it is a monotonically increasing function of path length, and approaches zero as the path length approaches zero.
The absorbance is widely used because it allows the expression of the Lambert-Beer absorption law.
- Wikipedia. Absorbance. https://en.wikipedia.org/wiki/Absorbance