Absorbance vs. Absorptance

Main Difference

The main difference between Absorbance and Absorptance is that the Absorbance is a common logarithm of the ratio of incident to transmitted radiant power through a material; the optical depth divided by ln(10) and Absorptance is a ratio of absorbed to incident radiant flux

• Absorbance

  In chemistry, absorbance or decadic absorbance is the common logarithm of the ratio of incident to transmitted radiant power through a material, and spectral absorbance or spectral decadic 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 use of the term “optical density” for absorbance is discouraged.

  In physics, a closely related quantity called “optical depth” is used instead of absorbance: the natural logarithm of the ratio of incident to transmitted radiant power through a material. The optical depth equals the absorbance times ln(10).

  The term absorption refers to the physical process of absorbing light, while absorbance does not always measure absorption: it measures attenuation (of transmitted radiant power). Attenuation can be caused by absorption, but also reflection, scattering, and other physical processes.

• Absorptance

  Absorptance of the surface of a material is its effectiveness in absorbing radiant energy. It is the ratio of the absorbed to the incident radiant power. This should not be confused with absorbance and absorption coefficient.