It looks like you're using Internet Explorer 6: Features on this site are not supported by that browser version. Please upgrade to the latest version of Internet Explorer.

You are viewing a version of the Ocean Optics Web Book site specially-formatted for printing.

# Absorption

## Measurement of Absorption

 Page updated: Aug 18, 2017 Principal author: Collin Roesler

#### From Theory to Reality

Consider a scenario where the goal is to measure the absorption spectrum of a thin layer of material (Figure 1A). The incident radiant power is given by , in the form of a collimated beam. The radiant power transmitted through the layer, , is detected. If , there is no loss of radiant power and therefore no attenuation. If however the medium absorbs some quantity of radiant power, , then , and (Figure 1B). In the case of material that absorbs and scatters, the scattered radiant power is given by , and .

To quantify the absorbed radiant power only, it is necessary to measure both the transmitted and scattered radiant power. This is a requirement for an absorption meter. Consider first a nonscattering material. The measured dimensionless transmittance, , is the fraction of incident power transmitted through the layer:

The absorptance, , is the fraction of incident radiant power that is absorbed ( ):

The absorption coefficient, , is the absorptance per unit distance

which, for an infinitesimally thin layer can be expressed as:

Rearranging this expression and taking the limit as yields:

Integrating the function over the layer:

This equation provides a guide toward designing instruments to accurately measure absorption. The Level 2 pages beginning at Benchtop Spectrometry of Solutions give the specifics on techniques to measure absorption by dissolved and particulate constituents in seawater.