ECE532 Biomedical Optics © 1998 Steven L. Jacques, Scott A. Prahl Oregon Graduate Institute |

Consider a chromophore idealized as a sphere with a particular
geometrical size. Consider that this sphere blocks incident light and
casts a shadow, which constitutes absorption. This description is of
course an incorrect and schematicized version of the real situation.
However, it does provide a simple concept which captures the essence of
the **absorption coefficient**, the
parameter we use to describe the effectiveness of absorption.

The size of the absorption shadow is called the **effective
cross-section** (σ_{a}
[cm^{2}]) and can be smaller or larger than the geometrical size
of the chromophore (A [cm^{2}]), related by the proportionality
constant called the **absorption efficiency** Q_{a}
[dimensionless]:

The **absorption coefficient** µ_{a}
[cm^{-1}] describes a medium containing many chromophores at a
concentration described as a **volume density** _{a} [cm^{3}]. The absorption
coefficient is essentially the cross-sectional area per unit volume of
medium.

Experimentally, the units [cm^{-1}] for µ_{a}
are inverse length, such that the product µ_{a}L is
dimensionless, where L [cm] is a photon's pathlength of travel through
the medium. The probability of survival (or transmission T) of the
photon after a pathlength L is:

This expression for survival holds true regardless of whether the photon path is a straight line or a highly tortuous path due to multiple scattering in an optically turbid medium.