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

Consider a scattering particle idealized as a sphere with a
particular geometrical size. Consider that this sphere redirects
incident photons into new directions and so prevents the forward on-axis
transmission of photons, thereby casting a shadow. This process
constitutes scattering. This description is of course an oversimplified
and schematicized version of the real situation. However, it does
provide a simple concept which captures the essence of the **scattering coefficient**, a parameter
analogous to the absorption coefficient
discussed previously.

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

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

Experimentally, the units [cm^{-1}] for µ_{s}
are inverse length, such that the product µ_{s}L is
dimensionless, where L [cm] is a photon's pathlength of travel through
the medium. The probability of transmission T of the photon without
redirection by scattering after a pathlength L is: