TPDSci Attenuation coefficient

Attenuation coefficient

Attenuation coefficient of light in a medium, also referred to as the extinction coefficient, customarily denoted by c [length^-1], is defined by the following equation:

dΦ(z) = -cΦ(z) dz

(1)

where Φ [power] is the power of light flowing only in the positive direction of the z-axis, z [length] is the distance. This equation leads to the Lambert law of attenuation of light in a medium. Note that the attenuation coefficient may depend on position in the scattering medium. In the medical optics literature, the attenuation coefficient is frequently denoted by symbol ľ_t.

The inverse of the attenuation coefficient is referred to as the attenuation length. It is the average distance between photon attenuation events (scattering and/or absorption).

By requiring the power of light to flow only in the direction of the z-axis, we refer here to a unidirectional flow, as in a travelling plane light wave, whose equal-phase surfaces are perpendicular to the z-axis. Such a situation exists at the very axis of a parallel, collimated beam of light of a finite thickness. Away from the axis, diffraction curves the equal-phase planes.

The attenuation coefficient represents the combined losses due to light absorption and scattering of light power propagating in a turbid medium. Hence, it is a sum of the absorption coefficient, a, and the scattering coefficient, b:

c = a + b

(2)

The attenuation coefficient is sometimes referred to as the extinction coefficient. The latter name is also used in referring to the imaginary part of the refractive index. In the oceanographic literature the attenuation coefficient is also referred to as the beam attenuation coefficient (for example, Voss KJ 1992).

Topics in Particle and Dispersion Science