Biophotonics_Concepts_to_Applications

is transferred from the photon to the molecule, or the molecule can transfer some
energy to the photon.
The molecular transitions occurring in these two processes are shown in
Fig.6.17. Here hν 0 is the energy of the incident photon and hνRis the energy
gained or given up by the photon through the Raman scattering event. In the
process of inelastic collisions between photons and vibrating molecules, an extre-
mely short-lived transition state orvirtual stateis created. This transition state
brings the collective quantum energy state of the molecule and the photon to a high
energy level, which is called thevirtual level. From this virtual level the molecule
can then relax to an energetically excited molecular level (e.g.,ν= 1) if the original
molecular state was the ground state, or to an energy level (e.g.,ν= 0) below its
initial state if the original molecular state was already in a higher excited state (e.g.,
ν= 1). Thefirst process (from its lowest vibrational energy level to a higher
vibrational state) is calledStokes scattering. If the incoming photon interacts with a
molecule that already is in a higher vibrational state, the molecule can transfer some
of its energy to the photon during the scattering event. This process is calledanti-
Stokes scattering.

6.4 Scattering with Absorption

The previous sections addressed absorption and scattering independently. However,
in actual tissues these two processes are present simultaneously. In this case the
total extinction coefficientμt(also referred to as thetotal interaction coefficient) for
the combined absorption coefficient and scattering coefficient is given by

lt¼laþls ð 6 : 19 Þ

which is the probability of photon interaction with a medium per infinitesimal path
length. This leads to thetotal mean free pathLtof an incident photon

Lt¼

1

lt

¼

1

laþls

ð 6 : 20 Þ

In some spectral regions eitherμaorμscan be a dominant attenuation factor in
tissue. Both mechanisms are always present, but generally occur in variable ratios.
Another factor of interest when considering both absorption and scattering is the
optical albedo a. This parameter is defined as the ratio of the scattering coefficient
to the total attenuation coefficient, that is,

a¼

ls

lt

¼

ls

laþls

ð 6 : 21 Þ

6.3 Scattering 169