6.2 Absorption
Light absorption in tissue depends on the molecular composition. Molecules will
absorb light when the photon energy equals an interval between quantum
mechanical energy levels of the molecules. In biological matter the constituent
atoms of molecules are principally elements such as H, C, N, O, and S. After light
has entered a tissue, interactions between the electromagneticfields of the incoming
photons and the molecules of the medium will cause absorption of some of the light
energy. This absorption results from the conversion of a portion of the light energy
into thermal motion or an increase in molecular vibrations in the material. That is,
photons will give up their energy to move the vibrational state of a molecule to a
higher vibrational state. The degree of the absorption depends on factors such as the
electronic structure of the atoms and molecules in the material, the wavelength of
the light, the thickness of the absorbing layer, and the temperature.
6.2.1 Absorption Characteristics.
According to the theory of quantum mechanics, the vibrational energy of a mole-
cule is quantized into discrete energy levels. For an ideal system, the molecular
motion can be described by a harmonic oscillator, which obeysHooke’s Law. This
law assumes that when a system is displaced from equilibrium by a distance d, then
there is a restoring force F that is proportional to the displacement by a factor k, that
is, F = kd. The discrete energy levels for a harmonic oscillator are evenly spaced.
However, this idealized situation does not hold in a real system, because the
restoring force is no longer proportional to the displacement. In this case the
molecular bonding energy can be described in terms of ananharmonic oscillatoror
aMorse potential, as shown in Fig.6.6[ 19 – 21 ]. In this description, the energy
separation between vibrational levels decreases with increasing energy until a
continuum state is reached where molecular breakup (dissociation) takes place (see
Sect.6.5.5for the results of this effect).
In Fig.6.6, at a specific vibrational level the displacement d is the oscillating
variation of the distance between the nuclei of the constituent atoms of the molecule
in an excited state and the equilibrium bond distance re. For example, at the
vibrational levelν= j the interatomic distance d vibrates between dj,minand dj,max.
Thus as the vibrational energy level approaches thedissociation energyDe, the
increasingly large oscillating distances between the atoms eventually causes the
molecule to break apart. In Fig.6.6, the dissociation energy Deis defined relative to
the dissociated atoms and D 0 is the true energy required for dissociation of the
molecule.
Light absorption is quantified by a parameter called theabsorption cross section
σa, which is the absorbing strength of an object. This parameter gives the propor-
tionality between the intensity I 0 =P 0 /A of an incident light beam of power P 0
154 6 Light-Tissue Interactions