Dðk 1 Þ¼Seoðk 1 ÞcdþðÞ 1 Sedðk 1 Þcd ð 6 : 7 ÞDðk 2 Þ¼Seoðk 2 ÞcdþðÞ 1 Sedðk 2 Þcd ð 6 : 8 ÞBy eliminating the factorcdand letting D 12 =D(λ 1 )/D(λ 2 ), then solving these
two equations for S yieldsS¼
edðÞk 1 edðÞk 2 D 12
½eoðÞk 2 edðÞk 2 D 12 ½eoðÞk 1 edðÞk 1ð 6 : 9 Þwhere D(λ 1 ) and D(λ 2 ) are the measured optical densities atλ 1 and λ 2 ,
respectively.6.3 Scattering
Scattering of light occurs when photons encounter a compact object having a
refractive index that is different from the surrounding material. The scattering pro-
cess will redirect the photons to follow diverted paths. From a physics point of view,
scattering in tissue is the same process as the scattering of sunlight by the atmo-
sphere, which gives the sky a blue appearance and makes clouds appear white.
However, scattering in biological media is much more involved due to the complex
intermingled structure of tissues. Because most human tissues are heterogeneous
materials that have spatial variations in their optical properties, they are considered
to be cloudy or opaque materials known asturbid media. The consequence of these
spatial density and optical variations is that tissues strongly scatter light. If the
decrease in optical power due to absorption is small compared to scattering losses,
then a large fraction of the photons in a light beam entering a tissue are scattered
multiple times. This process, which results in a diffuse distribution of the light beam,
is being used for imaging methodologies such as those described in Chap. 10.
Light scattering in tissue depends on the wavelength of the light together with
the sizes, structures, and refractive indices of tissue components that are responsible
for the scattering, for example, cell membranes, collagenfibers, and organelles.
These scattering processes are widely used in both diagnostic and therapeutic
biomedical photonics applications. In diagnostic applications such as imaging,
disease-induced changes in the tissue can affect scattering properties, thereby giving
diagnostic indicators for distinguishing healthy tissue from diseased tissue. In
therapeutic applications using the principles of laser-tissue interactions (see
Sect.6.5), photon scattering data is useful for functions such as determining light
dosage levels and for assessing progress during a light therapy session. Chaps. 9
and 10 have more details on these applications.
The light scattering elements are contained within the four basic tissue types,
which are epithelium tissue (a sheet of cells that covers a body surface or lines a
160 6 Light-Tissue Interactions