Biophotonics_Concepts_to_Applications

6.1.3 Diffuse Reflection

Diffuse reflectionresults from the reflection of light off of microscopically uneven
surfaces for which the sizes of the surface imperfections are comparable or larger
than the wavelength of the incident light. This type of reflection tends to send light
in all directions, as Fig.6.5shows. Because most surfaces in the real world are not
smooth, most often incident light undergoes diffuse reflection. Note however that
the path followed by each individual light ray impinging on an uneven surface still
obeys Snell’s law as it hits an incremental surface area. Diffuse reflection also is the
main mechanism that results in scattering of light within biological tissue, which is
a turbid or random medium with many different types of intermingled materials,
such as cell nuclei, organelles, and collagen, that reflect light in all directions. Such
diffusely scattered light can be used to probe and get images of spatial variations in
macroscopic optical properties of biological tissues. This is the basis of elastic
scattering spectroscopy, also known as diffuse reflectance spectroscopy, which is a
non-invasive imaging technique for detecting changes in the physical properties of
cells in biological tissues (see Chap. 10 ).
When a beam of photons illuminates a diffusely scattering tissue sample, the
diffuse reflectanceis defined as the photon remission per unit surface area from the
tissue. Monte Carlo simulation methods can predict the diffuse reflectance value
accurately, but this is a computationally intensive process [ 15 ]. A faster but still
mathematically involved computational approach is to usediffusion theory. Details
of this method are presented in the literature [ 16 – 18 ].

50º

60º 70º

R=25.4%

R=8.5% Air

Tissue

Fig. 6.4 Angular
dependence of reflectance of
light from a tissue with
n = 1.400

θi

Parallel rays incident

on a rough surface

Reflected rays scatter

in all directions

Uneven surface

Fig. 6.5 Illustration of
diffuse reflection from an
uneven surface

6.1 Reflection and Refraction Applications 153