Biophotonics_Concepts_to_Applications

components depend strongly on wavelength, there are significant variations in the
penetration depth of the light from lasers operating at different wavelengths. In
Fig.6.18examples of the light penetration depths are shown for different lasers
across the 190-nm to 10-μm spectrum. For example, light from a 193-nm ArFl laser
or a 2.96-μm Er:YAG laser is totally absorbed in thefirst few micrometers of tissue
owing to high absorption from amino acids in the UV and water absorption in the
IR. In contrast, because of an absence of strongly absorbing chromophores, colli-
mated light in the 600–1200-nm region can penetrate several centimeters into tissue
and the associated scattered light can travel several millimeters through the tissue.
As the collimated beam passes through tissue, it is exponentially attenuated by
absorption and generates a heating effect in the tissue. Scattering also attenuates the
collimated beam and the scattered light in the tissue forms a diffuse volume around
the collimated beam as the light travels through the tissue.
Note that some terminology confusion can occur because the termsintensityand
irradianceboth are used in the literature to describe the power level falling on a
specific tissue area [ 1 – 8 , 31 ]. This terminology variation often is traditional and
depends on the particular topic. In this section, the termirradiancewill be used to
designate the light exposure rate given in J/s/cm^2 or W/cm^2. The important
parameters of tissue exposure include the following:

• The wavelength (or the photon energy) of the incident light


• The optical power emitted by the source


• The irradiance (the power per unit area usually measured in W/cm^2 )


• The spot size (the area irradiated on the tissue)


• The spatial profile of the spot (the irradiance variation across the beam)


• The irradiation time for a CW laser

XeF

XeCl

CO 2

0.001

0.01

0.1

1

10

100 1000 10000

Wavelength (nm)

Penetration depth (mm)

KrF

UV Visible Infrared

ArF

Ar Ho:YAG

HeNe GaAs; Nd:YAG

Alexandrite; Ti:Sapphire; GaAlAs

Fig. 6.18 Tissue penetration
depth of some common
medical lasers

6.5 Light-Tissue Interaction Mechanisms 171