sterilizing instruments, great care must be exercised to avoid human exposure to
this light.
Example 1.2Show that photon energies decrease with increasing wave-
length. Use wavelengths at 850, 1310, and 1550 nm.Solution: From Eq. (1.3) it follows that E(850 nm) = 1.46 eV, E
(1310 nm) = 0.95 eV, and E(1550 nm) = 0.80 eV. Thus, the energy
decreases with increasing wavelength.In relation to Fig.1.5, biophotonics disciplines are concerned mainly with
wavelengths falling into the spectrum ranging from the mid-UV (about 190 nm) to
the mid-IR (about 10.6μm). Generally, because optical properties vary from one
tissue type to another, specific lightwave windows are needed to carry out most
therapeutic and diagnostic biophotonics processes. Thus, knowing the details of these
properties allows the specification and selection of photonics components that meet
the criteria for carrying out a biological process in a selected optical wavelength band.
The interaction of light with biological tissues andfluids is a complex process
because the constituent materials are optically inhomogeneous. More details on
these effects are given in Chap. 6. Owing to the fact that diverse biological tissue
components have different indices of refraction, the refractive index along some
path through a given tissue volume can vary continuously or undergo abrupt
changes at material boundaries, such as atflesh and blood vessel interfaces. This
spatial index variation gives rise to scattering, reflection, and refraction effects in the
tissue. Thus, although light can penetrate several centimeters into a tissue, strong
scattering of light can prevent observers from getting a clear image of tissue
characteristics beyond a few millimeters in depth.
1.4 Light Absorption
Light absorption is another important factor in the interaction of light with tissue,
because the degree of absorption determines how far light can penetrate into a
specific tissue. Figure1.6 shows the absorption coefficients as a function of
wavelength for several major tissue components. These components include water
(about 70 % of the body), proteins, whole blood, melanin (pigments that give skin,
hair, and eyes their color), and the epidermis (outer layer of the skin).
Most tissues exhibit comparatively low absorption in the spectral range that
extends from 500 nm to about 1500 nm, that is, from the orange region in the
visible spectrum to the near infrared (NIR). This wavelength band is popularly
known as thetherapeutic windowor thediagnostic window, because it enables the
best conditions for viewing or treating tissue regions within a living body by optical
means. Note that although the absorption coefficients for tissue components such as
1.3 Biophotonics Spectral Windows 11