Biophotonics_Concepts_to_Applications

hair, and eyes their color. As shown in Fig.6.8, the absorption coefficient of
melanin increases monotonically when moving from the visible toward the UV
spectrum. Melanin is an effective absorber of light and is able to dissipate over
99.9 % of absorbed UV radiation. Because of this property, melanin can help
protect skin cells from UV-B radiation damage.
Hemoglobinis the fundamental protein that transports oxygen from the lungs to
the body and also functions as an antioxidant and regulator of iron metabolism.
Each hemoglobin molecule within the red blood cells has four iron-containing heme
groups plus the protein globin. Oxygen atoms readily bind to the iron atoms with
the result that hemoglobin molecules carry 97 % of the oxygen in the blood.
Hemoglobin saturationdescribes the extent to which hemoglobin is loaded with
oxygen molecules. The hemoglobin is fully saturated when oxygen is attached to all
four heme groups. Hemoglobin with no oxygen atoms bound to it is called
deoxyhemoglobinand is denoted by Hb. When oxygen binds with the iron atoms,
the resulting hemoglobin molecule changes its structure and now is calledoxyhe-
moglobin(denoted by HbO 2 ). This structural change results in a change of the
absorption spectrum. The spectra of deoxyhemoglobin and oxyhemoglobin are
shown in Fig.6.9in terms of the molar extinction coefficient [ 26 ]. Both hemo-
globin forms absorb strongly up to 600 nm and have absorption peaks around 280,
420, 540, and 580 nm, which are used for analyses of blood health. At the 600-nm
wavelength the HbO 2 spectrum drops sharply by almost two orders of magnitude
and then stays near the new low level for longer wavelengths. The absorption of the

Wavelength (μm)

Absorption coefficient (cm

-^1 )

1

10

100

1000

0.10 1.00 10.0

KrF

ArF 249 nm

193 nm

Er:YAG

2940 nm

Diagnostic or

therapeutic

window

Water

Whole

blood

Melanin

Water

0.20 0.50 2.00 5.00

CO 2

10.6 μm

Proteins

Fig. 6.8 Absorption
coefficients of select
biological tissues and some
common biological laser
wavelengths

Table 6.1 Absorption coefficients of three amino acids at their local peak absorption wavelengths

Amino acid Peak absorption wavelength (nm) Absorption coefficient (cm−^1 )

Tryptophan 280 5400

Tyrosine 274 1400

Phenylalanine 257 200

158 6 Light-Tissue Interactions