Biophotonics_Concepts_to_Applications

An important factor to keep in mind is that biological tissue has a multilayered
characteristic from both compositional and functional viewpoints. Specific bio-
logical processes and diseases occur at different levels within this multilayered
structure. Thus when selecting an optical source, it is necessary to ensure that the
specific wavelength of the light that is aimed at the targeted biological process or
disease can penetrate the tissue down to the desired layer.
The medicalfield also uses many types of surgical lights and a variety of lamps
for illumination and visual diagnostic purposes in operating rooms, medical offices,
and health clinics. In general these sources do not relate directly to biophotonics, so
they will not be covered in this chapter. A key factor in selecting an optical source
is the range of wavelengths over which the source emits and the radiant power per
unit wavelength over the emission spectrum. The radiant power spectrum can range
from being nearly monochromatic (as emitted by certain lasers) to being broadband,
that is, the emission covers a wide spectrum, which is the characteristic of a light
source such as an incandescent lamp. Other types of sources, for example
light-emitting diodes, have spectral ranges between these two extremes. Further
details on specific optical sources can be found on manufacturers’data sheets.
For biophotonics the radiometric wavelengths of interest range from about
190 nm to 10μm, which covers the ultraviolet, visible, and infrared regions.
A selection of sample light sources and their emission wavelengths are shown in
Fig.4.1and summarized in Table4.1. To cover a wide range of applications, the
source types include arc lamps, semiconductor light-emitting diodes (LEDs),
superluminescent diodes, and various lasers including excimer, gas, liquid,
solid-state crystal, semiconductor, and opticalfiber lasers. The following sections
describe these sources and their operating characteristics.
In this chapter, Sect.4.1first defines terminology used in radiometry, which
deals with the measurement of optical radiation. Understanding the significance of
this terminology is important when determining and specifying the degrees of
interaction of light with tissue. Then the material in Sects.4.2–4.4describes a
selection of optical sources used in biophotonics and presents the characteristics of

100 200 400 700 1000 2000 4000 10000

Wa v e l e ngth (nm)

20000

UV Visible Near infrared Mid-infrared

Nd:YAG

1064 nm

Er:YAG

2.94 μm

Ho:YAG

2.10 μm 10.6 μmCO^2

Ar

488 nm

ArF

193 nm

KrF

249 nm

HeNe

632 nm

GaAlAs

780-870 nm

Free-electron

5.4,6.1,6.45,7.7 μm

Optical fiber

1030-2100 nm

Fig. 4.1 Examples of light sources used across the biophotonics spectrum

92 4 Fundamentals of Light Sources