Biophotonics_Concepts_to_Applications

broad-spectrum ultraviolet radiation is the strongest and most damaging to living
things. Ultraviolet radiation (UVR) is classified into three standard wavelength
ranges identified as UV-A (315–400 nm), UV-B (280–315 nm), and UV-C (100–
280 nm). Among the uses of UV radiation are the following:

• 230 – 400 nm: optical sensors and instrumentation


• 240 – 280 nm: sterilization of surface areas and water


• 250 – 405 nm: bodyfluid analysis


• 270 – 300 nm: protein analysis, drug discovery


• 300 – 320 nm: medical light therapy


• 315 – 400 nm: curing of dental bonding material


• 390 – 410 nm: cosmetic sterilization.
  Uses of LEDs in the visible and near infrared spectral regions include photody-
  namic therapy (skin cancer and acne treatments),fluorescence spectroscopy, blood
  and oxygen sensors, endoscopic illumination of internal organs with simulated nat-
  ural light, cosmetic treatments (e.g., wrinkle removal and hair growth), treatment of
  prenatal jaundice, promotion of wound healing, and optogenetic neural stimulation.

4.3.3 Modulation of an LED

A variety of biophotonics measurement techniques use pulsed or modulated light
signals. These systems include optical spectroscopy, tomography, modulated
Raman spectroscopy, and neuroimaging. Both LEDs and laser diodes are advan-
tageous for such uses because of the ability to directly modulate the optical output
with a time-varying electric current. Theresponse timeorfrequency responseof an
optical source dictates how fast an electrical input drive signal can vary the light
output level. The maximum possible modulation frequency depends largely on the
recombination lifetime(also calledinjection carrier lifetime)τiof the electron-hole
pairs in the recombination region (the active light-producing region) of the device.
This time can vary widely between different LED types. If the drive current is

1480 1520 1560 1600 1640

Wa v e l e ngth (nm)

Peak wavelength (1546 nm)

Spectral widths

broaden with

increasing wavelength

FWHM

Fig. 4.9 Typical spectral
patterns for edge-emitting and
surface-emitting LEDs at
1546 nm showing that the
patterns are wider for surface
emitters

102 4 Fundamentals of Light Sources