Biophotonics_Concepts_to_Applications

amplification and analog-to-digital conversion electronics, and a computer for
calculating the Fourier transform.
The FTIR spectrometer simultaneously collects spectral data in a wide wave-
length range. The resulting detected spectrum represents a molecularfingerprint of
the material sample, because similar to an actualfingerprint no two unique
molecular structures produce the same infrared spectrum. The designationFourier
transformis in the FTIR name because this mathematical procedure is used to
decode the individual spectral frequencies in the measured data in order to obtain an
interpretable plot of intensity versus frequency.

9.12 Brillouin Scattering Spectroscopy.

Brillouin scattering spectroscopyis defined as the inelastic scattering of light by
thermally excited acoustical phonons. Basically it is an optical technique for
noninvasively determining the elastic moduli or stiffness of materials. Because
diseases can alter the elasticity of bodyfluids or tissues, Brillouin scattering
spectroscopy can be used to distinguish healthy from diseasedfluids or tissues.
Brillouin spectroscopy is similar to Raman spectroscopy because the physical
scattering processes are identical. However, whereas Raman scattering involves
high frequency molecular rotational and vibrational modes, Brillouin scattering
involves the scattering of photons by low frequency phonons. The Brillouin scat-
tering process thus provides information regarding elastic properties of the scat-
tering medium. Optical phonons measured in Raman spectroscopy have
wavenumbers on the order of 10–10,000 cm−^1 , whereas wavenumbers of phonons
involved in Brillouin scattering are on the order of 0.1–6cm−^1. This biophotonics
tool has been used for functions such as in vivo measurements of the rheological
properties of the eye lens, screening for increased total protein in cerebrospinalfluid
during bacterial meningitis, and assessing changes of the microscopic viscoelas-
ticity associated with skin injury [ 59 – 62 ].

Laser

Computer

Fixed mirror

Movable mirror

Beam

splitter

Sample

compartment

Detector unit

and electronics

Fig. 9.21 Basic FTIR
spectrometer configuration

9.11 Fourier Transform Infrared Spectroscopy 285