Biomedical Engineering Reference
In-Depth Information
1.4.3 Electromagnetic Radiation: Optical Devices
Virtually the full gamut of physics offered by optical science has been employed
over the years for the detection of fundamental biophysical processes,
biochemical binding events and species of bioanalytical interest such as
biomarkers for disease. Techniques include those based on measurements of
absorption, luminescence, interference, reflectance, scattering (including Raman
spectroscopy) and refractive index phenomena. The wide variety of techniques
employed in optical sensing are reviewed well in ref. 91. The use of electro-
magnetic radiation in medicine (e.g. diseased and damaged tissue detection by
infrared and Raman spectroscopies) is ubiquitous
92,93
andveryexcitingasthe
technique pertains to the characterization of tumors or tissue damage.
94-96
However, here we concentrate on optical transducers which appear most often
in terms of bioanalytical sensor technology. The field from this perspective has
been reviewed, especially in terms of the advantages presented by the possibility
for label-free detection.
97
In many cases there is a specific focus in view such as
the interesting discussion of clinical diagnostic technology.
98
In contrast with conventional instruments for optical spectroscopy, the
delivery of radiation in biosensor technology takes an alternative pathway and
often involves alternative
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structures. Accordingly we begin with the
'confinement' of light energy.
1.4.3.1 The Optical Wave Guide and Fibers
At the heart of a number of biosensor optical devices is the optical fiber, a
structure ubiquitous in the world of communications technology. In essence,
light transmission along the fiber is produced via a guided wave through an
integral process of internal refection. As depicted in Figure 1.20, light can
transmit along a fiber with complete internal reflection or via some 'loss' of
electromagnetic energy though the effect of reflection/refraction at the interface
where materials of different refractive indices are involved. The physics behind
these processes are elegantly described in the beautiful classic text by Lisa
Figure 1.20
Segment of optical fiber depicting conditions for reflection of radiation at
the internal interface.
