Biomedical Engineering Reference
In-Depth Information
4. Scattering
The process of re-emitting or redirecting the light that impinges on the body, so
that it is scattered to directions that are different than the original trajectory of
the beam of light that illuminates the object. A good example is the Rayleigh
scattering of sunlight that is responsible to the blue color of the sky and the
scattering of light by nano-size metal spheres also known as Mie scattering that
allows to observe even small particles at high intensity [ 5 ].
5. Luminescence
Is the emission of light from matter, originating not from the thermal emission
(incandescent) of the bodies. For such a process to occur, some source energy
must excite the material. There are different types of luminescent processes due
to the nature of the excitation, such as electroluminescence which results from
electrical excitation, photoluminescence where the matter is excited by light at
a higher energy than the luminescent light, and chemiluminescence where the
matter is excited by chemical reaction and others.
Spectral imaging relies mostly on the visible-light range which is relevant for the
electronic transitions of atoms and molecules, but the infrared spectrum (usually
2-20 m) is also heavily used in spectroscopy. The infrared light is much less
energetic, and it is usually absorbed by molecules while exciting vibration and
rotation modes, also called vibronic modes. These transition energies are highly
specific to the interatomic bonds and molecule state, and therefore, it is an excellent
fingerprint of its structure [ 6 ].
It is important to distinguish the processes of photoluminescence and absorption.
In fluorescence measurement, fluorescent molecules (or other fluorescing entities
such as quantum dots) are excited by an intense power source at a relatively high
energy and light at lower energy emitted from the material. In many cases, there is
a direct functional relationship between the concentration of fluorescent molecules
and the amount of fluorescence intensity. At low concentrations, this relationship is
linear and therefore quantitative analysis is possible.
Fluorescence measurements are very useful in the life sciences. Mostly due to
developments in biochemistry that allow labeling almost any type of a biological
entity with fluorescent molecules, so that labeled entities can be viewed on top
of a dark background (to increase sensitivity). In addition, when the fluorescent
material concentration is not too high, the signal intensity is a good measure of its
quantity, and the method is therefore quantitative. Moreover, fluorescent proteins are
available nowadays. Their genes can be inserted to a living cell that will produce the
fluorescent probes by itself, a Nobel-prize award invention that revolutionized the
life sciences [ 7 ]. Due to abovementioned advantages, fluorescence became one of
the most abundant research and diagnostics method. In fluorescence, it is essential to
distinguish the weak emitted light from the strong excitation light. This requires the
use of color filters, excitation barrier filters, dichroic mirrors, and emission barrier
filters that allow one to distinguish the two light sources. These filters must have
adequate transmission ranges and high rejection ratios at the required wavelength,
and the spectrum that is measured is, therefore, usually different from the real
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