Biomedical Engineering Reference
In-Depth Information
radiation, usually results from the interaction of light with matter, and therefore,
it carries a fingerprint of the material, or structure.
The spectrum can be a result of different interactions of light with matter.
Light can be described as a flux of particles, or photons, that each one carries a
well-defined energy. The amount of energy depends on the wavelength of light as
described by
hc
E
D
(4.1)
where h Š 6:626 10 34 ŒJ s is Planck constant and c D 2:998 10 8 Œm=s
is the speed of light. Therefore, knowing the spectrum provides a direct access to
the energy level transitions of the atoms and molecules that constitute the material
being measured. A simple substitution of constants leads to a convenient form of
this equation, E D 1;240=, where substituting the wavelength in nanometer units
gives the energy in electronvolt units (eV).
In addition, the spectrum is affected by the geometry of the material being
studied. The geometry depends on the shape and dimensions of a structure, its
index of refraction, as well as the index of refraction of the boundary material. Such
properties can be calculated by solving Maxwell's equations, for which there are
now a handful of excellent commercial programs.
We therefore see that generally, a measured spectrum results both from the
intrinsic properties of the material from which it is made and from its geometry and
boundaries. This fact should be taken into account when performing a measurement.
It also makes the interpretation more complex.
The most important processes that probe the energy level transition of atoms,
molecules, or bulk materials are:
1. Emission
The process of transforming energy of some sort onto electromagnetic radiation.
All the light sources are emitting photons, including light bulbs, fluorescent
molecules, lasers, a blackbody, and the sun. In a blackbody the spectrum is a
result of the temperature, the macroscopic geometry of the body and the quantum
nature of light (e.g., the Sun). In contrast, the emission of lasers, a fluorescent
molecule and an atom is a result of electrons' quantized well-separated energy
levels that are being excited by some sort of energy and fall back to lower-level
energies, usually the ground energy state.
2. Absorption
An electron is excited from the ground state to an empty higher-level energy
band. The absorption of simple entities such as atoms and molecules is rather
simple, as the number of energy bands is not large. When the structure of the
absorbing material becomes more complex, such as for biological tissue, the
absorption can be very complex [ 4 ].
3. Reflection
The light is reflected from the sample according to the reflection law of light,
the geometry of the material, and the index of refraction of the matter and its
boundaries.
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