handbook of modern sensors

112 3 Physical Principles of Sensing
The frequency of light waves in vacuum or any particular medium is related to its
wavelength λ by Eq. (3.128), which we rewrite here as
where c is the speed of light in a medium.
The energy of a photon relates to its frequency as
ν = c λ , (3.146)
E = hν, (3.147)
where h = 6.63 × 10 −34 Js(4.13 × 10 −15 eV s) is Planck’s constant. The energy E is
measured in 1.602 × 10 −19 J=1 eV (electron volt).
Ultraviolet and visible photons carry relatively large energy and are not difficult to
detect. However, when the wavelength increases and moves to an infrared portion of
the spectrum, the detection becomes more and more difficult. A near-infrared photon
having a wavelength of 1 µm has an energy of 1.24 eV. Hence, an optical quantum
detector operating in the range of 1 µm must be capable of responding to that level
of energy. If we keep moving even further toward the mid- and far-infrared spectral
ranges, we deal with even smaller energies. Human skin (at 37 ◦ C) radiates nearand
far-infrared photons with energies near 0.13 eV, which is an order of magnitude
lower than red light, making them much more difficult to detect. This is the reason
why low-energy radiation is often detected by thermal detectors rather than quantum
detectors.
The electromagnetic wave (now we ignore the quantum properties of light) has the
additional characteristic that is polarization (more specifically, plane polarization).
This means that the alternating electric field vectors are parallel to each other for all
points in the wave. The magnetic field vectors are also parallel to each other, but
in dealing with the polarization issues related to sensor technologies, we focus our
attention on the electric field, to which most detectors of the electromagnetic radiation
are sensitive. Figure 3.48A shows the polarization feature. The wave in the picture
is traveling in the x-direction. It is said that the wave is polarized in the y-direction
because the electric field vectors are all parallel to this axis. The plane defined by the
direction of propagation (the x axis) and the direction of polarization (the y axis) is
called the plane of vibration. In a polarized light, there are no other directions for the
field vectors.
Figure 3.48B shows a randomly polarized light which is the type of light that is
produced by the Sun and various incandescent light sources; however, the emerging
beam in most laser configurations is polarized. If unpolarized light passes through
a polarization filter (Polaroid), only specific planes can pass through and the output
electric field will be as shown in Fig. 3.48C. The polarization filter transmits only
those wave-train components whose electric vectors vibrate parallel to the filter direction
and absorbs those that vibrate at right angles to this direction. The emerging light
will be polarized according to the filter orientation. This polarizing direction in the filter
is established during the manufacturing process by embedding certain long-chain
molecules in a flexible plastic sheet and then stretching the sheet so that the molecules
are aligned in parallel to each other. The polarizing filters are most widely used in the