Biomedical Engineering Reference
In-Depth Information
resistance to UV radiation and chemicals, better resistance to temperatures (up to
C 400 ı C), and a large acceptance angle, as high as 20 ı .
Beamsplitting polarizers split the incident light into two beams with orthogonal
polarization states by one of the following mechanisms: reflection, birefringence,
thin film coating, or wire-grid metal coating. The thin film beamsplitting polarizer
is based on interference effects. A thin film stack of alternating high- and low-
index materials of appropriate thickness is coated onto one surface of a plate
polarizer or onto the hypotenuse of a right angle prism that is then cemented to
a second right angle prism to form a cube. The thin film polarization beamsplitter
can tolerate relatively high light levels of 500 W=cm 2 . One major limitation of thin
film polarizers is that their performance has a strong dependence on wavelength and
angle of incidence.
Birefringent polarizers exploit the birefringent properties of crystals such as
calcite and quartz. When the unpolarized ray strikes the surface of the crystal, it
is split by refraction into two linearly polarized rays with orthogonal polarization
states: an ordinary ray (o-ray) and an extraordinary ray (e-ray). There are several
types of birefringent polarizers, such as the Nicol prism, Glan-Thompson prism,
Glan-Foucault prism, Glan-Taylor prism, and Wollaston prism. The angle of
incidence in birefringent polarizers is limited, and the aperture is smaller than 30
mm in diameter due to the limited availability of large crystals.
A wire-grid polarizer consists of an array of fine parallel metallic wires deposited
on one surface or imbedded into a glass substrate. The transmitted wave has an
electric field purely perpendicular to the wires and is, therefore, linearly polarized.
The direction of polarization is defined by the wire direction, not by the plane
of incidence. The performance changes very little with angle of incidence; it can
be used at acceptance angles of 20 ı without depolarization of the skew rays. The
transmitted wavefront error of a wire-grid polarizer is small and suitable for most
imaging applications. However, astigmatism and coma introduced by the titled
polarization beamsplitter are a concern for the transmitted beam in some optical
systems. Because it is made from metal wire and glass, a wire-grid polarizer
can withstand temperatures as high as 200 ı C and power densities greater than
50 kW=cm 2 . One limitation of the wire-grid polarizer is its surface quality. Its
scratch-dig is around 80-50, which is relatively poor due to current manufacturing
methods.
Choosing the right polarization element is one of the key steps in designing
a polarization imaging system. Each polarization element has its own niche in
polarization imaging with applications that it best serves. By understanding the
requirements of an imaging system and the key requirements of the polarization
elements, one can identify the polarization elements that best fit the specific
application. One of the first parameters to consider is wavelength range [ 34 ]. This
can easily limit the types of polarizers to be considered. For example, for UV light
with wavelengths between 270 and 400 nm, only a few polarizers, such as wire-grid
and crystal polarizers, are available. Other factors to consider include acceptance
angle, size, working temperature, and the transmission/reflection wavefront.
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