Multimodal Biomedical Imaging Systems - Biomedical Optical Imaging Technologies: Design and Applications - page 325

reflected light is blocked by the analyzer, thus, only the cross-polarized component
of the scattered light contributes to image formation. This technique is an effective
tool for functional diagnostics and for imaging subcutaneous tissue layers. It has
been also applied extensively in biomedical imaging to eliminate specular reflection.
To design a polarization imaging system, we need to understand the properties of
different polarizers and polarization effects in optical systems [ 34 ].

9.3.3.1 Polarizer

Polarizers are essential in polarization imaging systems to precondition the illu-
mination light and to filter the light from the tissue. A polarizer usually converts
unpolarized or mixed-polarization light into light with a single polarization state
or transforms light between polarization states. There are two types of polarizers:
absorptive polarizers and beamsplitting polarizers. An absorptive polarizer absorbs
light with the unwanted polarization state and transmits light polarized along its
transmission axis. A beamsplitting polarizer splits the incident beam into two beams
having orthogonal polarization states.
Polarizers are generally specified by the following parameters: transmission,
contrast ratio, acceptance angle, wavefront quality, beam deviation, laser damage
threshold, temperature range, surface quality, and geometric specifications. Trans-
mission is the percentage of light that passes through the polarizer, and the contrast
ratio is the ratio between the transmission of the desired polarization and the
undesired orthogonal polarization. The acceptance angle is the range of incidence
angles over which the polarizer has acceptable performance. Wavefront quality is
the measure of distortion when a plane wave passes through the polarizer. Beam
deviation is an angular measure of how much the light direction changes when
passing through the polarizer. The laser damage threshold is the amount of laser
power that the polarizer can handle. Surface quality defines the level of defects
to be allowed on the surface. Geometrical specifications include thickness, outside
diameter, and the clear aperture of the polarizer.
The sheet polarizer is the most common, inexpensive absorptive polarizer. It
consists of a matrix of oriented dye molecules or crystals on a polymer substrate.
A sheet polarizer works throughout the visible spectrum but deteriorates in IR light,
and it is totally ineffective in UV light because polyvinyl alcohol (PVA) absorbs
strongly in the 200-400-nm region. A sheet polarizer has 45-90 % transmission
for polarized light and a contrast ratio ranging from 100:1 to 1,000:1. Its acceptance
angle is relatively large, up to 20 ı . The transmitted wavefront is poor, on the order of
few waves. The advantages of a sheet polarizer include a large angular acceptance
and extreme flexibility for custom shapes and sizes. However, the sheet polarizer
can only be used in low irradiance applications. The typical power limit is 1 W/cm 2 .
Other limitations include low working temperature range and low transmission in
short wavelengths. Another type of absorptive polarizer is made of elongated silver
nanoparticles embedded near the surface of a glass. It is more durable and can
polarize light much better than a sheet polarizer. Other advantages include more