PerkinElmer Spotlight 400 Series, User Manual

Page: 73 / 120

Techniques for Collecting IR Images and Spectra . 73
Polymer films
Although they may appear identical when using normal light, the film and inclusion present in
a polymer film may exhibit different degrees of birefringence when viewed using polarized
light. If this is so, you can visibly identify the inclusion and collect an infrared spectrum to
determine its composition.
Rocks minerals and crystals
Most crystals are characteristically birefringent and thus are ideally suited to this technique.
Viewing a mixture consisting of crystals, such as an artificial sweetener, pharmaceutical
powder, or an illicit substance, enables you to visibly separate the components by their
relative size, shape and birefringence.
Fibers
Polarization may enable you to identify and separate fibers in a clump or to view a
bicomponent fiber. Most fibers in their natural state are optically thick and their cylindrical
shape can cause lensing effects. For these reasons, fibers are usually flattened in preparation
for infrared microspectroscopy. This flattening affects the birefringence of the structure and
may degrade the usefulness of this technique.
Biological substances
Birefringence can occur in some biological substances. You can collect infrared spectra of thin
sections of these materials. In some cases, polarized light can reveal structures or chemical
disparities in these structures, and infrared spectra can be collected from the different
regions of interest.
Equipment
The equipment for visible polarization studies consists of two parts: the polarizer and the
analyzer (Figure 26).
The polarizer polarizes the incoming beam from the illuminator and the analyzer contains a
polarizing element that you can rotate to any orientation. It is placed in front of the camera.
The polarizers for both transmittance and reflectance are built into the imager, and are
automatically switched into the beam when the analyzer is inserted.
Figure 26 The analyzer