PerkinElmer Spotlight 200, User Manual

Page: 73 / 132

Techniques for Collecting Spectra . 73
The second situation occurs because the light transmitted by the first polarizer oscillates in
exactly the plane that is blocked by the second polarizer.
Figure 24 Polarizers Parallel (top) and Polarizers Crossed (bottom)
Some materials are
anisotropic
(or
birefringent
), that is, their refractive index depends on
their orientation. These materials can alter the polarization of light passing through them;
this is dependent on the wavelength of the transmitted light.
When you look at an anisotropic sample using polarized light, the change in polarization
caused by the sample means that some light leaks through the second polarizer. Because the
change in polarization is dependent on the wavelength, the color of the light emerging
changes with the distance traveled through the sample and the amount of birefringence
encountered.
Applications
Differences in the birefringence of an object or area may be an indication of chemical
disparity. This can be useful in visibly separating or identifying an object or area of interest
before collecting an infrared spectrum. Some examples are given below.
Laminates
Many polymer structures consist of different layers of material and adhesives of varying
thicknesses bonded together in order to meet physical requirements. If you view a cross
section of the structure using polarized light, you can identify the individual layers and set
the apertures to collect a separate infrared spectrum from each layer. This is useful for
identifying the materials used to create the structure.