Page 194 - Chalcogenide Glasses for Infrared Optics

170 Cha pte r Se v e n

based on the experimental evidence demonstrating the potential of
using fiber to measure temperature, AMI suggested to Raytek that its
devices could also be made with an option to use a fiber rather than a
lens for direct instant temperature measurement of particular spots.
AMI modified several Raytek devices to demonstrate the potential
appeal to the customer. In the final analysis, Raytek rejected the idea
primarily from the standpoint of the durability of a fiber probe in the
field.
One application for which the fiber was ideally suited was chemi-
cal analysis. The development and use of Fourier transform infrared
spectrophotometers have revolutionized the application of infrared
analytical techniques to chemical analysis. The fact that all generated
spectra are digital and can be directly compared electronically to a
digital reference scan has led to analytical techniques never dreamed
of before. The added ability to scan many times the sample and refer-
ence before the comparison is made leads to extreme sensitivity. The
instrument can be used to measure transmission (absorption) directly
through a sample or with attachments, reflection from the surface of
a solid sample. All measurements are carried out after a prerecorded
reference scan.
An early, very useful FTIR technique involved the use of a flat
plate of an infrared material transparent in the wavelength range of
interest. Analysis by this method is usually referred to as using an
ATR (attenuated total reflection) plate. The plate is polished and most
likely will have edges on both ends polished with a 45° bevel relative
to the flat surface. The purpose of the bevel is to ensure the light
traveling down the plate bounces off the inner plate surface many
times before reaching the other end. On each bounce, the evanescent
light wave couples with the substance covering the plate surface. An
optical arrangement will be provided for the light entering the sample
region to strike the plate end and travel through the plate to the other
end. On leaving the plate, optical arrangement will be provided to
insert the light back into the optical path of the instrument to be con-
verted to an energy versus wavelength digital signal.
First, the plate is scanned a number of times with no liquid in
contact or with a liquid containing no solute or sample. Next, the
plate is scanned the same number of times in contact with the liquid-
sample solution. The two scans are then compared and the results
displayed. The absorption due to only the liquid used will be elimi-
nated in the comparison with only the absorption due to the sample
remaining. ATR plates are supplied by instrument companies, pre-
cisely fabricated and expensive. Several materials are used such as
germanium, silicon, zinc selenide, and sometimes Amtir 1.
Figure 7.13 shows a diagram of methods used at AMI with our
Perkin Elmer FTIR to perform the same kind of analysis based on
AMI fiber and including extruded chalcogenide glass rods. Extrusion
will be discussed in a later section. In the diagram, AMI Amtir 1