Page 139 - Chalcogenide Glasses for Infrared Optics

116 Cha pte r F o u r

A 1-µm-wavelength gallium arsenide light emitter is placed off-axis at
the focal point of a 10-in telescope mirror. Focal lengths of both mirrors
are 48 in. The mirrors are aligned facing each other. A NIR camera is
placed off-axis in the opposite direction at the second mirror’s focal
point. The result is a beam of parallel light passing through an aperture
and focused on the camera with the image shown on a TV display. The
polished plane parallel plate is placed against the aperture. Any regions
in the plate where the refractive index is varying will show up in the
image darker than the rest of the plate due to phase cancellation in
the parallel wavefront. Also, cracks or large bubbles or particles will
be visible. The process is carried out in a darkened room. The operator
may mark on the plate with a grease pencil any areas that are not
homogeneous and should be avoided. When the plate is sawed or core-
drilled for blanks, these marks are used to guide the operator.
In the 1980s, AMI used an image spoiling test as ordered by the
Army to verify the optical quality of each lens blank produced. The
process was used to measure the modulation transfer function (MTF)
in the 8- to 12-µm range of a high-performance FLIR test module and
then to remeasure the module with the blank in the optical path. The
decrease of the MTF score was used to pass or reject the blank. The
process was time-consuming and expensive. Some blanks failed
because of the quality of the polish, not striations. After a period of
time, AMI was able to demonstrate to the Army that the AMI striae
scope method was better than the image spoiling test and the MTF
test should be discontinued. The cost of the lens blanks was reduced
by 20 percent. To emphasize the usefulness of this technique, two
striae scope photographs are shown in Fig. 4.19. The top photograph
is taken of an early Amtir 5 plate, number 10. One can clearly see the
variation in index in the plate, striae. The second photograph is plate
number 19, showing a striae-free plate due to the success of the pro-
cess adjustments.
Use of the striae scope is somewhat subjective and yields no abso-
lute number useful for comparing the homogeneity of different infra-
red optical materials. The classical method is to use an interferometer
to measure the optical wavefront distortion (OPD) when light is
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transmitted through a plate of the material. Rosberry reported
perhaps the first homogeneity results for infrared optical materials.
The materials tested were silicon, zinc sulfide, magnesium fluo-
ride, and calcium fluoride. Although other companies continued
using MTF image spoiling tests, AMI decided it would be a good
idea to have AMI materials evaluated by an interferometer test. A
plate of Amtir 1 was sent to R. M. Ranat of Pilkington. A MTF
image spoiling test as well as the interferometer showed that Amtir 1
was about 2 times as homogeneous as single-crystal, annealed
germanium. Bill Spurlock of Exotic Materials found a similar value
for Amtir 1. Later Spurlock measured Amtir 3 and Sullivan of
Exotic Materials measured gallium arsenide for AMI. The results