Page 155 - Chalcogenide Glasses for Infrared Optics

CHAPTER 6

Unconventional

Lens Fabrication,

Aspheric Surfaces,

and Kinos

n Chap. 5 the fabrication methods producing spherical optical
surfaces in use for many years were discussed. The methods were
Iapplied in the fabrication of visible as well as infrared lenses. Two
technical advances brought about a dramatic change. The first was
the direct application of computer control of machine tools that led to
diamond point turning. The second was the development of inexpen-
sive uncooled infrared detector arrays, the most expensive part of
1
the cost of infrared cameras. Now the most expensive cost of the cam-
eras became the optics. Attention turned to lowering the optics cost.

6.1 Optical Designs
In Chap. 5 an optical element designed by Louis Fantozzi of Diversi-
fied Optics was described as a test element in an image spoiling test
for Amtir 1 blanks. The lens had three elements, two germanium and
one Amtir 1. Each lens had all spherical surfaces. When tested in the
wavelength range of operation, 8 to 12 µm, the design produced an
MTF score of 70 percent at 10 line pairs per millimeter and 50 percent
at 20 line pairs per millimeter. The performance was more than ade-
quate for the then Army common module FLIR. Lens designs for high
performance with all spherical surfaces required the use of three or
more elements and two or more different optical materials. It is true
that aspheric designs could be made to improve performance and
produced, but with great difficulty using the old spherical methods.
The goal to lower cost became the search for new designs using only
two lens elements and still maintaining the required performance.

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