Selecting Infrared Optical Materials

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Selecting Infrared Optical Materials
Jay Vizgaitis University of Arizona Optics 521
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Selecting IR Materials
How do you select the proper materials for your
infrared system? Design with the materials that
will best meet your needs May result in
trade-off between cost and performance
-Performance MTF, weight, transmission,
operating environment -Cost molding,
diamond turning, bulk material Best materials
to design with, tend to be the most expensive!!
Know your key system requirements!!
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Atmosphere Transmittance
LWIR covers a very wide spectral band 6.5 ?m
Broadband IR is considered 3.0 14.0 ?m
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Transmission Range for Common IR Materials
Only a handful of usable materials, most dont
cover MWIR and LWIR
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Table of Infrared Optical Materials
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Comparison of MWIR and LWIR Dispersion
-LWIR color correction is easily achieved with
Germanium optics only -Dispersion of Germanium in
LWIR is significantly different from that of
MWIR -Color correction in MWIR can be quite
challenging -Broadband correction is extremely
challenging due to lack of materials and changing
v-number of Germanium
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Reflective Optics
-Reflective optics are very useful for broadband
performance -Diamond turned mirrors are feasible
due to long wavelengths -Obscurations can have
significant performance impact -Off-axis designs
are often used instead
Cassegrain
Obscuration degrades diffraction limited MTF
Three Mirror Anastigmat
Unobscured aperture results in ideal MTF
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Special Material Considerations
Germanium -LWIR designs can best be achieved
with solely germanium optics -Large index
(n 4.0) and high v-number (gt1200) However
-Temperature gt 57C will cause germanium to
start to go opaque -Large dn/dT values
require athermalization after 10 ?T
-Germanium can be very expensive
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Special Material Considerations
Chalcogenide Glasses -Chalcogenide glasses can
provide the most cost effective solution -Some
chalcogenide glasses are moldable (AMTIR-4, 5,
GASIR 1,2) However -Inhomogeneity is
greater than crystalline materials
-Hardness is much less than crystalline
materials -Lower melting points of some
glasses make coating more difficult
-Dispersion characteristics arent as good as
other materials
Molded GASIR lenses from Umicore
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Conclusions
Lens material choices in the infrared is both
easier and harder -Fewer materials, but higher
indices -Aspheres and diffractives
are easily fabricated Every choice of material
has its penalty and advantage Spectral band and
cost tend to dictate what materials are best