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Selecting Infrared Optical Materials

Selecting Infrared Optical Materials. Jay Vizgaitis University of Arizona Optics 521. 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

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Selecting Infrared Optical Materials

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  1. Selecting Infrared Optical Materials Jay Vizgaitis University of Arizona Optics 521

  2. 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!!

  3. Atmosphere Transmittance LWIR covers a very wide spectral band ~6.5 m Broadband IR is considered 3.0 – 14.0 m

  4. Sapphire MgO Silicon As2S3 CaF2 BaF2 GaAs AMTIR-1 AMTIR-3 AMTIR-4 GASIR1 GASIR2 Germanium ZnSe ZnS (Cleartran) 0.1 0.5 1.0 5.0 10.0 50.0 Wavelength (m) Amorphous Glasses Crystalline Materials Transmission Range for Common IR Materials Only a handful of usable materials, most don’t cover MWIR and LWIR

  5. Table of Infrared Optical Materials

  6. 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

  7. 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

  8. Special Material Considerations Germanium: -LWIR designs can best be achieved with solely germanium optics -Large index (n = 4.0) and high v-number (>1200) However… -Temperature > 57°C will cause germanium to start to go opaque -Large dn/dT values require athermalization after ~10° T -Germanium can be very expensive

  9. 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 aren’t as good as other materials Molded GASIR lenses from Umicore

  10. 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

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