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Athermalization Techniques in Infrared Systems

Outline. Effect of temperature on focusAthermalization of focusOptically passiveMechanically passiveElectromechanically activeConclusionReferences . OPTI 521 Fall 2010Daiker Tutorial2. Effect of temperature on focus. Defocus with temperature change due to the nature of IR lens materials

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Athermalization Techniques in Infrared Systems

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    1. Athermalization Techniques in Infrared Systems OPTI 521 – Fall 2010 Tutorial Jeffrey T Daiker

    2. Outline Effect of temperature on focus Athermalization of focus Optically passive Mechanically passive Electromechanically active Conclusion References

    3. Effect of temperature on focus Defocus with temperature change due to the nature of IR lens materials g the thermo-optical coefficient of the lens is positive for most IR lens materials and indicates a negative change in focal length with temperature

    4. Tolerable temperature change

    5. Athermalization – optically passive Combine suitably chosen lens materials which together compensate for thermal defocus Athermal achromat Total power Achromatism Athermalization

    6. Athermal achromat Example: Si, Ge, ZnS Optimum three-material solution

    7. Multiple Lens Systems Optically passive athermalization very complex Zoom lens is extreme case where completely passive athermalization generally not possible Utilize optical design software

    8. Thermal Modeling in ZEMAX Before doing any thermal modeling Set TCE of air spaces

    9. Thermal Modeling in ZEMAX Built-in tool in the Multi-Configuration Editor Add different temperature configurations

    10. Thermal Modeling in ZEMAX MCE should look something like this

    11. Athermalization in ZEMAX Carefully construct merit function Minimize RMS wavefront between two temperatures, for example Optimization using glass substitution Create a glass catalog from a short list (Si, Ge, ZnS, ZnSe, MgO, KRS5, AMTIR1, CaF2 for 3-5 microns) No guarantee solution is global optimum Use design practices, experience, and resources

    12. Athermalization – mechancially passive Involves some method of moving a lens element or elements by an amount that compensates for thermal defocus By using two different materials with very different TCE arranged as either differential expansion cylinders or rods, it is possible to move the compensating element directly Rods or cylinders must be of sufficient length

    13. Differential expansion Combine spacers of length L1 and L2 with TCE a1 and a2 respectively, then to athermalize over distance L Using materials with a > 0 requires L < 0

    14. Athermalization – electromechanically active Relies on compensator elements driven in a temperature controlled manner using information from separate temperature sensors Brute force solution Most suitable for complex systems such as zoom lenses where an electomechanical focus mechanism already exists

    15. Avoidance of the Problem Example: all-reflective system consisting of aluminum mirrors in an aluminum housing

    16. Conclusion

    17. References Povey, V. “Athermalisation Techniques in Infra Red Systems,” Proc. of SPIE Vol. 0655, Optical System Design, Analysis, and Production for Advanced Technology Systems, ed. Fischer, Rogers (Apr 1986) Rogers, P. “Athermalization of IR Optical Systems,” Critical Review Vol. CR38, Infrared Optical Design and Fabrication, ed. R. Hartmann, W.J. Smith (Apr 1991) Jamieson, T. “Athermalization of optical instruments from the optomechanical viewpoint,” Critical Review Vol. CR43, Optomechanical Design, ed. P.R. Yoder, Jr. (July 1992) Rayces, J. “Thermal compensation of infrared achromatic objectives with three optical materials,” SPIE Vol. 1354, International Lens Design Conference (1990) Riedl, M. “Optical Design Fundamentals for Infrared Systems,” tutorial texts in optical engineering; v.TT20, SPIE Rogers, P. “Thermal Compensation Techniques,” Handbook of Optics, Volume I, Part 9, Chapter 39. McGraw-Hill, 1995. Zemax Knowledge Base. 2010. http://www.zemax.com/kb/articles/106/1/How-to-Model-Thermal-Effects-using-ZEMAX/

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