OPTI 521 – Fall 2010
Tutorial
Jeffrey T Daiker
Effect of temperature on focus
Athermalization of focus
Optically passive
Mechanically passive
Electromechanically active
Conclusion
References
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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
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Tolerable temperature change for a Ge lens
100
delta T (deg C)
F/4
F/2
F/1
10
1
0.1
0
0.1
0.2
0.3
lens diameter (m)
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Combine suitably chosen lens materials which
together compensate for thermal defocus
Athermal achromat
Total power
Achromatism
Athermalization
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Example: Si, Ge, ZnS
Optimum three-material solution
Config 1
Config 2
0.0000 (deg)
+20⁰C
+60⁰C
40.00
-20⁰C
Config 3
Surface: IMA
Layout
Total Axial Length:
Configuration Matrix Spot Diagram
Units are µm.
1.01268 mm
athermal achromat 216 v1.ZMX
Configuration 3 of 3
OPTI 521 – Fall 2010
Scale bar : 40
Daiker Tutorial
Airy Radius: 19.02 µm
Reference : Chief Ray
athermal achromat 216 v1.ZMX
Configuration: All 3
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Optically passive athermalization very complex
Zoom lens is extreme case where completely
passive athermalization generally not possible
Utilize optical design software
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Before doing any thermal modeling
Set TCE of air spaces
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Built-in tool in the Multi-Configuration Editor
Add different temperature configurations
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MCE should look something like this
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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
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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
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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
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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
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Example: all-reflective system consisting of
aluminum mirrors in an aluminum housing
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Optically passive
Single FOV
Dual FOV
Zoom
Performance
X
Mechanically
passive
X
Very good
Good
Reliability
Very good
Fair to good
Weight
Very light
Can be heavy and
bulky
Power requirements
Environmental
Very good
stability
Ease of maintenance Excellent
Cost
Cheap
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Good
Good
Fairly cheap
Daiker Tutorial
Electromechanically
active
X
X
X
Depends on
technique
Depends on
components
Heavy
X
Concerns under
vibration
Fair to good
Expensive
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1.
2.
3.
4.
5.
6.
7.
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-toModel-Thermal-Effects-using-ZEMAX/
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