Passive athermalization has become a key-technology for automotive and other outdoor applications using modern uncooled 25 and 17 micron bolometer arrays. For high volume applications, passive athermalized optical designs with only two lenses reduce costs. A two lens solution requires a careful choice of lens and housing materials. A first order approach to thermal drift uses the RAYLEIGH criteria for depth of focus. It can be seen that narrow field of view lenses are the most sensitive to defocus with temperature. The different methods used to achieve stable performance over the required Temperature Range can be compared, namely passive optical athermalization and passive mechanical athermalization. GASIR® possesses inherent properties enabling optical passive athermalization. High resolution, two element designs for different field angles are presented. Each lens category is present: Super Wide Angle, Wide Angle, Standard, Tele and Super Tele. All examples are designed for 17micron VGA-detectors. These designs use aspheres and diffractive structures. The impact of temperature on all these parameters can only be determined by ray tracing. The proposed metric is the average of the tangential and sagittal MTF versus image height at Nyquist frequency. A very nonlinear impact of temperature on MTFA at different image heights is clearly visible. Examples are shown. An MTF based criteria for judging athermalization is proposed. It contains two values: the admissible MTF-drop ▵MTF in % and the resulting Temperature Range ▵T in Kelvin. The procedure to get these values is demonstrated. Values of 9 lens assemblies are listed. A comparison with results of first order approach shows limitations of this approach. A general quantification of athermalization is proposed. The pair of values (▵MTF, ▵T) is independent of other lens indexes. The limitations of this method are discussed.

Date Published: 31 May 2012
PDF: 10 pages
Proc. SPIE 8353, Infrared Technology and Applications XXXVIII, 835325 (31 May 2012); doi: 10.1117/12.918112

Published in SPIE Proceedings Vol. 8353:
Infrared Technology and Applications XXXVIII
Bjørn F. Andresen; Gabor F. Fulop; Paul R. Norton, Editor(s)