Passive athermalization of two-lens designs in 8-12micron waveband

Abstract

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 ΔMTF in % and the resulting Temperature Range ΔT 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 (ΔMTF, ΔT) is independent of other lens indexes. The limitations of this method are discussed. Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/23/2015 Terms of Use: http://spiedl.org/terms Proc. of SPIE Vol. 8353 835325-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/23/2015 Terms of Use: http://spiedl.org/terms

Development of a fast aperture, high resolution, wide-angle, optically passive athermalized LWIR lens for driver vision enhancer systems

“…It is also optically passive athermalized without the use of any optomechanical mechanisms in an aluminum housing with a thermal expansion coefficient of 23.6 x 10 -6 K -1 . The athermalized temperature range is -20 to +60°C, with a maximum absolute focal plane deviation of 17 µm, which is smaller than the depth of focus of the given design according to the Rayleigh criteria [19]:…”

Comparison of the thermal effects on LWIR optical designs utilizing different infrared optical materials

“…We start by noting that the change in focus Δf with temperature ΔT of an ideal lens of nominal focal length f o may be calculated by the use of a thermal glass constant γ T related to the refractive index n, the temperature coefficient of refractive index dn/dT, and the coefficient of thermal expansion (CTE) of the lens material α L as follows: 4,8,10 …”

“…The aforementioned lack of an athermal image quality standard has enabled these approximations to persist in literature and replace a more a rigorous methodology for athermalization analysis. 1,3,4,8,9,10 The scope of this study is to begin with the well-known analytical approximations, and then compare to more realistic expectations from modeling of practical systems. This will form the proper foundation for comparing a select group of available IR lens materials to reveal advantages that can be utilized for achieving realistic design goals.…”

“…The first issue is that there is no agreed-upon quantitative definition for an athermal lens, such that lens providers may choose to call any lens "athermal" without specifying the actual lens performance over temperature 4 . The athermal characteristics of a lens system may be evaluated in various ways, ranging from theoretical approximations ("rules of thumb" analytical formulas) to predictive modeling of expected performance through rigorous optomechanical design of the lens, housing and detector components.…”

Passive athermalization of doublets in 8-13 micron waveband

“…(1) Additionally, the housing expands at higher temperatures by thermal expansion coefficient α H and increases the defocusing. Supposing that the housing length is similar to the focal length ' f of lens assembly, then the thermal focus drift becomes: (2) with T Δ temperature difference. Referring to Fig.…”