Advantages of using engineered chalcogenide glass for color corrected, passively athermalized LWIR imaging systems

Abstract

Long wave infrared (LWIR) optical systems are prone to defocus with changes in temperature. IR refractive materials are more thermally sensitive compared to conventional visible glass due to their larger therm-optic coefficients. LWIR systems can be designed to be passively athermal (little or no change to focus with varying temperatures). Chalcogenide glasses provide additional material choices for IR lens designers. In particular, AMTIR5 has been engineered so its therm-optic coefficient matches the coefficient of thermal expansion (CTE) of aluminum, allowing for an athermal singlet. This paper explores the benefits of using engineered chalcogenide glass for color corrected, passively athermal systems.Initially, we present color corrected and passively athermal doublets that are designed with different materials and / or diffractive surfaces. Their thermal and color performance are cataloged for axial beams only. These are intended to be starting components, which readers may then insert into common design forms, such as Petzval, Double Gauss, Telephoto, and Inverse Telephoto.A F/1.3, 20° full field of view, aspheric Petzval lens design form is explored and the MTF is evaluated for -50°C to 85°C in an aluminum housing. From this design, we explore the tradeoffs between using chalcogenide versus crystalline materials, diffractive versus pure refractive surfaces, and engineered chalcogenide (AMTIR5) versus "catalog" materials.

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.…”

Calculation of heat-stabilized achromatic IR-lenses using a graphic method for choice of optical materials

“…В работах [1][2][3][4][5][6][7] рассмотрены требования, которым должны удовлетворять оптические материалы двухкомпонентной системы без воздушного промежутка, обеспечивающие дости-жение ахроматической коррекции при сохранении оптической силы объектива в заданном диа-пазоне рабочих температур с учетом материала корпуса. Согласно этим требованиям соотно-шение характеристик оптических материалов двух компонентов должно быть следующим:…”

Diffraction efficiency change of multilayer diffractive optics with environmental temperature

“…In the MWIR and LWIR, many other materials also can be used as substrates for MLDOEs besides ZNSE and GE. The available materials include Zinc Sulfide (ZNS), AMTIR1 [22], Silicon (SI), Gallium Arsenide (GAAS) [16], GASIR1 [23], IG6 [24] for both MWIR and LWIR. The thermal parameters for the above materials are listed in table 5.…”