Having designed refractive optics for SWIR, MWIR, and LWIR imagers, I’m familiar with the unique challenges associated with the mid and longwave spectral bands. In the SWIR, IR fused silica and a number of other glasses from the Schott catalog can be used, but in the MWIR and LWIR, the selection is much more limited.
In aerospace applications, the need to design for unstable environments can reduce “glass” options further. For example, while germanium is a good choice in terms of hardness and transmission well into the LWIR, its application is limited by how much its refractive index changes with temperature. Materials with more moderate dn/dT values often have high coefficients of thermal expansion, so it can seem impossible to find adequate materials. Fortunately, lenses can be combined with each other and structural materials in a compensatory arrangement to maintain focus over temperature. I’ve designed systems successfully with this approach. Additional compensation can be achieved if needed by mechanically adjusting the position of the optics and detector to maintain focus. If the lenses will be motorized for zoom anyway, then that may not add much effort to the mechanical design.
A stop or shield is often used to block stray infrared radiation from reaching the detector. In my experience, placing the stop close to the detector tends to make a system larger, so I evaluate the necessity for it early in the design. In the MWIR, the sensor generally needs to be cooled, in which case, a “cold stop” is located at the cryocooler window.
Another issue with stray radiation is “narcissus”, which is detector radiation reflected back to it by an optical surface. This kind of stray radiation isn’t prevented by a cold stop. It can be minimized by identifying the offending optical surfaces and either adjusting their curvatures or specifying better anti-reflection coatings to reduce the intensity of the backreflections. The coatings needed to achieve even moderate anti-reflection in the IR are more complex than the quarter-wave MgF2 coatings commonly used in the visible, due to the high refractive indices of IR materials. Also, instead of designing for a 0.2-µm visible range, the range might be 2 or 4 µm. That said, infrared materials tend to be less dispersive, which helps.
In time, I plan to create some examples of these types of designs, which I’ll add to this page.