Due to advances in material and manufacturing technologies, the vast majority of lenses used on long-wave infrared (LWIR) cameras are simple: One- or two- element designs with an emphasis on low-cost manufacturing. One effect of this is that most of these cameras are supplied without any means for the user to focus them. This means that the lens must stay in focus over a broad range of temperatures. Since the standard temperatures range for use outdoors is -40°C (really cold) to +85°C (much too hot!), this can provide some interesting design challenges.
The Problem
Most materials have properties that change with temperature, and all the materials that can be used to make lens for the LWIR waveband have properties that change. For optical performance, there are two relevant properties to consider: the thermal expansion (α), and the way the refractive index changes with temperature (dn/dT). The refractive index (n) defines the optical power of a lens —he higher the index, the stronger the lens — and this changes with wavelength. Figure 1 shows how these combine to cause a change (Δs) in the distance from the lens to the focal plane.