The next challenge in thermal is reflectivity and thermal opaqueness. In Hollywood movies, it is represented that a thermal camera can see through walls. In the real world, a thermal camera can’t even measure through clear glass or the surface of a puddle of water since the glass/water is thermally opaque.
Furthermore, thermal imaging lenses cannot see very well in a highly humid environment because the water in the air has a temperature that transmits in the infrared spectrum, thus masking the thermal signature behind the water molecule. While normal dry air is thermally transparent, steam or fog is somewhere between thermally translucent to thermally opaque. This makes thermal imaging lenses difficult in a humid environment or a spraying environment. Beyond that, a shiny piece of metal is generally thermally reflective, meaning that it reflects the thermal signature of the items in its optical path rather than the thermal signature of the metal itself.
Strangely enough, this is why tinfoil works well in the oven – it creates a partial thermal shield in the IR spectrum, splitting the thermal environment into two where only convection/conduction transmission of thermal energy is possible while the IR radiation is rejected.
So, thermal imaging lenses is very different than visual imaging, but there are elements that are the same. Thermal imaging lenses is time dependent, humidity dependent, surface reflectivity dependent, and resolution dependent. That is a lot for an engineer to deal with, but there is more.