When I read IR in the post, I thought, "well that's not thermal, is it?" and the thought was incorrect. FLIR's page [1] has a more understandable to me summary than StackExchange [2]. Also interesting is how lensing works with radiation in the IR spectrum [3].
Thermal cameras cannot use regular glass lenses, as glass will reflect thermal radiation rather than allowing the radiation to pass through the lenses. Commonly used materials for thermal lenses are Germanium (Ge), Chalcogenide glass, Zinc Selenide (ZnSe) and Zinc Sulfide (ZnS).