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).
These materials provide good transmission for wavelengths in the range of 8-15um, i.e. LWIR (Long-wavelength infrared). Sometimes LWIR is also called far infrared.
The TA-LENS standard
Thermal lenses do not use C- or CS-mount lenses which are commonly used on standard cameras. Instead they use other types of mounts. The first documented thermal lens standard is TA-LENS. The letters "TA" stand for Thermal A, where "A" stands for the first documented standard.
TA-LENS lenses provide a male thread which connects with a female thread on the camera. The thread is nominally 34 mm in diameter, with a pitch of two threads per mm (M34x0.5mm).
The lens mount is large enough for sensors with a diameter up to at least 13mm, which means that it works for commonly used LWIR sensors.
As the thread size is relatively large, it is possible to make adapters for other, smaller lenses for example lenses using M24x1 threads or M25x0.5mm threads.
Sample test chart images captured by an LWIR camera using (left) a 60 mm lens and (right) using a 10 mm. Both images are captured by a camera using the TA-LENS standard.