For visible light, glass lenses are typically used to focus light on a camera sensor, however glass is not transparent to thermal radiation. Instead, thermal lenses are constructed from a special metal called Germanium (Ge). This is a relatively rare element and thus is quite costly, with raw prices often as high as $2000 per kilogram. Depending on the type of sensor, lenses of different specifications are required.
Our cooled sensor thermal cameras are designed to he the best long-range detection. We he a wide variety of long-range continuous zoom lenses, allowing the operator to smoothly transition between wide angle and long range. Our germanium lenses are ailable in lengths of up to 1400mm, allowing us to reach detection ratings of over 50km.
Since uncooled thermal imagers are inherently less sensitive than cooled sensors, we maximize the quality of those images by using lenses with an extremely wide aperture of ƒ/1.0. This wide aperture allows more thermal energy through to the sensor for detection; twice as much energy as that of a lens with an aperture of ƒ/1.4, or four times as much as ƒ/2.0 (if you’d like to understand how those numbers work, see this excellent explanation of how f-stops are measured: http://www.uscoles.com/fstop.htm).