I'm trying to figure out whether lenses for thermal (long-wave infrared, i.e., 7-14µm) cameras are inherently more expensive or less capable than the lenses for visible cameras.
Production lenses for digital cameras often incorporate a dozen or more individual glass elements up to perhaps 50mm in diameter. With surface coatings these systems still transmit over 90% of light in the visible spectrum, and are able to nearly eliminate all forms of distortion while providing a wide focal (zoom) range. These systems can be mass produced for a few hundred dollars per unit.
In the limit (i.e., assuming the same economies of scale were provided), is similar performance and/or price possible with current uncooled thermal lens technology?
I am having trouble determining the state of the art in thermal lenses. I know that for production cameras at least the front element is typically made of coated Germanium, or Ge-As, and a single Ge element in real-world temperatures (i.e., below 50°C) transmits over 90% across the LWIR spectrum. (My understanding is that Zn-Se and Zn-S are also very good in the thermal spectrum, but they are too soft to be exposed, so they could work as interior elements, but not the front element.) But I don't know, for example:
- Is transmission through these materials so low that using more than a couple of elements is not practical?
- Are these materials inherently significantly more expensive to manufacture than the glasses used in visible light lenses?
- Are the physics of refraction in these materials in the LWIR spectrum too variable to build lenses with many elements, or with "zoom" capabilities?