Whiskers wrote:
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There are of course purpose-built thermal-imaging cameras, and IR
'remote thermometers'. Some 'night vision' devices operate using
'passive IR'. Oddly, these seem to tend towards the 'stupidly
expensive' end of the price spectrum, being targeted at the armed
forces and emergency services, or for industrial or medical uses.

Photons at the "thermal imaging" wavelengths (3-5 and 8-14um) have much
less energy than visible and near-IR photons, and may even require cooled
detectors to see them with as much clarity as is required for a
recognisable image. Cooling may even be to liquid nitrogen temperatures.
Hence the cost. Using 3-5um is cheaper, and requires less cooling. As
glass stops long-wave infra-red radiation, optics may have to be made from
germanium or other exotic materials - again not cheap!

David