How does IR thermal vision work?
What I'm really trying to wrap my head around is (firstly) how a sensor can "see" colours that I can't. (I understand it's probably like trying to explain colour to somebody born without sight).


The sensor is just converting some physical measurement &mdash; in this case, the temperatures of a whole lot of little thermal sensors &mdash; into a 2-dimensional array of data. The thermal sensors (microbolometers) get their temperature from the fact that a point on some distant object is focused on each one by a lens that is transparent to long-wave IR.


  And (secondly) how it can "pitch-shift" to a visible wavelength/frequency. (I understand that terminology is probably incorrect when talking about light as opposed to sound, but you get the idea)


The sensor isn't doing that at all. Any 2-D array of data can be represnted visually by mapping colors (or grayscale values) to the numbers. It is the display device (eg., LCD) that is producing the visible light that you see.