Infrared Thermography

Random thought ... in some industries temperatures are judged by the "colour" of the emissions - e.g. metalworkers looking for "cherry red" - obviously that's for much higher temperatures to push thing into the visible part of the spectrum, but does the same principle still holds in the IR range - i.e. is it the wavelength/frequency of the IR emissions that are key to reading its temperature, rather than the "amount" of the emissions?

   - Andy.

yes - the spectral shape of the emissions  is a far better gauge of temperature than the simple intensity at any one wavelength - that is how we know the temperature of stars, almost independent of their distance. Unfortunately, as far as I know, no simple hand held IR imager is anything other than effectively monochromatic and fairly broadband..

"Hyperspectral" cameras, that act rather like a colour camera but for non-visible wavelengths,   do exist, and end up on space stations, spy satellites and so on, but are complex beasts, and well beyond the price  point of what we are discussing here.

However much simpler non contact  thermometers detecting  the spectrum peak do exist that are in effect act like a prism to split the incoming signal over an arc, with the angle of deflection being wavelength dependant  (actually usually for infra-red a diffraction grating) and an array of photo sensors rather like a single strip of a digital camera so each sensor gets a particular wavelength range. Similar games are used in the machines that do thousands of automated blood tests per day, where it is no longer a bored lab tech looking for the right sort of colour change in the test tube, but a machine machine looking at the critical part of the spectrum and a much more controlled and smaller sample can be used.

The technique is still not perfect, at some wavelengths that correspond to the resonances of specific atomic bonds and electron states there is absorption and more or less re-radiation ,that may be on the same wavelength or another so the theoretical curve has odd extra nulls and peaks in it and at some wavelengths the atmosphere is not clear, and at others the sky is pitch black even during the day. 

Mike.

To complement mapj1's response. Most of the cheaper (i.e. trade handheld) IR thermometers use a single waveband (in the way that black/white/grey cameras cover a colour range) and measure the intensity in that band. The IR bands, for the typical 0C to 100C temperature range, are very much closer to the colour brightness effects that cherry red has for metalworkers. What the IR camera sees is the glow of the surface and any reflections of other hot 'lighting'.

To complement mapj1's response. Most of the cheaper (i.e. trade handheld) IR thermometers use a single waveband (in the way that black/white/grey cameras cover a colour range) and measure the intensity in that band. The IR bands, for the typical 0C to 100C temperature range, are very much closer to the colour brightness effects that cherry red has for metalworkers. What the IR camera sees is the glow of the surface and any reflections of other hot 'lighting'.