# What is the difference between thermal and infrared imaging?

Is there any sort of difference between thermal radiation and infrared radiation at least when detected by a camera? What are the differences between the two?

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The thermal radiation and the infrared radiation are the same thing if the sources of the radiation have temperatures comparable to the room temperature.

In general, the two terms are different: the infrared radiation is defined by having the wavelength in the fixed interval 0.7 - 300 micrometers (a convention)

while the thermal radiation of an object depends on its absolute temperature $T$. While any object at nonzero temperature emits at all wavelengths, the wavelength at which the emission is maximized is inversely proportional to the temperature of the source $T$ in Kelvins. See:

For ordinary cold and lukewarm objects, the thermal radiation is mostly emitted in the infrared. For much hotter objects, thermal radiation may be dominated by the visible light (or even ultraviolet light, at even higher temperatures). On the contrary, much colder objects emit radiation at a much longer wavelengths; for example, the empty Universe has temperature 2.7 Kelvin so it is filled by the "cosmic microwave background" which is dominated by much longer electromagnetic waves which are microwaves - similar long waves to those in the microwave oven.

http://en.wikipedia.org/wiki/Electromagnetic_spectrum

which explains all kinds of electromagnetic radiation. In all cases, they only differ by the wavelength (or the energy or frequency which is inversely proportional to it).

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I think there is scope for confusion. Near Infrared (.8 to 1.5microns) is a lot easier to make a camera for, but is too high frequency to catch thermal emissions for ordinary temperatures. True terrestrial thermal radiation is about ten times longer in wavelength, but thermal cameras cost several thousand dollars. –  Omega Centauri Mar 14 '11 at 16:30
@ Omega Centauri - depends on the camera! I spent a reasonable slice of M bucks building a 0.8-1.6um camera (for astronomy) while you can get thermal imagers built into firefighting helmets for 1000. It all depends on sensitivity and resolution. –  Martin Beckett Mar 15 '11 at 15:47
Near infrared has enough energy per photon to knock an electron loose in a detector. Thermal not so much. If it is bolometric, it measures the heat from the total amount of radiation. But perhaps there are other detection mechanisms that are used for thermal (i.e. 5-25 microns)? –  Omega Centauri Mar 15 '11 at 17:51
@Omega - thats a good engineering difference. You can push photovoltaics to about 6um (InSb). In the mid-far IR you can also use Josephson effect and soon Quantum Well detectors but most are bolomters –  Martin Beckett Mar 15 '11 at 18:33