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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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2 Answers 2

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)

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

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:

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

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.

If the comments about the "wavelength" of the infrared radiation were new to you, I recommend you to read some basic texts about the electromagnetic spectrum

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

Infrared imaging can refer to any imaging system that operates in the infrared, which extends from about .7 to 300 microns wavelength, as Lubos stated. Thermal imaging, as the term is commonly used, refers to an infrared imaging system designed specifically for the portion of the infrared range which is emitted by object at or just above room temperature. This is typically in the neighborhood of 10 microns. At this wavelength you will see images that people typically associate with "thermal vision." People will be brighter than room temperature objects, running car engines will stand out, etc.

Infrared imaging is useful as well, of course. Most "night vision" goggles work by imaging visible and infrared light, and amplifying the resulting image to a level that the user can detect. Using infrared in addition to visible simply means that the goggles have a stronger signal to work with before amplification, because they are sensitive to a broader range of wavelengths.

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