7
$\begingroup$

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?

$\endgroup$
10
$\begingroup$

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).

$\endgroup$
  • 2
    $\begingroup$ 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. $\endgroup$ – Omega Centauri Mar 14 '11 at 16:30
  • $\begingroup$ @ 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. $\endgroup$ – Martin Beckett Mar 15 '11 at 15:47
  • $\begingroup$ 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)? $\endgroup$ – Omega Centauri Mar 15 '11 at 17:51
  • $\begingroup$ @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 $\endgroup$ – Martin Beckett Mar 15 '11 at 18:33
4
$\begingroup$

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.

$\endgroup$
1
$\begingroup$

Rest has already been covered however for basic students....the crux to understand is....Infra Red or commonly referred as IR detects the heat generated by a body( which of course is there due to atomic vibrations), and generates a picture through a IR conversion tube. whereas Thermal Imaging device or commonly referred as TI detects the picture based upon temperature difference between the body and its immediate surroundings.....thus generating a picture. it involves use of high speed thermal scanner coupled with an IR detector, amplifier and Video Display. thus TI is not only eye safe as it offers indirect view and provides facility of storage and re production of video images as well

$\endgroup$
0
$\begingroup$

It seems to me that your title and the body of your question could be interpreted as asking two different things. One asks about imaging, and the other about the radiation the image is made from.

Others have talked about the frequencies and wave lengths you might infer from "thermal" and "infrared". I'm more interested by what is implied when you talk about the image.

I'm not sure what products on the market are actually producing, but there's a difference between producing an image of the colour temperature of the surfaces in the image, vs the quantities of radiation received from different parts of the image. I.e a monochrome or false colour image might be based on the colour of the light reserved or it's brightness. Due to differences in the heat conductivity of materials being photographed, they are not the same thing. Brightness of the infrared received is not entirely a simple quantity either when you start to think of the sensitivity to different frequencies of radiation.

If you are producing a thermal image for the purpose of auditing energy leakage from a building, then you want an image that shows you the rate of heat emission of surfaces in the image. Timber, glass, concrete and metal will all radiate at quite different rates while at the same temperature, and in context with air at the same temperature. That rate of radiation, with sensitivity to frequencies in proportion to the energy carried is what you are after here, not colour temperature, though product manufacturers may try to tell you that's what you are looking at, and in theory it might even be true.

If you are trying to asess heat of components in a circuit, then colour temperature probably is what you want, though it's likely not to be what your camera gives you, and you might find yourself sticking plastic tape over some of the metal surfaces.

If your purpose is to photograph the activities of wildlife, then you might want sensitivity at the frequency of the IR LEDs you use to light the scene, while sensitivity to the background thermal radiation in other frequencies just reduces the contrast of your images. Or you might not be illuminating the scene, and want to see the quantity of radiation in the range of temperatures likely for a warm body.

I can't tell you what various cameras actually give you, though these sorts of variations no doubt account for much of the wide variation in prices.

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.