Talk:Infrared thermometer
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Hi,I'm Marwan Fouad I noticed that there's not enough focus on Infrared thermometer work theorem . how the infrared radiation transmits the body's temperature to the detector, I can imagine that as the infrared radiation has its thermal effect, I can conclude that it's affected and heated by the hot body and the detector measures the infrared beam temperature.Am I right?
Reply from James Knope:
I will try to answer your question. Infrared radiation is light energy at wavelengths greater than what can be observed with the human eye (~0.7 microns, perceived as red light). There are two primary types of detectors, a thermopyle detectors, and photodetectors.
The thermopyle detector works in a manor you describe, the light energy heats the detector and the amount of heating is used to determine the intensity of the radiation. The radiation intensity is then used to determine the temperature of the source. These devices are generally responsive to radiation at 8 - 14 microns.
Photodetectors utilize what is called the photoelectric effect and respond to radiation by freeing electrons (electrical solar energy panels do this also). The freed electrons generate an electrical current which is measured to determine the intensity of the radiation. The type of photodetectors define the wavelengths of light energy they will respond to. Silicon detectors generally respond to 0.4 to 1.05 microns, Indium Gallium Arsenide detectors usually respond to 0.7 to 1.7 microns.
From StuFifeScotland: That's very helpful, but what is not clear from the current article is how these devices differentiate between a hot body some distance away and a slightly colder body nearer to the thermometer. If all they measure is the power received in some narrow infra-red frequency band (which is what the article implies) then how is the distance allowed for? Is some approximation assumed, which therefore limits the accuracy of the device? If so, it should be detailed. On the other hand, if there is some clever way of adjusting for distance, it would be helpful to read what it is. Is it in the optics and the way the received infra-red is focused, for instance?
- I'd certainly agree that the article is lacking in technical detail, but on the one point about distance the devices are not designed to measure small objects. The objects need to fill the whole beamwidth. Inverse square law effects only apply when the source is small compared with the beam width. -- David Woolley 12:45, 13 November 2006 (UTC)
- Thanks. That wasn't clear from the article. Given that some of these devices project tiny laser spots to mark out the object whose temperature you wish to measure, it would be easy to overlook the need for the body to be much larger than the laser spot. It would be helpful to users if this potential pitfall could be made clear in the article. StuFifeScotland 11:57, 18 November 2006 (UTC)
It is commonly believed that these thermometers identify the peak in the black-body spectrum and so infer the temperature. However, that's quite a challenging task, requiring power measurements in multiple narrow wavebands. It would seem from this article that none of these devices actually work that way. StuFifeScotland 11:57, 18 November 2006 (UTC)
[edit] Why these devices need to measure ambient temperature
An example of one of the technical details that this article ought to be addressing (as well as the different types of sensor already mentioned on this talk page) is why these devices typically also have thermistor sensors to measure the device temperature.
Based on some clues I've found and some guesswork, so none of this meets the traceability requirements for inclusion in the main article, it seems to me that one reason is that any object with an emissivity of less than 1, e.g. the typical value of 0.95 for dielectric materials that the devices are normally calibrated for, will reflect a proportion of the general environmental background infrared radiation into the device. Measuring the device temperatures assumes that it is in equilibrium with that radiation, which may or may not be a valid assumption.
A possible secondary reason is that the detector is almost certainly seeing a signficant amount of radiation from the inside of the device itself, which, as a closed structure may be closer to a black box.
The overall measurement will therefore be a mixture of all the above, but can be compensated if you know the source emissivity and you are confident that the measuring device is in radiative thermal equilibrium with its surroundings.
I also suspect that radiation pyrometer is a synonym, even though the current article for these suggests a minimum temperature that is rather high compared with typical use of IR thermometers (which are typically used for food and central heating system temperatures).
David Woolley 13:09, 13 November 2006 (UTC)
[edit] Fair use rationale for Image:InfraredShellScanningExample.jpg
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BetacommandBot (talk) 16:43, 2 January 2008 (UTC)
