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Operational method of Radiation thermometers

  1. Jan 20, 2016 #1
    Is there anybody on the forum who is up to speed with the ins and outs of these devices?

    99% of research papers and all manufacturers comment I have read say these devices are working because the distant object emits radiation that the detector absorbs and therefore this absorbed energy *heats* the detector. In fact these devices seem to be radiatively coupled to the observed scene in a two way energy exchange and are either heated or they are cooled by the observed scene depending upon its temperature.

    Is there though some peculiar property of these devices that enables the detector to always only be heated?
     
    Last edited: Jan 20, 2016
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  3. Jan 20, 2016 #2

    russ_watters

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  4. Jan 20, 2016 #3
    A visible light camera does not produce any visible light. The thermal camera produces the very radiation the detector must measure. The two methods are not really comparable at the level of the detector even if many of the micro engineering and electronic elements are the same.

    I just googled this link (it might not work for you now)

    http://www.npl.co.uk/reference/faqs/how-does-a-radiation-thermometer-work-(faq-thermal)

    it says "
    Radiation thermometers measure the thermal energy emitted by a source and relate this to its temperature by means of the Planck law of radiation. They consist of optics (generally lenses) to collect and focus the emitted energy onto a detector. The signal from the detector can either be measured directly, or it can be converted to a temperature using a system of electronics. Filters are usually used to define the wavelength or wavelength band over which the emitted energy is measured.

    Many types of radiation thermometer are available for different applications. For measuring high temperatures a thermometer should be chosen that operates at a short wavelength, where the rate of change of emitted radiation with temperature is very high. However, for low temperature applications where the amount of emitted radiation is low, a broad-band device operating at longer wavelengths is required. NPL can calibrate infrared thermometers between -40 °C to 3000 °C."


    It is typical of the simplified information available where emission is emphasised and it is suggested there is some way of knowing the temperature only by receiving emission using the laws of physics.

    http://www.laserfocusworld.com/articles/print/volume-48/issue-04/features/microbolometer-arrays-enable-uncooled-infrared-camera.html [Broken]

    This one goes deeper but you still find the claim they work via heating rather than emphasising temperature change.

    "The lower the mass of the illuminated pixel, the less IR energy is needed to increase its temperature a given amount, and the more sensitive it is."

    99% of research papers are saying the same thing.

    There are, by the way, basically two different types of detectors used in uncooled bolometers, 1. thermal detectors and 2. photon detectors. Thermal detectors firstly measure a change in temperature and these are the kinds of detectors used in cheap radiation thermometers and many of the more expensive multi pixel cameras.

    People seem to be mixing up ideas. Ie the idea of heat absorption which is going to be a one way energy transfer in either direction which in reality happens via a two way process and then the idea of absorption emittance which is also a two way process.
     
    Last edited by a moderator: May 7, 2017
  5. Jan 20, 2016 #4

    russ_watters

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    The first link isn't working for me, but the second is talking about cameras and mentions two different technologies, one of which is indeed (and surprising to me) measuring the temperature of the detector and the other is measuring the photons directly, ccd style [edit, actually that's photovoltaic style - but ccds also capture photons from their own heat).

    I own both a thermal camera and an infrared thermometer. The camera closes it's shutter every few seconds to recalibrate, suggesting it is measuring it's own temperature. The thermometer doesn't have a shutter, suggesting it uses direct photon measurement.
     
  6. Jan 20, 2016 #5
    However the idea of direct photon measurement that is selecting photons coming from an object and rejecting all others is still a bit challenging for the reason you mentioned.

    I would guess your thermometer has a bog standard thermal detector rather than a photon detector. I wonder though if you will get similar results either way.

    Presumably your thermometer has no emissivity setting. You can try the following. Get a nice shiney saucepan and put 40C water in it. Notice how hot it feels to your hand on the outside of the pot and measure the temperature. It will be about 27C or so. Now put water of about 8C say (my tap water!) and check the pan feels pretty cold and measure it while making sure water is not condensing on it. it will be almost room temperature. If the device measures emissions it will always under read hot or cold shiny objects. Instead we get what we expect if it either heats the detector or cools it, where the saucepan reflects the rooms room temperature emissions more than it can emit directly from the metal.

    I think for detectors with emissivity setting they recognise the detector is cooling so they reverse the correction for emissivity. It would be really nice to get that confirmed from a designer but I can find nothing so far online.

    By the way this thread got created because I am having an insane conversation about cold Ir not being absorbed by hot objects! :-)
     
    Last edited: Jan 20, 2016
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