Wikipedia:Reference desk/Archives/Science/2022 October 21

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October 21

Black body and its temperature measurement

In a black body experiment, where and how do we measure its temperature? Malypaet (talk) 05:21, 21 October 2022 (UTC)

If you can get close enough, you can use an infrared thermometer. If you can make contact, a regular thermometer can work, but you will want to calibrate it to be sure what you are reading. Graeme Bartlett (talk) 09:13, 21 October 2022 (UTC)

Infrared thermometers measure thermal radiation in the infrared portion of the spectrum and fit the measurement to the temperature-dependent black-body radiation curve (see also Wien's displacement law). Using an infrared thermometer to test Planck's law is circular; what you are testing then is instead the thermometer's calibration.  --Lambiam 10:54, 21 October 2022 (UTC)

• It should be noted that an actual black body doesn't exist. It's an idealized construct (i.e. a spherical cow) used to simplify a situation and eliminate confounding variables so that a particular phenomenon can be isolated and studied theoretically. All real objects are reflective to some degree, are likely not isotropic radiators, and all real materials have frequency dependent radiation (especially when we consider things like emission spectra). Most black body experiments don't use a black body, per se, the most common experimental analogue is a well-calibrated, well-insulated oven with a pinhole in it. The pinhole itself is the blackbody analogue; because and radiation incident on the hole is highly unlikely to re-emerge, and all light exiting the hole is roughly isotropic (it radiates equally in all directions from the hole). See Hohlraum. --Jayron32 11:18, 21 October 2022 (UTC)

  If you're looking for the actual device used to measure the distribution of radiated energy as a function of wavelength, it's a spectrometer.

  If you want to measure the temperature of an object, it's a thermometer, as observed above. PianoDan (talk) 18:34, 21 October 2022 (UTC)

In a distant past on a planet not unlike this one, special test cards were made for adjusting cameras in TV studios and home receivers. Such cards are rare now that test patterns are digitally generated, home receivers hardly need adjustment and sending time for advertisements is too profitable to waste. Nevertheless I was involved in handling a set of test cards of various patterns that were all made of Paperboard - except one. The exception was a card much thicker than the others, with something like a matchbox glued on the back. This card needed special care, it provided a Grayscale for an analog TV camera that had a very High dynamic range. The "matchbox" was a compartment lined internally with black fur, with a small window facing forwards. This device ensure a more extreme "black" (low reflectance or ideal "Black body") swatch than obtainable by an ordinary pigment. Philvoids (talk) 00:00, 23 October 2022 (UTC)