IR thermometers and Blackbody Radiation

[roam]

My question is about using an IR thermometer for measuring the temprature of a blackbody that emitts a spectrum that peaks somewhere in the visible region.

I think since these kind of thermometers measures the amount of infrared radiation emitted by the object, they can only determine the temprature of objects of moderate temprature that radiate in the mid-infrared region or colder ones that radiate in the far-infrared. So, does this mean we can't use them to determine the temprature of a blackbody that is about the same temprature as a star? And what happens if we use them to measure such hot objects?

Do we need a spectrometer for measuring the temprature of a blackbody which is extremely hot? (I've read this in an old thread, not sure if it is true)

I appreciate any clarification.

 

[mfb]

Hot objects emit infrared radiation as well - they emit even more than colder objects. The large amount of visible light should not harm in that respect. So in principle, it is sufficient to measure the infrared spectrum to measure your temperature range. If they are designed to calculate with spectra in this temperature range is a different question.

 

[Andy Resnick]

My question is about using an IR thermometer for measuring the temprature of a blackbody that emitts a spectrum that peaks somewhere in the visible region.
<snip>

Without knowing any details, it's hard to provide an explanation. Yes, non-contact temperature measurements (IR thermometers) can and will give erroneous results- especially if the object is not a blackbody or even a greybody. As for measuring the temperature of stars, you have to take into account atmospheric absorption.

However, one can calibrate an IR thermometer with a blackbody cavity, and if the IR thermometer uses several wavelengths instead of a total radiated power, they can be fairly useful as measuring devices.

 

[Bobbywhy]

What is emissivity, and how is it related to infrared temperature measurements?

Emissivity is defined as the ratio of the energy radiated by an object at a given temperature to the energy emitted by a perfect radiator, or blackbody, at the same temperature. The emissivity of a blackbody is 1.0. All values of emissivity fall between 0.0 and 1.0. Most infrared thermometers have the ability to compensate for different emissivity values, for different materials. In general, the higher the emissivity of an object, the easier it is to obtain an accurate temperature measurement using infrared. Objects with very low emissivities (below 0.2) can be difficult applications. Some polished, shiny metallic surfaces, such as aluminum, are so reflective in the infrared that accurate temperature measurements are not always possible.

There are five ways to determine the emissivity of the material, to ensure accurate temperature measurements:

1. Heat a sample of the material to a known temperature, using a precise sensor, and measure the temperature using the IR instrument. Then adjust the emissivity value to force the indicator to display the correct temperature.

2. For relatively low temperatures (up to 500°F), a piece of masking tape, with an emissivity of 0.95, can be measured. Then adjust the emissivity value to force the indicator to display the correct temperature of the material.

3. For high temperature measurements, a hole (depth of which is at least 6 times the diameter) can be drilled into the object. This hole acts as a blackbody with emissivity of 1.0. Measure the temperature in the hole, then adjust the emissivity to force the indicator to display the correct temperature of the material.

4. If the material, or a portion of it, can be coated, a dull black paint will have an emissivity of approx. 1.0. Measure the temperature of the paint, then adjust the emissivity to force the indicator to display the correct temperature.

5. Standardized emissivity values for most materials are available. These can be entered into the instrument to estimate the material’s emissivity value.

For more information, see also:
http://www.allqa.com/IR.htm
http://www.metrisinst.com/faqs-and-tutorials
https://www.physicsforums.com/showthread.php?t=511254
http://www.coleparmer.com/TechLibraryArticle/377
http://www.wintron.com/infrared/guideir.htm

 

[Andy Resnick]

<snip>

There are five ways to determine the emissivity of the material, to ensure accurate temperature measurements:

<snip>

That's fine if you have a piece of the material to work with. What about remote sensing operations? I don't think Kim Jong Un is going to lend you a piece of missile to experiment with.