It is not an oxymoron, nor a quote from Yogi Berra.
Real Blackbodies do not exist, at least on Earth. Only approximations or simulations are real. We use them to calibrate IR Thermometers, Radiation Pyrometers and Thermal Imagers.
Technically they should have a spectral emissivity very close to 1.0. How close, you might ask? Read on.
Max Planck needed the concept of a perfect absorber of electromagnetic, thermal radiation to develop his theory of Thermal Emission of Radiation in 1899. Fortunately, Gustav Kirchhoff had already develped the foundation for them forty years earlier.
A perfect blackbody is perfectly absorbing to all the thermal radiation incident upon it. For that reason it had, necessarily, to be opaque and non-reflecting.
By logical reasoning, it was also clear that the same device had to be a perfect emitter of thermal radiation related to its absolute temperature, that is, temperature on the Absolute or Kelvin Temperature Scale.
There are several radiation equations or “Laws” that have been developed to describe the physics of thermal emission properties. They are well explained in a number of texts and shown in some detail in the online Hyper Physics website.
In an online Java applet, one can see visually also the three main radiation laws in graphic action; the temperature on the screen is shown on a column in a thermometer on the right side, and you can change it by clicking and/or dragging on it with your mouse.
If someone asks about the color of a blackbody, you can always refer them to this great set of webpages by Mitchell Charity at MIT.
They show both the temperature from 1000 K to 29,800 K (of course below about 700 K blackbodies actually look black to the human eye) . As can be seen on this page, red, white and blue blackbodies are possible!
There aren’t many 29,800 K blackbodies on Earth, but astronomers & AstroPhysicists see them all the time. How do you think they measure the temperatures of stars?
So, now you know, there can be both Red and Blue Blackbodies!
The devices used by calibration laboratories to calibrate and check the calibration of IR Thermometers, Radiation Thermometers and Infrared Thermal Imagers are not perfect (and seldom Blue, but often appearing Black, Red, Orange, Yellow and even White), but they can be very close to perfect.
The closer to perfection, the higher the cost of them also.
A blackbody having a spectral emissivity of 0.99 would have, at best, an error of about ± 1% in emitted thermal radiation or radiance, at a stable operating temperature and could be used to calibrate Infrared Thermometers.
The thermometers would be limited in their calibration uncertainty, since the radiance they emit would be uncertain to at least ± 1%.
Depending upon the radiance to temperature relationship for the temperature in question, that could mean a bigger or smaller effective temperature calibration uncertainty that could be assigned to a thermometer being calibrated.
That’s another issue for another time, but , if you can’t wait, one of the best explanations (and a lot more) that we have seen on that subject is in a 547 KB, downloadable PDF file from Land Instruments.
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