The blackbody color spectrum up to about 11,000 degrees Farenheit looks like this:

enter image description here

However, when iron or steel is heated it produces light in the following colors:

enter image description here

So it starts to luminesce at about 1000F and gets white hot at 2500F, not 11,000F. Another difference is that before it reaches white hot, it turns yellow (the image showing green is offcolor, it is actually yellow). However, the blackbody color is more like orange, not yellow.

So, I guess iron cannot be considered a "blackbody", but what is the reason for the difference?

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    $\begingroup$ Can you include a link to your source for these images? $\endgroup$ – rob Aug 31 '17 at 18:13
  • $\begingroup$ Do you know what a blackbody is? Do you have a reason to expect metals to fulfill the definition of a blackbody? $\endgroup$ – DanielSank Aug 31 '17 at 18:21
  • $\begingroup$ You should make the two plots have the matching x-axes and color scales to pose a meaningful question. Right now it is not possible to quantitatively answer your question I'd say. $\endgroup$ – KF Gauss Aug 31 '17 at 18:49

The colors do match. The differences you see are due to the differences in the scale and color index. If you create accurate charts, the colors would be very close. This of course does not include various colors of iron oxides on the surface after it is cooled off.

  • $\begingroup$ Well, aside from the color issue, the blackbody white is 11,000F and metal turns white hot at 2500F. That is a pretty big difference. $\endgroup$ – Ambrose Swasey Aug 31 '17 at 19:08
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    $\begingroup$ This is not true. "White" is a perception of the human eye that is evolutionary adjusted to the spectrum of the daylight (the sun plus sky). So white is the color of the black body at the temperature of the sun surface. Iron at 2500F is yelliwish-white like soft light bulbs as opposed to daylight bulbs that have a cooler tint. Again, the discrepancy is only in your data not being precise, but not in the actual phenomena. $\endgroup$ – safesphere Aug 31 '17 at 19:15

As mentioned by safesphere the perceived white may be misleading as this only tells you something about the relative reaction of the different receptor cells in your eyes.

Besides this I want to add that nearly no object can be considered to be a black body. "black body" denotes an object that has very extreme properties. Black bodies do not reflect or transmit any incident light, regardless of the associated wavelength. Most realistic objects are better described by the term "gray body". These are objects that feature a frequency-dependent light absorption. Of course, this also leads to an emission spectrum that deviates from the idealized black-body radiation. Especially the emission of light at specific wavelengths may be reduced. This can lead to deviating colors in comparison to black-body radiation.


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