What color is a blackbody?

A blackbody, in this context, is just an object glowing with its own heat in a way unaffected by its composition. Things like hot iron or the interior of a glass furnace.

As the temperature rises, a blackbody starts to get brighter very rapidly. But its color also changes. We are familiar with red going to yellow going to white as an object gets hotter, but is that all that happens? What does it look like? (1/n)


In principle this isn't too hard to calculate: our color vision is based to the responses of three photopsins (light-sensing pigments) to the incoming light; if you simply write down these amounts as X Y and Z you get a description of the color as received by the human eye. The response of each photopsin to light of various frequencies is tabulated.

Since we know the spectrum of blackbody light, we can easily calculate the X Y Z color we'd see. Convert this to RGB and you have it. (2/n)

There are perectly reasonable colors that can't be represented in RGB (they are out of the "gamut"). In particular, for very cool blackbodies the color you'd see is a deeper red than exists in RGB. But we're used to cramming the world's vibrant colors into the few that our screens can reproduce.

I have done these calculations and produced a color gradient (attached) that captures the heating of a blackbody from deep red through yellow and white up to a sort of baby blue. (3/n)

One odd thing you'll notice is that as the blackbody keeps getting hotter, the color settles down to a particular baby blue.

Although blackbodies this hot are actually putting out loads of ultraviolet and X-rays, the part our eyes can actually see is far below the peak of the blackbody spectrum. And in this (Rayleigh-Jeans) regime, the spectrum just looks like a straight line of a particular slope regardless of temperature. This means it looks the same color. (4/5)

@anne I'm still thinking about this part. So fascinating.

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