1
$\begingroup$

With black body radiation, light of different wavelengths are emitted in various proportions depending on the temperature of the emitter.

Graphs of spectral radiance vs wavelength for different colour temperatures look like these:

enter image description here

It is said that the colour of the radiation (as a whole) changes in a predictable manner. Samples of colours at various temperatures:

  • 1000K Red
  • 1500K Reddish orange
  • 2000K Yellowish orange
  • 2800K Yellow
  • 3500K Yellowish white
  • etc

Others claim that the visible light appears yellow at 5000K.

Leaving aside the precise temperature, the consensus is that "[a]s its temperature increases further it becomes bright red, orange, yellow, white, and ultimately blue-white".

However, if we take a look at the graph, it seems that even when the 'yellow' temperature is reached, the black body should emit quite a bit of red and orange wavelengths in addition to yellow.

It might be that the more precise phrasing is that "it first turns red, then orange (red+yellow light), and finally white (red+yellow+blue looks white to the eye)".

But standard high school introductory material and the Wikipedia article seem to suggest that the perceived colour can be a (pure) yellow at some point (2800K in the linked article above or 5000K from the high school material).

Is this just a matter of perception (we see various wavelengths but our brain combines the RGB data from the cones in the eyes into 'yellow'), or is there some other explanation for why we can (in theory) see a pure yellow light emitted from black body radiation of a suitable temperature?

Related questions that do not answer my question:

That question addresses black body radiation generically but not the specific issue of how a 'pure' colour can be perceived when the visible emissions are of a collection of wavelengths (e.g. red + orange + yellow, together perceived as a pure yellow).

That question addresses colour perception through an interplay between emitted and absorbed/reflected light.

Improve this question
$\endgroup$
2
  • 3
    $\begingroup$ Pure yellow or whatever colour doesn't fit with BB emission anyway. There is no way to use "pure" the way you did in the question. All the rest is physiology. Rather than yellow, what we can't perceive from a BB is green. Somehow we need higher purity to see green (likely because it is in the middle). $\endgroup$
    – Alchimista
    Commented Oct 17, 2021 at 12:03
  • 1
    $\begingroup$ I'm afraid that I can't quote a source, but I once read that the eye cannot usually distinguish an impure colour (wide spread of wavelengths) from a pure colour (very narrow spread) centred on the 'right' wavelength in the wide spread. $\endgroup$
    – Philip Wood
    Commented Oct 17, 2021 at 12:05

2 Answers 2

Reset to default
2
$\begingroup$

Your "more precise phrasing" is indeed exactly right.

Improve this answer
$\endgroup$
1
$\begingroup$

You can plot the track of a blackbody on a colour chart. Here it is (from the wikipedia page on black bodies). Yes the range of colours goes from reddish to blue-ish white, through a yellowish/orangeish region. There is no "pure colour" and blackbody radiation is emitted over a wide (infinite) range of wavelengths at any temperature.

I don't think there is a consensus on what names to give to the appearance of blackbody radiation, not least because the colour that is perceived by the eye will also depend on the intensity of the light (i.e. photons per unit area) received at the back of the retina - the difference between photopic and scotopic vision. However, if the blackbody surface is big/close enough such that it is resolved by the eye, then this number will be roughly constant and I guess is what the colour chart claims to represent in that case.

Planckian curve on colour chart

Improve this answer
$\endgroup$
17
  • $\begingroup$ For a given temperature, the intensity of a black-body is fixed. The color percieved by the eye can only depend on the temperature. $\endgroup$
    – Alfred
    Commented Jan 12, 2022 at 21:55
  • $\begingroup$ @Alfred not really: chromatic adaptation will influence perception. $\endgroup$
    – Ruslan
    Commented Jan 12, 2022 at 23:30
  • $\begingroup$ @Alfred the relevant phenomena are photopic, mesopic and scotopic vision. These do indeed depend on the flux received at the eye. Your statement is only true for looking ar a blackbody "wall" where indeed that flux would be constant no matter how far away the wall was. In the real world, blackbodies do not have infinite size, so when they are far away the flux received at the eye diminishes. As a result, the colour perceived changes. That is why faint stars appear white and brighter stars may appear coloured. physics.stackexchange.com/a/169986/43351 $\endgroup$
    – ProfRob
    Commented Jan 13, 2022 at 10:00
  • $\begingroup$ @Alfred i.e. it is the specific intensity (Watts per square metre per Hertz per steradian) that is fixed by the Planck function, not the flux (Watts per square metre per Hertz) at the eye. $\endgroup$
    – ProfRob
    Commented Jan 13, 2022 at 10:04
  • $\begingroup$ @Profrob OK, I was wrong but not for the reason you give. The color of any part of the sun is the same, and looking at just a part of the sun through a small opening about 1m away will burn a smaller area of your retina, but will burn it as thoroughly as if you were looking at the whole sun. What would indeed change the perception is to be able to contract your pupil even more to reduce the solid angle of the flux reaching each neuron of your retina. So not the flux received by unit of area at your eye, but the solid angle reaching your retina from the opening of your pupil. $\endgroup$
    – Alfred
    Commented Jan 13, 2022 at 10:36

Not the answer you're looking for? Browse other questions tagged or ask your own question.