How to Combine an Emission spectrum into a colour?

Question

I am trying to wrap my head around the flame test, and for the most part, it is making sense. My question is their a way to figure out the colour of a flame of an unknown element given the emission spectrum. In other words, how does the eye combine the mess of an emission spectrum into a single colour? I have been researching for a while now, so I am prepared for some pretty heavy maths. Some links would also be helpful for further reading.

Your brain receives information from your eye’s rods and cones and you see colors. What you see with an element’s flame atomic emission is just whatever your brain makes it. The element thallium gets its name from a Greek word that means “green twig” because it has a dominant strong emission at 535 nm. The story goes that it was identified spectroscopically, but the discoverer was color blind, so they called over their assistant to see it. Then the assistant apparently tried to lay claim to the discovery, being first to see the green emission. They lost! — Ed V, Nov 21 '21 at 23:23
I am afraid this is not really a question about chemistry, but about the physiology of vision (probably better suited for Biology StackExchange). Anyhow, take a look at various colour spaces that connect spectrum to the perceived colour, such as CIE 1931. — Domen, Nov 21 '21 at 23:24
Lots of information at physics stack exchange. Example: https://physics.stackexchange.com/a/488695/313612. Notice at the linked answer that the answerer says the purest yellow is 571 nm and that is shown in the figure. But sodium’s famous yellow D lines are at 589.0 and 589.6 nm and and no other sodium lines are nearly as intense. They look pure yellow to me. — Ed V, Nov 21 '21 at 23:26
Sorry, I just want to turn an atomic emission spectrum into a wavelength. What spectral line does a flame choose, and why? — Yak, Nov 22 '21 at 00:07
It does not pick a wavelength and is not a single color! What you perceive depends on your eyes and your brain. Look at some questions and answers such as this one: https://physics.stackexchange.com/q/564362/313612. — Ed V, Nov 22 '21 at 00:12
I am still learning, so sorry if this is a weird question, but how could you describe this sort of colour. I want a connection between atomic emission spectra and the colour, It doesn't need to be too specific, just this atomic emission spectra produces the colour yellow because... Also, as Domen answered previously, could we plot this on a colour space like CIE1931. Sorry if this is repetitive, I'm just struggling to wrap my head around it. — Yak, Nov 22 '21 at 00:37
There is no way to do what you want and trying to wrap your head around it can only result in a headache. It is like asking someone to summarize their favorite song by stating the violin (or piano or guitar or …) note to which it is equivalent. Look at the screen you have before you: the white background is from tiny red, green and blue LEDS. Use a magnifying glass to see this. Each emits a range of colors. By carefully adjusting those ranges and intensities, we perceive that mixture as white. — Ed V, Nov 22 '21 at 00:52
Thanks a lot for your help, Although you where a bit late for the headache! — Yak, Nov 22 '21 at 01:10

score 5 · Answer 1 · answered Nov 21 '21 at 23:36

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It is not simple -- for example, to many people, red + blue spectral lines appear purple/violet, i.e., shorter wavelength than pure blue! Though you could assign a color name to a pure spectral line, as you state, the appearance of mixed spectral lines becomes much more difficult to describe.

In this color wheel, names are assigned to mixed colors.

An interesting case is didymium glass, used in photography and by glass-workers to block the bright sodium yellow lines. Under incandescent illumination, the glass has a distinctly pink appearance, and under skylight, it's decidedly mauve.

So take description of spectral appearance with a grain of salt... oops, that's definitely yellow!

answered Nov 21 '21 at 23:36

DrMoishe Pippik

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(+1) Nice answer! Glad you see the sodium D lines emission as yellow and my Nd:glass laser rod behaves exactly as you describe in terms of colors. Also, if I remember correctly, Isaac Newton saw only 5 colors in the rainbow, so even our terms and names for colors are not so intrinsic as might be hoped. – Ed V Nov 21 '21 at 23:53
I would have thought we could predict the colour of a flame by the emission spectrum, is there any way to get an approximate colour. The colour between people with normal vision doesn't vary dramatically for example I don't think anyone sees a sodium flame as green, so can we pinpoint the colour with RGB or something. I am probably misunderstanding this so if you could clarify is this is impossible that would be benificial. – Yak Nov 22 '21 at 00:03
@EdV Hmm, I have never perceived sodium lines as pure yellow, always seeing there some warmer orangish/pinkish/salmonish tints. – Poutnik Nov 22 '21 at 07:14
@Yak I think the "forget" part was for you operatively. The Cie space in principle serves to the purpose for the average colour seeing people. Also, while the lines structure makes the prediction difficult for elemental emission, it is generally easier to predict the colour appearance by looking at the abs/em of molecules. Obviously roughly, is very hard to tell the exact nuance. – Alchimista Nov 22 '21 at 07:38
@Poutnik Maybe you are one of the very rare people with Tetrachromacy. ;-) Or we just perceive colors a bit different. – Ed V Nov 22 '21 at 12:46
Light mixed from 2 simple light color can be usually represented by a single light somewhere between. But mixed red and blue light triggers perception of magenta/purple color which is not present in the spectrum line range, but is the evaluation of the brain. It kind of joins the blue/violet and red ends of the spectrum into a closed circle. – Poutnik Nov 22 '21 at 12:46
@EdV Hmm, I am aware about the TCH, but I would rather go here: "Yellow: 570–590 nm, Orange: 590–620 nm." . The sodium doublet 589.0 + 589.6 nm is at the orange border. – Poutnik Nov 22 '21 at 12:51
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There are also impossible colors and so on. But getting back to chemistry, a digital photograph of a green-colored flame can have its intensity of green quantified by intensities of RGB pixels, yet this says nothing about what actually caused the green flame color, i.e., boron, barium, copper, thallium, etc. – Ed V Nov 22 '21 at 12:52
@Poutnik Orange border for you, but pure yellow for me! – Ed V Nov 22 '21 at 12:53
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@EdV, thanks for pointing out "impossible colors." The crossed-eye color illusions are interesting! BTW, seven is considered a "magical" number, so colors such as indigo were added to make the spectrum more "complete". There was no color orange in English until the fruit (naranj, نارنج‎, and then misnamed) was imported, – DrMoishe Pippik Nov 22 '21 at 19:53

score 3 · Answer 2 · answered Nov 22 '21 at 10:40

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As I understand your issue, you need to convert spectra into RGB triplets. Quantitative colorimetry theory addressing that issue is quite thoroughly stated in this wikipedia article on CIE 1931 color space

answered Nov 22 '21 at 10:40

Nicolas Peron

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score 3 · Answer 3 · answered Nov 22 '21 at 14:11

In other words, how does the eye combine the mess of an emission spectrum into a single colour?

Although your query is related to vision and others have addressed it, let me clarify a couple of things that might be causing confusion. First, you are talking about simple flame tests for alkali metals, alkaline earth metals, and a few elements like boron and copper. Bunsen flame spectra are not a forest of emission lines but a very simple spectrum (indeed not a mess at all). You may have been misled by Google images search. Most of them are just cartoons and fake. Bunsen flame is a low-temperature flame. Not many lines are expected.

If you have a pocket spectroscope or even a homemade CD spectroscope, you will see that sodium flame is just one orange-yellow line around 589 nm. Similarly, potassium flame spectrum is just one or two lines. If you are lucky with sensitive eyes, you might the deep red and violet lines.

See this excellent video of the true flame colors Flame emission spectra. The video shows the simple spectra. You will also note that the yellow sodium line is usually present in most spectra...Na is everywhere.

You can try making a CD spectroscope to try it yourself. Internet is full of such easy constructions, but the spectra on Google Images is disappointly cartoonish (or rather software generated) from at a first glance.

Another misconception to be noticed is that many people think that the Bunsen flame colors are always due to atomic emission. This point is not valid. Ca, Ba, Sr, Cu, B, flame colors are due to small molecular compounds of those elements, not their atoms. Only the alkali metal emission is atomic emission, and the rest are molecular emission in a Bunsen burner.

For real analytical work, Bunsen flame color is useless. We have to use a high-temperature from a spark or electric discharge or a plasma around 5000 K to see true atomic emission of most of the elements in the periodic table. Then, indeed, the emission spectrum is a mess, mainly in the ultraviolet, and you certainly need a device to separate wavelengths. The concept of color then vanishes because we cannot see ultraviolet.

Sorry, I cannot see sodium emission photos. Which link are you referring to? — AChem, Nov 22 '21 at 20:30
This link is useful: https://www.rapidtables.com/web/color/RGB_Color.html. I got it from an answer over at electrical engineering stack exchange. The question there is https://electronics.stackexchange.com/q/590739/223146. — Ed V, Nov 22 '21 at 22:22
One last very informative link: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/cie.html. As stated there, RGB space is smaller than human perception of colors. Pretty neat! — Ed V, Nov 22 '21 at 22:31

How to Combine an Emission spectrum into a colour?

3 Answers3