I On Mixing Colors of Light

[sophiecentaur]

I think it would be good to introduce it to the physics majors

An interesting topic, of course but how would it fit into any existing scheme of work / syllabus? When I taught UK A level Physics, it was so content-loaded that fitting extra bits was difficult

 

[Charles Link]

An interesting topic, of course but how would it fit into any existing scheme of work / syllabus? When I taught UK A level Physics, it was so content-loaded that fitting extra bits was difficult

Then I guess it is all the better that we treated the topic of the mixing of colors of light on Physics Forums. I think most every physics major would benefit by having at least a little familiarity with the subject. :)

Edit: and perhaps worth mentioning again, the CIE theory in a nutshell is that the laws of vector addition apply for any two sources with their red=X, green=Y, and blue=Z components, so that $\vec{R}_{total}=\vec{R}_1+\vec{R}_2=(X_1+X_2) \hat{i}+(Y_1+Y_2) \hat{j}+(Z_1+Z_2) \hat{k}$.
Meanwhile the CIE color coordinates $(x,y,z)$ are found by where the vector $\vec{R}$ crosses the plane $x+y+z=1$, so that $x=X/(X+Y+Z)$, etc. (I'm repeating post 38).

 

[Charles Link]

and now for an observation of interest regarding the TV and computer screens that I have: On an older flat screen TV, perhaps 30 years old, I could clearly see the array of blue, green, and red LED's with my telescope eyepiece as a magnifying glass. With a newer TV and with this Chromebook computer screen, the LED array has much finer structure. I couldn't even tell for sure if they were using 3 LED's or possibly 5 or 6 or more to make the picture. Perhaps someone can supply some detail here=I could see with the eyepiece that there was some structure, but it was really too fine to totally resolve. It looked like there might even be a white LED in the structure, and perhaps even a yellow one, but I couldn't say for sure.

Edit: a google seems to suggest that they now might be using a white LED along with the red, green, and blue, but it wasn't real clear or definitive on the topic. Having a white one would provide an operating point near the center of the CIE chart, and make things easier for the other 3 LED's.

Edit 2: an additional google mentions things such as a white LED backlight, as well as things getting much smaller, (pixels, etc.), so that the older TV is actually much more interesting and definitive for viewing with the eyepiece from a telescope. Some of this nano type technology has gotten rather complex and much more difficult to figure out.

 

[Charles Link]

In reviewing this thread, I still encourage physics students to see posts 37,38, 46, and 55 (all on page 2) of this thread to learn about the CIE color map. From what we (@sophiecentaur and I ) concluded around post 90, the CIE color coordinates which came around 1931 is in many cases not being presented in the standard undergraduate physics curriculum, because there may be no place where it is a really good fit.

The CIE color map has withstood the test of time though, and should be considered good physics, and today's students would do well to have some familiarity with it. It should help students to understand how the right combination of red, green, and blue light can allow us to perceive most but not all of the colors that the human is able to recognize.

In the OP, a very simple experiment is presented that shows the yellow light you can get with the right mixture of green and red light. The physics student might also find that of interest. The yellow light can actually come in two forms as this experiment shows=pure yellow at around 580-590 nm (600 nm might be starting to be orange=a minor correction), and also from a mixture of green at 550 nm and red at 650 nm or thereabouts.

 

[sophiecentaur]

The yellow light can actually come in two forms as this experiment shows=pure yellow at around 580-590 nm (600 nm might be starting to be orange=a minor correction), and also from a mixture of green at 550 nm and red at 650 nm or thereabouts.

No one would ever use spectral line red or green; for good colour mixing, the primary sources (phosphors or equivalent) would never be monochromatic. In order to get a high output, efficiently, the primaries are broad band. You seem to be looking for something 'significant' about R + G = Y but we already know that the 'yellow' on a T V display is not formed by equal weights of the R and G phosphors. The yellow colour that is produced with that mixture is way off Sodium Yellow but Sodium Yellow is just 'any old colour' for a TV system. My point has always been that this particular mixing demo was first used long before there were TV primary phosphors and was used to 'make a point'. You surely can't think that the whole Colour TV system was invented around synthesising Sodium Yellow. You are putting the cart before the horse; the original choices of primary phosphors will (almost certainly) have been chosen for practical reasons so that a fair range of colours could be matched.

From what we (@sophiecentaur and I ) concluded

You are overstating the claim that we concluded anything. Sorry but I really do not want to be associated with your personal version of colourimetry. Some of what you say is fair enough but does it make total sense when you look at the spectra of three example phosphors? On a plasma screen Red is a difficult one and they are all very wide band .

The only place where they are three 'dots' on a CIE chart.


Plasma Screen Phosphors

 

[Charles Link]

@sophiecentaur Very good. You are supplying much needed detail on the state of the art. I'm also trying to address what may be an audience of freshman undergraduate physics students, but I don't even know for sure because I'm getting very little feedback. From what I can tell, Physics Forums doesn't get the wide audience like it did five or ten years ago or more, because the google search engines aren't directing very many to our posts, but that is another subject. In any case, your feedback is very good, and maybe we really didn't conclude anything, but I wonder how many students get exposed to the color mixing of light topic and the CIE chart as part of their curriculum. Cheers. :)

 

[Klystron]

As the OP requests feedback, kudos for an interesting thread on color mixing and colorimetry. Back when I taught electronics and radar science, students became fascinated with the electromagnetic spectrum with visible light but a small, though important, segment of the continuum.

Physics students at uni put much effort into veiwing and understanding interference patterns derived from interferometers and crystal diffactrion images as much as visible light spectra, supporting the OP's thesis that this subject may be underappreciated or undertaught in physics departments. One found art students, particularly painters, fascinated by color charts, color mixing, pigments, dyes and paint constituents with interest in human perception of visible light.

 

[sophiecentaur]

One found art students, particularly painters, fascinated by color charts, color mixing, pigments, dyes and paint constituents

That is virtually the life blood of most artists and there is a lot of discussion and information; With a few exceptions, artists are dealing with subtractive colour mixing and the CIE chart wouldn't be of any use. With additive mixing, the primaries don't 'tread on each other's toes' but three broad band pigment primaries inherently cause crosstalk. That calls for a different approach. Too hard guv'.

I wonder how many students get exposed to the color mixing of light topic and the CIE chart

Which ten hour university course could include one hour on colourimetry? Who would even teach it? There are so few obvious slots in mainstream science threads and, of course, it's more psychology than Physics.

I could imagine an entertaining and informative extra curricular presentation Physics Club meeting?) working very well. Colour projectors could provide three 'primaries' for mixing and comparison with a laser monochromatic source. Boy, but the preparation time for such a presentation (and clearing up afterwards).

However, apart from the guys I worked with, briefly and many years ago, I never met anyone with a burning ambition to spread the gospel and with sufficient depth of knowledge.

 

[Charles Link]

Just a couple additional comments to post 95 from @sophiecentaur : It may be in practice that most sources have somewhat complex spectral characteristics, but part of the purpose of showing how two monochromatic sources add together, as vectors in the CIE 3-D color space with their coordinates defining the direction of the vector, is it shows the fundamentals of how the CIE system works. The complex spectral sources then have their color coordinates found by performing integrals, but those are nothing more than linear summations of many monochromatic sources. I didn't present the integrals in the above posts, but they can be found in the Wikipedia link of post 32. $X=\int x(\lambda) I(\lambda) \, d \lambda$, and similarly for $Y$ and $Z$.

Even though the human perception ultimately is not entirely linear and perhaps even very non-linear, the linear system that the CIE color coordinates provide can give some very good estimates to what the individual will see for these more complex sources.

 

[sophiecentaur]

but part of the purpose of showing how two monochromatic sources add together, as vectors in the CIE 3-D color space with their coordinates defining the direction of the vector, is it shows the fundamentals of how the CIE system works

I don't know where this is taking you. The analysis of the eye gives three signals, each of which is the correlation of the admitted light and the three analysis curves. What the eye does with monochromatic light is just by the by. We didn't evolve to deal with monochromatic sources so it's no surprise that we cannot actually identify them. Worse than that, when we look at a reasonably saturated colour our brain sees them as much more saturated than they are - hence my comments about the subjective effect of a rainbow; we fool ourselves.

Before the Enlightenment no one ever saw pure spectral sources so why would be ever needed to spot one?

I think you should look at the whole thing again and take on board the fact that both analysis and synthesis are involved. If you want a well matched TV picture then we need to include both in our channel. I think you are a bit too obsessed with spectral colours. Can you apply your personal theory to explain how we see all those non-spectral colours that lie between the straight portion of the CIE chart and the central White Point? Nothing spectral in that area.

 

[Charles Link]

Can you apply your personal theory to explain how we see all those non-spectral colours that lie between the straight portion of the CIE chart and the central White Point? Nothing spectral in that area.

The monochromatic sources of the visible spectrum make a horseshoe around the border of the CIE chart. We can get any point in the interior with a couple of points from this outside ring in the right combination. In the OP, I happened to pick a couple of points that lie on the straight line of the upper right portion of the outer ring, but that was completely by chance. It was only after I wrote the OP, around post 20 and after, that I figured out some of the details of the CIE chart, including the mathematics upon which it is based.

I do not think my inputs fall into the category of "personal theory". I'm simply taking what is there, and explaining it in very simple terms. Cheers. :)

Edit: and note that even broadband sources are represented by a single point on the CIE chart. With TV screens, if they can get 3 sources that have points sort of spread out around the horseshoe of the CIE chart, they can the cover anything inside the triangle that has vertices at these 3 points, so they can cover most of the interior of the CIE color map. They find the combination of a red source, a green source, and a blue source, even with each being somewhat broadband, works very well.

 

[sophiecentaur]

Edit: and note that even broadband sources are represented by a single point on the CIE chart.

That's the whole point of the system; it can only produce colour matches for colours that lie within the triangle of the chosen primaries and that actually excludes all the spectral colours in any practical TV system. It excludes a huge area of colours in the region of Cyan to Blue. After passing through the system, all those colours end up in the 'right direction from White but sit along that side of the triangle. Luckily ,the eye just doesn't care too much about those colours so the error is accepted. That's because evolution 'realised' that such colours are not in our world or, when they are present, the information is not needed.

even with each being somewhat broadband,

Exactly. If the analysis were not broadband, with contributions from all spectral content, then most things would be invisible.

I do not think my inputs fall into the category of "personal theory".

Sorry. "Theory" was the wrong word. The word I was looking for is "agenda". You seem preoccupied with the spectral content of a 'colour'; you keep bringing that up all the time - you even seem to think I agree with that idea. The eye is not a spectrometer; it's only aware of 'colour'.

Many birds and insects have colours which are not all due to pigments but use interference filtering. They are 'startling' and grab our attention because they are not common.

 

[Charles Link]

"Theory" was the wrong word. The word I was looking for is "agenda".

I did find it interesting to read about the mathematics of the CIE color chart from the Wiki article, and I thought I had a couple of useful things that could add to their write-up, (such as the color coordinates (x,y,z) are where the $X,Y, Z$ crosses the plane $x+y+z=1$), but in any case the audience seems to be so limited that I may be as well off just doing calculations for myself in my neighborhood Starbucks. These days I find there are very few people I meet there who even know how to work the Pythagorean theorem for the simple case of 5, 12 and 13. I still think my OP is a post that is worth reading, but the coffee is still good in Starbucks, as well as the people-watching, even if I can't find very many, if any, who have even a little interest in the things my generation learned when we were in school=many years ago. Cheers. :)

 

[renormalize]

I did find it interesting to read about the mathematics of the CIE color chart from the Wiki article, and I thought I had a couple of useful things that could add to their write-up

According to https://en.wikipedia.org/wiki/Help:Editing anyone can edit a Wikipedia topic. Why don't you concentrate on updating their CIE entry and thereby help a much broader audience?

 

[Charles Link]

According to https://en.wikipedia.org/wiki/Help:Editing anyone can edit a Wikipedia topic. Why don't you concentrate on updating their CIE entry and thereby help a much broader audience?

Sounds good, but I really don't have a lot of interest in making an anonymous edit. I thought I would have a much wider audience on Physics Forums than I did=we had it a few years ago, but it seems for some reason the search engines aren't steering people to the Physics Forums posts nearly as much as they did previously. My plan is to continue to stick with the Physics Forums for better or worse. Cheers. :)

 

[DaveC426913]

Sounds good, but I really don't have a lot of interest in making an anonymous edit. I thought I would have a much wider audience on Physics Forums

It doesn't have to be anonymous.

Still, it's strange that you would turn your nose up at contributing to the quintessential Library of the Internet, where you will reach countless people who have come there specifically out of interest in the very topic you're writing about.

 

[Charles Link]

To respond to the above=I was glad I was able to figure out what the CIE chart is all about, but from what I can tell from the limited positive feedback that my explanation received on the Physics Forums, to me it would be somewhat pointless to put much effort into posting it in other places. This forum IMO should be the one where people would have the most interest in hearing what may be another way to look at something. I'm presently retired=I've seen the very competitive world for many years at both the university and at the workplace. The coffee was good again today at Starbucks. Cheers. :)

 

[scottdave]

This is actually a different phenomenon completely.

Your brother's night vision is comprised mostly of activated rod photoreceptors - which do not detect colour. The cone photoreceptors - which do detect colour - are not activated in dim light.
He is seeing in black, white and shades of grey.

His camera does not suffer from this affliction and sees colour at low illuminations no problem.

There is something else going on with digital cameras - they can capture light outside what humans can see, but then display that light as visible light. You can see this if you point your phone camera at a TV remote and push some buttons on the remote. In the phone display, you can see the LED's light up, but if you look at it, nothing is apparent.

I also notice if I am photographing or videoing with my camera, the blues tend to look more purple when viewed back.

I am not sure how much of this phenomenon plays into the Northern Lights effect that you described.

 

[sophiecentaur]

In the phone display, you can see the LED's light up, but if you look at it, nothing is apparent.

Regular CMOS sensors have an extended response into the IR region. There is a filter placed on top of the camera sensor which brings the sensitivity range more into the visual red spectrum. It's a fairly easy job to modify a regular DSLR for astrophotography by removing that filter. You can then put filters of choice in front of the sensor That gives you more flexibility and selectivity for bringing features out in your images. Astrophotographs seldom show the 'true' colours but that's ok because your eye is not capable of seeing colours accuratly / at all out there because, at that light level it's only the rods that work. A solid state sensor can be left to cook for many minutes and multiple images can be stacked with softwre. Also, the colour pallette that's normaly used for astro images is not tied to human sight but to enhance various features which show the presence of elemts in the different structures out there.

 

[sophiecentaur]

the blues tend to look more purple when viewed back.

It's usually possible to adjust colour balance to get rid of that common problem, either to a permanent setting or for individual images.

 

[Charles Link]

I would like to make a couple further comments regarding the mixing of colors of light. My comments here are made more at the beginner level=someone who already has an in-depth understanding might find my inputs perhaps even somewhat boring.

When mixing red, green, and blue light together, the result in general is white light. In the CIE color map of post 16 I'm a little surprised that I don't see a larger section of white in the middle of the color map, but there again, I think the map of post 16, at least what I get on my computer screen, does not have very accurate colors in at least a couple places.

To me it is somewhat remarkable that these 3 colors can generate what we see as white light=even as remarkable as red and green able to generate what we perceive as yellow. This may be obvious, but I will state it in any case=if we put the white light into the prism of the OP, we get the colors of the rainbow. If we combine the colors of the rainbow onto a white sheet of paper, we will see a bright white. If we send this white light into the second prism, we once again get the colors of the rainbow. Thereby, the "white" isn't really a color, i.e. it is not light of a single wavelength, like red, green, yellow, or blue can be.

Once again, my inputs here are basically for the beginner, and also for the sake of completeness. It may be somewhat obvious, but if you don't state the obvious, at times it can become the oblivious. Perhaps a viewer or two will find my latest input here of some benefit.

 

[Gleb1964]

Pure spectral colours are placed on the outside line of the CIE color map ( excluding the strait purple line connecting blue and red, which is a combination of blue and red). All other colours that are inside outer line are represent a mixture of pure spectral colours. White colour in that respect has no difference from the other.

 

[sophiecentaur]

Thereby, the "white" isn't really a color, i.e. it is not light of a single wavelength, like red, green, yellow, or blue can be.

"white" is not a specific location on the CIR chart. It is chosen ad-hoc in the specification of the illuminant that's used for the system. But it's right there in the middle of the CIE chart so how can it not be 'a colour'?

Thereby, the "white" isn't really a color, i.e. it is not light of a single wavelength, like red, green, yellow, or blue can be.

You really are locked into the idea of colours all being spectral. Is the CIE chart not crammed full of 'colours'? Are the 'colours sitting on the Red - Blue line not really colours. By your argument you would have to say that you haven't seen a single colour since you woke up this morning, unless you were looking at the output of a spectrometer. Can you quote a passage that agrees with your view? (You would need a quality source.)

 

[Charles Link]

What I was trying to say in the above, and I was trying to address a beginner audience, is that "white" light is not a single wavelength. The concept is simple enough that I think the more advanced should be able to see what I was referring to, and could perhaps say it in a better way, rather than putting a lot of effort into finding a lot of fault with the statement. I did think it was necessary for completeness to mention white light in this thread.

The concept of monochromatic (single wavelength sources of some $\lambda$ around a narrow $\Delta \lambda$ ) is also needed early-on for students. I do think I am presenting this concept in a reasonably good way, but @sophiecentaur , you may disagree. Both in college and at the workplace I did a fair amount of spectroscopy work, and for doing any kind of interference, such as from a diffraction grating or a thin film interference filter, one works with single wavelengths in the calculations, because light of two different wavelengths, except in rare cases, does not interfere. This is also a concept that is taught early-on in the Optics courses.

Edit: and to say the above in another way, there is no such thing as "monochromatic" white light, where monochromatic means single wavelength. Red, blue, green, yellow, orange, and violet can all come in monochromatic form, but white light can not. It was shown in the OP that yellow can have a monochromatic form with $\lambda \approx 585$ nm, or it can come as a mixture of red and green light, but white light is never of a monochromatic form. I thought this is indeed a useful concept for the beginner=I thought it should have been fairly clear what I was trying to get across to the reader.

In any case, it is worthwhile to get some feedback in an ordinary thread, rather than to try to write the topic up as an Insights article, and then find there are a couple who disagree, perhaps even strongly, with how it is presented.

 

[sophiecentaur]

Edit: and to say the above in another way, there is no such thing as "monochromatic" white light,

Did we need 114 posts to establish that? If you are anxious to get a worthwhile message over to students then get then to distinguish between Wavelength and Colour. Colour is a totally subjective experience and wavelength refers to just one measurable spectral line.
Monochromatic is a term for single wavelength and, as I have pointed out many times, we just do not see examples of monochromatic / spectral light in everyday life. None of the colours which we can recognise and name in a normal day is monochromatic. Likewise, the term 'white light' is not precise enough; two people in two different labs are likely to be using different 'white points' unless they make a point of sharing the spec of the white source they are using.

except in rare cases,

which cases are you referring to?

Thereby, the "white" isn't really a color, i.e. it is not light of a single wavelength, like red, green, yellow, or blue can be.

If your aim is to avoid confusing students then you should try to avoid sentences like that one. What message were you trying to get across with it? Is my new blue shirt 'not' a colour because the blue is not spectral blue?

 

[Charles Link]

which cases are you referring to?

Something called heterodyning. (For two different wavelengths interfering)

 

[Charles Link]

Did we need 114 posts to establish that?

I thought some of the posts were fairly productive. This is just an extra detail that we hadn't discussed, (that white light can never be a single wavelength). For the more advanced ones it is obvious, but perhaps there are readers who would benefit by mentioning it.

On another note, things don't need to be spectral (or a monochromatic) blue to be blue, but the blue will generally be dominated by wavelengths between 425-475 nm or thereabouts. Green 500-560 nm or thereabouts , and reds 625-750 nm. White light will normally be a balanced mixture of all three. It is a step in the right direction IMO for students to start to think in terms of wavelengths. Thinking in these terms to me is even much more important than to know some or any of the details of the CIE color map.

 

[sophiecentaur]

(that white light can never be a single wavelength)

How could it ever be if it doesn't lay on the spectral curve?

Something called heterodyning. (For two different wavelengths interfering)

Have you the remotest idea what that's about? It is a 'thing' involving non-linear interaction of two or more signals of different frequencies but it doesn't involve "interference".

On another note, things don't need to be spectral (or a monochromatic) blue to be blue, but the blue will generally be dominated by wavelengths between 425-475 nm or thereabouts. Green 500-560 nm or thereabouts , and reds 625-750 nm.

Can you repeat that 'theory' for magenta? What wavelength is magenta, by the way?

It is a step in the right direction IMO for students to start to think in terms of wavelengths

Those poor students.

 

[Charles Link]

Have you the remotest idea what that's about? It is a 'thing' involving non-linear interaction of two or more signals of different frequencies but it doesn't involve "interference".

With heterodyning the beat frequency (I also call that interference, besides the spatial interference patterns one gets from things like a diffraction grating) can be picked up between two optical sources closely matched in wavelength , typically somewhere in the acoustic range. It's been a while (20 years ago or thereabouts) that I worked with acousto-optic devices, but please give me a little credit for having seen some of the basics like Raman-Nath scattering. See https://www.rp-photonics.com/optical_heterodyne_detection.html

The write-up in this "link" tends to disagree with you=it says optical heterodyning simply uses a photodiode and not non-linear materials. I could present more of the work that I did with acousto-optics, but this doesn't need to turn into a contest of who is or isn't qualified to at least say something about the subject.

 

[sophiecentaur]

I also call that interference,

"Intermodulation"? Why not use the right term?

The write-up in this "link" tends to disagree with you=it says

A photodiode is non-linear. That's how it works. That link is about detecting light and not changing colours. It's not relevant to this thread at all.

And what about my question about Magenta? Does that have a wavelength?