I On Mixing Colors of Light

[sophiecentaur]

but it wasn’t a good medium.

The darkroom safe light was always over-optimistic. The image on paper would gradually come into view and it would look so promising. Total magic. You took it out of the Dev dish and dumped it in the Fix dish; safe to turn on the room lights. Nearly always, the result was tired, soft and low contrast but no chance to click the history button and start again.
Someone will point out that you need to be much better disciplined. True; the thermometer and timer should always rule.

 

[DaveC426913]

Nearly always, the result was tired, soft and low contrast but no chance to click the history button and start again.

Huh. Describes my high school experience exactly.

 

[Calstiel]

All three sets of cones have overlapping spectral sensitivity over more or less the whole spectrum. Without that, the eye would be blind to awkwardly placed and "narrow band" colours. This is why the three analysis curves are not described as Red Green and Blue but L,M and S . The diagram shows those curcespresented with 'spinach'.

Can you give me the name from the paper from this diagram?

 

[sophiecentaur]

Can you give me the name from the paper from this diagram?

Here is the link. Many others are available

 

[Charles Link]

See https://en.wikipedia.org/wiki/CIE_1931_color_space
There seems to be an open item on the LMS (long, medium, short) and the X,Y, Z response curves where there is a clear discrepancy between these two. I have limited search resources on why these two are so different, but they are. I tend to believe that the X,Y, and Z is more accurate.

 

[sophiecentaur]

The goal is to generate a perceptually accurate yellow

That is a very limited goal. It's not a good basis for spectrometer design. CIE were in the business of good colour reproduction and a two colour system is not sufficient. I cannot understand why Yellow seems to be taking up most of the comments on this thread. Making Yellow with a green and a red light is great for a reliable demonstration with crude equipment in school but not a lot else.

 

[Charles Link]

The concept here is more detailed, and to try to show why the primary colors are what they are. I found it very interesting that there is a spectral yellow from 580-590 nm, and alo a perceived one, made from green at 550 nm and red at 650 nm or thereabouts. With the mixing of light explained, and that with red and green, they do not physically make any 580-590 nm,= this to me is important.

Then there is the other part=why is yellow then a primary color in mixing paints? I thought @sophiecentaur and others did a very good job of answering that in the thread, why do yellow and blue light not make green light?, in the thread that was linked far above, which I will find again momentarily. Yes, see https://www.physicsforums.com/threads/color-theory-what-does-blue-yellow-make.1008903/

 

[Charles Link]

Just a comment or two: We really need to be willing to discuss fundamentals at times, how boring it may be to some, even things as basic as the Pythagorean theorem or the quadratic formula, or soon we will have a generation where nobody (of the younger generation) knows how to do anything any more. If their cell-phone doesn't give them the answer, they will be all thumbs, and it would be far better to have a generation that know much of the fundamentals once again.

 

[sophiecentaur]

do yellow and blue light not make green light?,

Yellow is not a primary colour in the additive world. To put yellow into the additive world you call it -B (minus B); it's what you get when you take blue away from white. Pigments and filters subtract appropriate wavelengths.
If you draw a line from the yellow spot to the blue primary spot, the resultant will pass from yellow to blue and go through a de-saturated bluey-green area. (always inside the triangle of primaries). Unfortunately (for image processing people) we all get taught in primary school about mixing paints but primary school teachers can't be expected to use the word 'caveat'. If the thread strays into subtractive mixing, I will get my hat and coat.

 

[Charles Link]

Yellow is not a primary colour in the additive world. To put yellow into the additive world you call it -B (minus B); it's what you get when you take blue away from white. Pigments and filters subtract appropriate wavelengths.
If you draw a line from the yellow spot to the blue primary spot, the resultant will pass from yellow to blue and go through a de-saturated bluey-green area. (always inside the triangle of primaries). Unfortunately (for image processing people) we all get taught in primary school about mixing paints but primary school teachers can't be expected to use the word 'caveat'. If the thread strays into subtractive mixing, I will get my hat and coat.

Subtractive mixing is the last thing I would want to see as well. One thing that I think could use some clarification, (and this has come up), is that the complete human perception that can be viewed is all from colors/wavelengths that lie on the outer edge of the tongue that is the CIE color map. You can go to the interior part of that region and get a color, but that color is simply a combination of other wavelengths/colors from the fringe, and a little study, not even a large study, shows that the composition that makes up the color in the interior is not at all unique. That's where, once again, the spectral wavelength view of things is really much more fundamental than the CIE chart.

 

[berkeman]

Thread is closed for Moderation...

 

[berkeman]

After removing a likely AI-assisted side conversation by a problematic member, this thread is now reopened. Thanks for your patience.

 

[sophiecentaur]

After removing a likely AI-assisted side conversation by a problematic member, this thread is now reopened. Thanks for your patience.

Scary stuff.

 

[Charles Link]

When you've got someone (@Crispybacon) who tells you you've got a good idea or two, and then you find out some of that opinion may have been formulated by a robot. I'm skeptical=I think this one may have been human, even if he did use artificial intelligence to help him write some of his inputs.

 

[DaveC426913]

When you've got someone who tells you you've got a good idea or two, and then you find out some of that opinion may have been formulated by a robot. I'm skeptical=I think this one may have been human, even if he did use artificial intelligence to help him write some of his inputs.

PF has a policy of not allowing AI-generated content. That's not to say one cannot use AI bots to help one's writing, but it is required that it be the human's words, not the words an AI merely barfed-up.

Further discussion should probably move to the Feedback section.

 

[Charles Link]

PF has a policy of not allowing AI-generated content. That's not to say one cannot use AI bots to help one's writing, but it is required that it be the human's words, not the words an AI merely barfed-up.

I remain skeptical. It was the best feedback I have gotten in the last two months, and I find it hard to believe that the robots can actually appear as intelligent as some of us. He liked my OP=he actually said he "loved" it=to date I think it has only gotten 2 "likes".

 

[DesertDog1830]

I want to reiterate something that for me was a major find with the CIE chart that I mentioned in posts 23 and 24 above. It basically treats the 3 color bands [Edit=color cone stimulus/response] in 3 dimensions with the X, Y, and Z being the intensity of each band. (X=red cone, Y=green cone, Z=blue cone). The light of a given luminance and color is a vector $\vec{R}=X \hat{i}+Y \hat{j}+ Z \hat{k}$ , and where the line $\vec{R} t$ crosses the plane $x+y+z=1$ is designated as the color coordinate. The CIE chart is a color map in this plane which crosses the axes at $(1,0,0)$, $(0,1,0)$ , and $(0,0,1)$. (They usually show it as an x-y graph, but they are actually showing the view from above of the map in the plane $x+y+z=1$.)

If you have two sources of some intensity with different color coordinates on this chart, making vectors of two different lengths, the resultant vector will be in the plane containing these two vectors, and its color coordinates are found by where the line made from this resultant vector passes through the plane $x+y+z=1$. The CIE chart is really a neat piece of mathematics. See also https://en.wikipedia.org/wiki/CIE_1931_color_space , but they don't seem to highlight these details.

Edit: It should be noted that the vector $\vec{R}$ above thereby has color coordinates of $x=X/(X+Y+Z)$, $y=Y/(X+Y+Z)$ , and $z=Z/(X+Y+Z)$.

Good!

 

[Charles Link]

To add a couple additional comments to post 160 above, it might be worth mentioning that the CIE diagram puts the visible spectrum simply on the edge of the tongue-shaped color map, essentially on a line/curve that has zero area. These spectral colors on the fringe are still the much more fundamental item here than the entire color space in the interior of the color map, which can be created from spectral sources on the fringe of the diagram. In fact it should be stated that any color light source is a combination of spectral sources from the fringe=perhaps I'm stating the obvious, but it might be worth mentioning.

One other item worth mentioning is that the spectral composition of any location in the interior of the CIE color map is not unique. Instead many, many different spectra can have the same color coordinates. Even a little arithmetic with the CIE color coordinates on the upper right section of the color coordinates map suggests that you can even generate colors on the fringe of the color map from pairs of adjacent spectral colors on the fringe. (e.g. red and green in the right combination can give yellow or orange). For the section on the upper right fringe, there is essentially no excitation of the blue cone, i.e. $z \approx 0$, because $x+y+z=1$, and in this case $x+y \approx 1.0$.

To reiterate what was mentioned in the OP, mixing these colors does not change their spectral composition, again something that may be obvious to some, but probably not to everyone. If the light is originally composed of red and green light, after mixing it is still simply composed of red and green, regardless of how it appears to the eye.

Hoping at least a couple of people find this useful reading. So far, the feedback has been rather limited, so it's difficult to assess whether anyone is finding this useful.

 

[Charles Link]

Just one additional comment: If it is going to be credible physics, I do have to give posts such as @sophiecentaur 's of post 146 much consideration. I am not infallible, but on this one I stick to what I have put forward in the OP. The CIE color chart also supports the concept of the OP=it is possible that is also an approximation and it certainly isn't a perfect model, (assumes linear responses of the color cones that can be added as vectors).

Note: I looked through a number of @sophiecentaur 's previous posts over the last 6 months, and there is no doubt that his physics is first rate, but on this item, I think I offer the better explanation. The concept in my OP is based on mainstream physics, but even the brightest ones don't always agree on everything. Everyone is entitled to their opinion. I do try to keep an open mind to others who don't agree with any physics that I promote, but on this one, I stick with my OP for better or worse. Cheers. :)

 

[sophiecentaur]

: If it is going to be credible physics,

But how much actual physics is involved here? The reason that the system works is human psychology. The Physics at work is as varied as the different technologies involved. You have repeated your view many times in this thread. I have just been saying standard chapter and verse on the topic. Can you quote any creditable source to justify what you have been saying?

 

[Charles Link]

Can you quote any creditable source to justify what you have been saying?

I am retired. I don't presently have access to much of the literature. You are welcome yourself though to do the calculation with the CIE color map that you presented early-on in post 16. Most of the upper right hand border ( the spectral fringe) lies very near the line $x+y=1$, with the result that $z=0$ for those spectral sources to a very good approximation. The result then is that the green at 550 nm and the red at 650 nm together in the right combination can generate something that looks almost exactly like something at 580-590 nm.

I really never intended to generate any controversy here=I really thought the OP might pick up a few likes and most might find it interesting and maybe even agree with it. It is also still a possibility that the CIE chart isn't entirely accurate. I don't know whether anyone has conducted a similar experiment like I proposed in the OP. Perhaps the human subjects would be able to tell a slight difference. That remains an open item.

Edit: and the human subject part of it, i.e. what the human perceives, I would categorize as biophysics or opthalmology, rather than psychology.

 

[sophiecentaur]

I don't know whether anyone has conducted a similar experiment like I proposed in the OP.

I seem to remember seeing an arrangement of prisms and slits in a work by Isaak Newton. He already sussed out the difference between 'colour' and 'spectrum' and he demonstrated how you can synthesise white light by adding colours. He didn't have the advantage of a handy CIE diagram, of course but what 'new' stuff is proposed here?

Edit: and the human subject part of it I would categorize as biophysics or opthalmology, rather than psychology.

A rose is a rose is a rose. I would say that Physics does not involve subjective testing or at least it tries to eliminate the human element when possible.

 

[Charles Link]

He didn't have the advantage of a handy CIE diagram, of course but what 'new' stuff is proposed here?

Probably nothing terribly new, but we do know about light as having wavelengths, something that Newton did not have in his time.

On a plus note, thank you for introducing the CIE color chart in post 16. If this thread served no other purpose, I did learn about how the CIE color chart works. Cheers. :)

 

[Charles Link]

and a follow-on: Basically I think I may have reinvented the wheel in the OP. The mixing of a colors of light might have been something kind of new about a hundred years ago, but today's technology with the flat TV screens with microscopic size pixels has advanced far beyond anything I presented. In any case, I still find the fundamentals of interest, and they do make for good discussion.

The same sort of thing applies to things like our cell phone technology. How they ever are able to manufacture things like the cell phones as well as memory cards and mass produce them is totally beyond me=it is possible that the science has advanced beyond what can be readily understood by most, but I still think it is worthwhile for students to know about things such as photodiodes and LED's (light emitting diodes). The best science IMO is of concepts that most can understand. The ideas might have been a little simplistic, but hopefully at least a couple of readers found the above thread of interest.

 

[sophiecentaur]

I don't know whether anyone has conducted a similar experiment like I proposed in the OP.

I seem to remember seeing an arrangement of prisms and slits in a work by Isaak Newton. He already sussed out the difference between 'colour' and 'spectrum'. It's certainly not new.

 

[Charles Link]

I seem to remember seeing an arrangement of prisms and slits in a work by Isaak Newton. He already sussed out the difference between 'colour' and 'spectrum'. It's certainly not new.

The idea I put forward in the OP two months ago was new to me, (perhaps not to you), but it has now gotten a little old. Right now it would interest me if someone could explain how they can make and mass produce several million or more p-n junctions (photodiodes) in a very tiny space, all with electrical connections, or how they can manufacture a billion or more memory cells on a small memory card, but that would be the subject of another thread.

Edit: The ideas in the OP may be a little simple, but they are more my speed. Science and technology has advanced so much in the last 20 or 30 years=perhaps even beyond our intelligence.

Edit 2: Just an additional comment or two=for those who already know all about the red, green, and blue color cone sensors of the human eye along with the CIE color map, that the original post=the OP might seem very simple, but for me I thought it to be good physics, and I still think they would do well to address the subject of the mixing of colors of light in the undergraduate physics curriculum=perhaps as part of the Optics course.

 

[Charles Link]

I just did a little googling on the subject of red, green, and blue color cones and came up with the following:
See https://en.wikipedia.org/wiki/Young–Helmholtz_theory

It looks like @sophiecentaur indeed has it correct, that the original post=the OP, presented nothing new= Thomas Young came up with a good part of this, and others have added to it since then.

If the mixing of colors of light along with the perception of the human eye is not taught as part of the standard curriculum, it seems it will be something that some may know very well, while others may be left with little or no expertise in that area. In any case, IMO this wiki article is a good one and others may find it of interest as well.

New or not though, I still think the OP that I posted is reasonably good physics. Even though I did a lot of work with diffraction grating spectrometers, mixing colors of light was a new topic for me, and one that I found very interesting.

 

[AlexB23]

TL;DR Summary: Doing a discussion with a Gedanken experiment of mixing green (550 nm) and red light (650 nm) and comparing it to that of yellow light at 600 nm

I want to illustrate what the color mixing that is done with a tv screen is all about, and how we can generate the appearance of color using primary colors, even though we don't actually generate any light of the color that we perceive. Consider beginning with a tungsten filament with a current running through it that creates a continuous spectrum that can be approximated by a T= 2500 K blackbody for a typical case where it generates white light.
Let's have that light be collimated by a lens and run it through a prism=basically a prism spectrometer that will split the visible light into the colors of the rainbow, because the index of refraction of the glass of the prism decreases with increasing wavelength.

Now let's sample the output at the locations where the spectrum is green (550 nm) and red (650 nm) and combine these with the necessary mirrors and or lenses onto a sheet of white paper. If we get the proportions right, we should observe some yellow light.

We can also sample the output of the prism at the angle about midway between where the green and red emerge, and there we will find yellow light at 600 nm. If we focus this onto a white sheet of paper, it is likely we can not tell the difference with our eyes between this and the green and red combination.

The next part is to send some of the yellow light that comes off the white paper into a second prism spectrometer. In the first case, we will find green and red light to emerge in the same proportion that we started with at the angles corresponding to 550 nm and 650 nm, but nothing at the 600 nm location. In the second case, everything will emerge at the 600 nm location. The light that is made of green and red light appears yellow, but the light doesn't change its composition when the green is combined with the red. It is still a mixture of green and red, even though it appears yellow.

Thought you might find this of interest. I welcome your feedback. I just came up with this the other day. Generally I don't think they treat this topic in very many of the textbooks in the manner in which I presented above.

Would the 650 nm light + 550 nm light appear yellow, as it would average out to 600 nm? I am not that good at optics.

 

[sophiecentaur]

Would the 650 nm light + 550 nm light appear yellow, as it would average out to 600 nm? I am not that good at optics.

Don't worry about your optics. Colourimetry is way outside mainstream optics; it's mainly to do with how the sensors in your eye work and how the brain recognises the result of mixtures of wavelengths and maps it onto 'colour space'.

When you say "average out" that would only be right for 'equal' levels of the two primaries you have selected. Controlling the levels of those two independently can produce a colour that looks the same as (a metemer) an approximate spectral colour. That colour will lie on a straight line between the two primariesUse the formulae in some of the posts higher up this post. Of course, the really interesting bit is how three primaries, in the appropriate relative levels can produce a 'match' of any colour within the triangle (nowhere near the spectral curve).

Imo it is risky to use the words 'colour' and wavelength together without being very aware that they are actually different things. Colour is what your brain makes of a range of different colours which only 'might' be monochromatic (spectral lines) Most of the colours we see are not from monochromatic sources. It is sad that so many 'experts' misuse the two terms. How can anyone be expected to get understanding of this topic in this desert of colourimetric misinformation?

 

[hutchphd]

(a metemer)

Metamer is how I learned to spell it
Wikipedia also does a pretty good explanation (graphs and all!)