On Mixing Colors of Light

[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!)

 

[pinball1970]

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

And metamerism in the phenomena. Trying to explain the difference between colour constancy and metamerism has been something of a feature at work for a while.

 

[pinball1970]

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

And metamerism in the phenomena. Trying to explain the difference between colour constancy and metamerism has been something of a feature at work for a while.

 

[sophiecentaur]

colour constancy

Never heard of of that one. I guess it means how much difference in the RGB values is noticeable(?). I remember the JND (just noticeable difference) which is different in various regions of the CIE diagram.

 

[pinball1970]

Never heard of of that one. I guess it means how much difference in the RGB values is noticeable(?). I remember the JND (just noticeable difference) which is different in various regions of the CIE diagram.

We use it for assessment of samples under different lights. Clients have different requirements and point of sale.
In short if they want a particular colour, they want that colour not to be different depending on where that setting is.
It presents a lot of challenges, as an example, say a car has interior grey, upholstery, seat belt and an exterior that matches.
Requirement is the car looks that colour in the show room and when it out in daylight.
The showroom will have a particular SPD and daylight has a different one.

 

[sophiecentaur]

Paints and pigments are a real pain. TV engineering is a piece of cake in comparison.

 

[pinball1970]

Paints and pigments are a real pain. TV engineering is a piece of cake in comparison.

Yes, three different sets of colourants for metal, leather and upholstery.
My old lab dealt with the interior side in the 90s so metamerism and colour constancy was a major part.

Life is simpler these days, but I miss it!

 

[AlexB23]

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?

Fascinating stuff. I do agree, as colorimetric stuff is not optics. Yeah, a lot of people including myself conflate color with wavelength or frequency, when color is just how we perceive these light waves.

 

[blue raven]

What does that mean? As you move between areas on the CIE chart, why shouldn't more than one value change?

Did you notice that there are very few objects in a scene with one of RGB being near zero? Saturated colours are rare in everyday scenes.

In the real world, when we made a change to one color, actually the other colors are affected as well by this change, but not in the same manner. Imagine we have sliders (values) for each color, and when you move one slider, the other sliders (values) will also start to move/change. So in the real world it is impossible to change just one color. For example, if you want to make a color redder, by adding red the green will be strongly affected, the violet will be a bit affected, the only one that won't be affected is the black, which has a kind of color inertia. When you look at such color charts, if you think you can move on one axis, in reality you can't, and you will move on two or more axes, creating a bit of a mess.

@Charles Link
I think the CIE graph has one big hole, namely the lack of black (or perhaps "darkness"? ) that appears in CIE Lab space as negative L, which reduces virtually any color, see this relatively accurate representation (others are not so good)

Why black (color) matters? Because using white and black you can produce 3D effects (and this might be quite interesting for physics, even thinking about black holes) and without black it does not work very well.
So many posts, and you haven't even scratched the surface of this interesting topic. You should be bolder.

 

[Charles Link]

@blue raven The Physics Forums rules only allow for mainstream physics, and do not allow for any personal theories where we are creating things that are outside of the realm of what has been established as credible science.

 

[sophiecentaur]

In the real world, when we made a change to one color, actually the other colors are affected as well

I think you need to re-think this. You would need to specify which part of the 'real world' you are discussing. Are you talking in terms of Analysis or Synthesis?

Synthesis: It's easy to vary the R primary signal to the display without changing the G and B primary voltages and you would get a result that lies along a line between the white point and one side of the triangle.

Analysis: It would be very difficult to change the pigment / paint / filter to change just one of the tristimulus values.

namely the lack of black

Your worry is misplaced. The whole point of the CIE (chrominance)chart is that it is independent of the Luminance. I mentioned earlier that 'skin tones' are virtually identical for pale skinned Nordics and very dark skinned Africans. The RGB signals have more or less the same ratios for both skins. You can have a lot of fun with a colour picker, writing down the RGB values for various faces in a crowd.

If you look at the old PAL / NTSC coding you see the levels of luminance , varying over each tv line andthe phase carrying a high frequency chrominance subcarrier signal which varies in amplitude (saturation) and

'hue (the phase of the subcarrier). This colour bar signal uses very saturated bars.

 

[blue raven]

@blue raven The Physics Forums rules only allow for mainstream physics, and do not allow for any personal theories where we are creating things that are outside of the realm of what has been established as credible science.

I don't recall such a topic "Mixing light colors" in theoretical physics, but in 2014 the Nobel Prize in Physics was awarded for the invention of the blue LED, and there you might find some related topics that can be explored without being accused of drifting apart from mainstream physics, such as how a photon is produced, the energy of a photon in quantum mechanics, valence, conduction and gap bands, see this video


You might find also interesting the topic about fluorescent and phosphorescent colors
https://en.wikipedia.org/wiki/Luminous_paint

 

[sophiecentaur]

I don't recall such a topic "Mixing light colors" in theoretical physics

Colourimetry is not 'theoretical' it's to do with very much applied Physics.

accused of drifting apart from mainstream physics

Who would be 'accusing' anyone? PF has a foot in all fields.

 

[hutchphd]

The "physics" of colorimetry is rather straightforward. The eyes report the "color" based upon the integrated intensity from several differently sensitive chromophors in the eye. (Note that this depends upon both the illuminant and the object under study, thus allowing metamerism) The assignment of color to an object is not therefore unique. In addition the reduction of a spectrum to a handfull of data is very "lossy". These are "known unknowns".
In addition there are a host of "physiologic" unknowns that have to do with processing this data into what we call "color". Many of these are "unknown unknowns".( https://en.wikipedia.org/wiki/There_are_unknown_unknowns)
But it represents a relatively arcane corner of physics (which paid my bills for quite a span)

 

[blue raven]

Synthesis: It's easy to vary the R primary signal to the display without changing the G and B primary voltages and you would get a result that lies along a line between the white point and one side of the triangle.

Analysis: It would be very difficult to change the pigment / paint / filter to change just one of the tristimulus values.

If you look at the old PAL / NTSC coding you see the levels of luminance , varying over each tv line andthe phase carrying a high frequency chrominance subcarrier signal which varies in amplitude (saturation) and

'hue (the phase of the subcarrier). This colour bar signal uses very saturated bars.View attachment 355873View attachment 355874

And have you wondered how accurate these colors were reproduced on the screen? Back in time, it was common knowledge that screens didn't accurately reproduce colors to match a quality print. There was quite a difference between the image on the screen and the print.
Any change at the input must be measured at the output, using a device such as a spectrophotometer like this one, X-rite i1 or cheaper ones. Nowadays, good monitors can be calibrated in the range of an acceptable error Delta E, where this error is given by the deviations from the target color on the color map/space, see what Delta E is. A calibrated monitor comes with a report that shows the values of this Delta E error for each color, if I remember correctly. So the challenge is to keep all errors below Delta E<1.5, this error is good, in this case you can be sure that all colors displayed on the screen will match a quality print, which also can be verified using a spectrophotometer.

 

[pinball1970]

And have you wondered how accurate these colors were reproduced on the screen? Back in time, it was common knowledge that screens didn't accurately reproduce colors to match a quality print. There was quite a difference between the image on the screen and the print.
Any change at the input must be measured at the output, using a device such as a spectrophotometer like this one, X-rite i1 or cheaper ones. Nowadays, good monitors can be calibrated in the range of an acceptable error Delta E, where this error is given by the deviations from the target color on the color map/space, see what Delta E is. A calibrated monitor comes with a report that shows the values of this Delta E error for each color, if I remember correctly. So the challenge is to keep all errors below Delta E<1.5, this error is good, in this case you can be sure that all colors displayed on the screen will match a quality print, which also can be verified using a spectrophotometer.

1.5 is huge EDIT: Big

 

[Charles Link]

@blue raven The video in post 191 is interesting, but there is a big step I am missing that I mentioned also in post 176=how do we go from having just one of these blue LED's to suddenly being able to manufacture millions of them on a computer screen? There seems to be some way of copying things atom by atom that was never taught in the classroom to students of my generation. That is in a way getting off topic, but at the same time it is relevant to this topic and would bring it up to date to how things are presently being done.