Why does the colour wheel exist?

[thegreenlaser]

A picture that seems to show up a lot in high school physics is one like this:

[PLAIN]http://www.kollewin.com/EX/09-15-03/electromagnetic-spectrum.jpg

There's the wide range of electromagnetic waves at different frequencies, of which we can only see a narrow band. My question is, why do red and purple seem to overlap so nicely? Why, for example, can we make things like below, where we go through the rainbow from red to purple and then back to red again?

[URL]http://www.color-wheel-pro.com/pics/color-wheel1.jpg[/URL]

From looking at the electromagnetic wave spectrum, it seems like there's no reason for purple to blend nicely with red at all, and yet it seems to in practice. Is there a good reason for this? What would happen with animals that can see different parts of the spectrum than us (like ultraviolet, for example)?

 

[dan_r]

I suppose it's a bit subjective really isn't it? If we were unable to see violet, then you might well ask why indigo seems to blend in so well with red. Personally I think that any colour can blend in nicely with another as long as there's transition between the two.

When it comes to perception which is outside our visual range, then I'd imagine the same sort of conditions would apply to the observer. Whether they'd appreciate the aesthetic of the blending of perceptible wavelengths is open to question.. I'll stick my cat betwixt a sun-lamp and rainbow wheel later on to see how she reacts :)

 

[Fantasmagoria]

The color wheel has to do with vision more than the physical properties of light. There are only 3 color receptors in the retina so it isn't a perfect reflection of the wavelength spectrum. (for example you can trick your eye into seeing yellow with light that is actually a combination of red and green light in the right proportions.)

As it's usually drawn, primary colors across from each other on the color wheel are the complementary colors that show color aftereffect. This is the effect that if you stare at a green square that quickly changes to white, you'll see the square as a violet color. This has to do with saturation of the green photoreceptors. When you stare at the green square they adapt, but the other two types of color receptors (blue and red) do not. So the violet color you see is a result of the stimulation of the other two relative to the green. That's why it goes from blue to violet to red - violet is the color we see when red and blue photoreceptors are stimulated at the same time.

 

[I like Serena]

Why does purple have a reddish tinge to it?
As opposed to red that does not have a bluish tinge to it.

Since it's on the other side of the spectrum, one would expect that the red photo receptor of the eye is stimulated the least by purple!
Weird huh?

Wouldn't this seem to suggest that the red photo receptor is also susceptible to purple?
But I've never read anything like that...

 

[nasu]

The color that seems to blend with red on that well is not the actual violet of the spectrum but rather a shade of magenta.
I don't think that the actual violet end of the spectrum looks like it has red in it. I mean, if you look at a violet laser spot. It does not really look like the magenta shade on that well.
The distortion may be intended or maybe comes from the color rendition on monitors. Some pictures of bright violet laser spots have this magenta hue too.

 

[I like Serena]

True enough!

But why then is there violet beyond blue in the spectrum?

 

[DaveC426913]

You know, this is a really good question. Why can we see violet at all?

There are various flavours to the idea that we can see magenta (the complement of green), which is a combination of red and blue.

But there is also a pure violet that is not a combination. Why can we see it? I guess because the blue receptor is sensitive to a range.

Does that mean that when we see blue, we are seeing the effects of stimulation of blue receptors and green receptors? It would suggest that, if we eliminated the green receptors, so that we could only see what is stimulating the blue receptors, it would appear violet, not blue.

Why does the blue receptor allow us to see violet beyond blue but the red receptor does not allow us to see some other colour beyond red?

 

[I like Serena]

An alternative explanation might be that our photo receptors are not nice band filters, but that they can be stimulated at other frequencies than their main frequency...

Edit: perhaps a resonance frequency?
I just looked it up: violet is 380–450 nm, while red is 620–750 nm.
So twice the wavelength of violet overlaps the red range...

 

[I like Serena]

I couldn't believe that when colors were inventoried and standardized this "problem" would not have been acknowledged.
So I researched a bit how colors are standardized and found this wiki article:
http://en.wikipedia.org/wiki/CIE_1931_color_space

It suggests that an observer has a response curve for red photo receptors that has a second peak in the blue part of the spectrum.
Btw, this is not at a resonancy frequency.

 

[Claude Bile]

The answer is pretty simple, our brains have adapted to it. We have 3 colour receptors, one for red/green/blue; it just happens that our red receptors have a small response in the blue part of the spectrum.

370-420 nm wavelength light excites primarily our blue receptors, plus a small amount of our red receptors, which our brain interprets as violet.

I encourage people to look up the spectral response of our different colour receptors; it answers a fair deal of these questions, and is also quite fascinating.

Claude.

 

[schip666!]

The answer is pretty simple ... it just happens that our red receptors have a small response in the blue part of the spectrum.

Well now, I was going to answer the same because I thought that was the case, but looked at the wiki version of the response curves and didn't see any blue-bump in the red receptor. In fact the green receptor response seems to be stronger than the red under and beyond the peak of the blue. Are those graphs not entirely to be believed? Or are we just hand-waving the violet-is-reddish-blue thing when it is "really" greenish-blue?

 

[nasu]

Looking for real research data, I found some actual measurements done on individual or small groups of cones,for humans and monkeys.
The paper on humans is
Spectral sensitivity of human cone photoreceptors
J. L. Schnapf*, T. W. Kraft & D. A. Baylor
Nature 325, 439 - 441 (29 January 1987)
The monkeys were done by the same group.

It seems that the red receptors have a small deep in the green-blue region and then a slightly increased sensitivity at the shortest wavelength end.
Unfortunately they did not measure the blue receptors.

 

[DaveC426913]

The answer is pretty simple, our brains have adapted to it. We have 3 colour receptors, one for red/green/blue; it just happens that our red receptors have a small response in the blue part of the spectrum.

I have always seen this kind of graph:

http://en.wikipedia.org/wiki/Color_vision

so I don't know about this 'responsive to blue' you mention.

 

[I like Serena]

Here's the one I found:

http://en.wikipedia.org/wiki/CIE_1931_color_space

As I understand it, the CIE institute has done a survey to determine how various people observe colors, resulting in standardized response curves.

 

[schip666!]

I guess all they say about wikipedia is right...even their own entries don't agree, vis:
http://en.wikipedia.org/wiki/CIE_1931_color_space
http://en.wikipedia.org/wiki/Color_vision
Don't believe anything you read there without independent corroboration.

Unfortunately, "Spectral sensitivity of human cone photoreceptors" is behind the wall of scientific pay-for-play at Nature so us pleebes may never know the one-true-response curve.
But thanks for the info!

 

[I like Serena]

I guess all they say about wikipedia is right...even their own entries don't agree, vis:
http://en.wikipedia.org/wiki/CIE_1931_color_space
http://en.wikipedia.org/wiki/Color_vision
Don't believe anything you read there without independent corroboration.

Unfortunately, "Spectral sensitivity of human cone photoreceptors" is behind the wall of scientific pay-for-play at Nature so us pleebes may never know the one-true-response curve.
But thanks for the info!

I believe both wiki articles to be true.

The Color_vision article describes the response of the 3 types of cones L, M, and S.
The CIE article describes how people "observe" colors with standardized x, y, and z curves.
These are different things.
And it is not clear why there would be a discrepancy between the two.

My 2 cents: perhaps the S-cones are cross-wired somehow to the L cones?

Does anyone have access to the "Nature" article?

 

[nasu]

http://books.google.ca/books?id=OcW...CDkQ6AEwAg#v=onepage&q=vision science&f=false

Here is a diagram of spectral sensitivities of cones from a book which is accessible on Google books. The page is 157.
Very interesting topic. I did not think about it too much. Just considered the Wikipedia type graphs with the two bell shaped curves.
The secondary maximum of red receptors does not seem to be confirmed by real measurements. Maybe is a subjective maximum, not a physical one.

 

[Drakkith]

Well, I typed up about half of a long response, but realized the situation was a bit more complicated that I was trying to describe. As such I'll just simply say that I think the reason that violet looks red is because your "red" receptor is activated somewhat by the color, whereas when we see red light and lower in wavelength the blue receptor does NOT activate at all. Hence violet looks like a mixture of red and blue, and in fact that is how we make the color when mixing paints or pixels from a TV. In contrast, your Blue receptor can never see into the red part of the spectrum, so there is no similar effect for red.

I would expect that if the Blue receptors overlapped the Red in a similar way then light that was a combination of the two could be perceived as violet as well depending on the amount and ratio of activation. Note that this is talking about monochrome light, as mixing paints still results in the perception of that specific wavelength range even though there is no light in the violet wavelength reaching your eye. A side effect in my opinion would be that a rainbow would go Violet, Red, Orange, Yellow, Green, Blue, Violet, making a "Color Wheel" with each color blending into the next in a circular fashion.

 

[DaveC426913]

...violet looks red because your "red" receptor is activated somewhat by the color, ...

But it isn't.

 

[Drakkith]

But it isn't.

How so? Everything I looked at here and elsewhere showed that the red receptor does have a response range in the violet wavelength. Did I miss something?

 

[nasu]

I had time now to read part of the Nature paper that I referred.
They measured the photo-current (with micro pipette) produced by the cone when illuminated with light of various wavelengths.
Their results are shown as relative sensitivity. The values sensitivity of red receptors near 400 nm are a little above -1 (log scale, with maximum at 0).
So it seems that the sensitivity is little less than 10 times smaller in blue than at the maximum (around 600 nm).
This may not be the same as perceived sensitivity.
Definitely they get some response at around 400 nm but it's not clear if this is enough for the brain to react. On the other side of the spectrum, at around 700 nm the sensitivity goes to -5 on the log scale, so much sharper drop.

 

[schip666!]

There does seem to be some overloading of terminology...but I don't see how cross-wiring would be an explanation without some kind of stimulation in both sensors. Of course I was never able to understand the explanation of the Land effect for more than a few seconds at a time, and can't even describe the setup now either.

And now you've got me wondering if I've every actually SEEN violet that is not aka magenta. I might have a tiny prism someplace with which to perform a science-like experiment.

I can perhaps prevail on some friends to 'jack the Nature paper.

 

[I like Serena]

...but I don't see how cross-wiring would be an explanation without some kind of stimulation in both sensors.

I'm thinking that we have 3 separate nerves that are connected to the 3 different cones.
Now suppose a cone for blue is also connected to a nerve for red, but with a reduced amplitude...
Wouldn't that account for what we see?

 

[nasu]

The paper about monkey cones is freely accessible. It's by the same group and the curves are very similar.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1192171/pdf/jphysiol00526-0162.pdf

This one contains results for blue receptors and even a table of values.
I did not have time to read through it yet. Maybe someone would be interested.
In the article about human cone they show that the human/monkey curves are very close, at least for green and red.

 

[I like Serena]

For the record, I started a new thread in Biology: https://www.physicsforums.com/showthread.php?t=509416
Just to see if someone might give some insight.

In retrospect, perhaps that was not such a good idea, since we'll get a similar discussion in 2 places instead of one!

@nasu: I see an interesting graph where the photo receptor for red first has a negative response for shorter wavelengths, and then becomes positive again for violet wavelengths!
Interesting!

 

[DaveC426913]

How so? Everything I looked at here and elsewhere showed that the red receptor does have a response range in the violet wavelength. Did I miss something?

I guess I did. I linked to a graph showing no such bump. I'm sure I've got it wrong. Can you point me at some material?

 

[nasu]

@nasu: I see an interesting graph where the photo receptor for red first has a negative response for shorter wavelengths, and then becomes positive again for violet wavelengths!
Interesting!

I am afraid you may have misread the graph. The negative values are due to the use of a logarithmic scale. Negative means less than one. They rescaled the sensitivity so that the maximum is one, corresponding to a value of zero on the log scale. Everything less than one is negative on the log scale.

However the graph seems to indicate a shallow minimum at some wavelength around 450 nm.
This is not a bump, as in the Wikipedia graph, but rather the opposite, isn't it?

 

[I like Serena]

I am afraid you may have misread the graph. The negative values are due to the use of a logarithmic scale. Negative means less than one. They rescaled the sensitivity so that the maximum is one, corresponding to a value of zero on the log scale. Everything less than one is negative on the log scale.

However the graph seems to indicate a shallow minimum at some wavelength around 450 nm.
This is not a bump, as in the Wikipedia graph, but rather the opposite, isn't it?

Well there's more than one graph.
Now that I look more carefully and read some of the text, I see the graphs show basically the same as the 2 wiki graphs.

That is, the red-cone-response-curve has a minimum at 450 nm, but goes every so slightly a very little bit up at shorter wavelengths.

But there is another graph showing color matching experiments of humans, which more or less shows what the CIE curves show as well. It is this graph that I referred to, with a negative minimum at 500 nm and a positive local maximum at 420 nm.

 

[sophiecentaur]

I guess all they say about wikipedia is right...even their own entries don't agree, vis:
http://en.wikipedia.org/wiki/CIE_1931_color_space
http://en.wikipedia.org/wiki/Color_vision
Don't believe anything you read there without independent corroboration.

Unfortunately, "Spectral sensitivity of human cone photoreceptors" is behind the wall of scientific pay-for-play at Nature so us pleebes may never know the one-true-response curve.
But thanks for the info!

At one time, all such information would cost you money. You had to buy books for it or go to a library. There is no justification for complaining that it's not all freely available. Would you, personally work all day, every day, writing useful information without expecting any pay? There is nothing very esoteric about those curves. They are in many textbooks on colourimetry, which you could, no doubt, read by going to a local library.

I am surprised that there seems to be some confusion about the apparent contradiction between the two sets of curves. There is no contacdiction and there should be no confusion. One set is labeled with the word "response" and the other is labeled with the word "matching". They are by no means the same thing. The red curve with the 'hump' in the blue is the result of what sense the brain makes of the three responses and what levels of three displayed primary colours will produce a match with the observed colour.

I have batted on a lot about this, I know, but the words Colour and Wavelength are treated, too frequently, as being synonymous and this has caused an awful lot of confusion for the casual reader.

 

[schip666!]

I'm thinking that we have 3 separate nerves that are connected to the 3 different cones.
Now suppose a cone for blue is also connected to a nerve for red, but with a reduced amplitude...
Wouldn't that account for what we see?

I don't think it could distinguish between the response on both sides of the blue peak. And given that the green response seems to be a bit higher than the red "one would think" that we'd "see" a combination of blue-green. But, as I said, whada I know? Digging further in the papers is indicated.

At one time, all such information would cost you money. You had to buy books for it or go to a library. There is no justification for complaining that it's not all freely available. Would you, personally work all day, every day, writing useful information without expecting any pay? There is nothing very esoteric about those curves. They are in many textbooks on colourimetry, which you could, no doubt, read by going to a local library.

Yes, usually libraries have access to many scientific journals. However what you pay for is the journal, not the science. Most scientific research is funded through grants and a good portion of those are public, i.e., derived from taxes. But the nagging thing for me is that the public is hoped to understand and make decisions based on "scientific evidence" to which many have no access. On a personal note, I've belonged to a number of IEEE societies for years and have only once actually gotten access to one of their papers because it was accidentally published in the right journal; and it turned out to have a misleading abstract so it was of no use to me.

 

[sophiecentaur]

The truth is that there is no 'hidden' information needed to resolve this particular question.

People keep going on about cross connections between receptors. There is no need for all that, if you want to explain the problem. The brain has only three signals to for its input and it can get identical combinations of signals for a whole range of received spectra. The 'non-spectral' colours, which cannot be produced by a single spectral line are not really different from the 'spectral' colours. No Artist ever painted a picture using spectral light sources to produce their fantastic coloured pictures of sunsets or flowers.
With additive colour synthesis, Red and green phosphors can produce a yellow colour which will match the Sodium D yellow - to an average Human Colour Vision and an appropriate combination of blue and red will give the same colour sensation as a spectral line that we would call violet. There is no contradiction or paradox in this. It's complex, in detail, because we are talking about human perception, but the basic principle is not hard and doesn't require anything 'fancy' from the hardware.
I have said this before but the human eye is not a spectrometer - it is (a fairly crude) colour matching engine and can be 'fooled' into thinking that the spectra from two sources are identical when, in fact they can be wildly different. The brain sees things in a way that can be conveniently and simplistically mapped onto the "colour wheel" which is mentioned in the OP. Better than the Colour Wheel is the CIE Chromaticity chart, which has a numerical scale associated with it and allows the design of good colour imaging systems.

 

[I like Serena]

Can you mix colors to a color that fools an observer into thinking it is spectral violet?
Or spectral red near the infrared?

 

[sophiecentaur]

What you need to do is to look at the CIA diagram. If you have phosphors way out at the violet corner of it, you could produce spectral violet. That means you would need a spectral violet phosphor. I don't think that is a practical proposition.
I have a feeling that you knew that already(?)
Haha

 

[I like Serena]

What you need to do is to look at the CIA diagram. If you have phosphors way out at the violet corner of it, you could produce spectral violet. That means you would need a spectral violet phosphor. I don't think that is a practical proposition.
I have a feeling that you knew that already(?)
Haha

Fair enough! :)
I guess I was only trying to make the point that violet can not be mixed from other colors than violet, or we would not perceive it as violet.

My problem remains that I'm still confused about the difference between blue and violet.

If I accept your explanation, then spectral blue would only be observed to be blue because it triggers the red/green cones.
And if the red/green cones were not triggered (by some means), we would be fooled into seeing violet.

Somehow, that seems to be off, since violet has a tinge of red in it.

 

[schip666!]

I decided against the "can I really see violet" experiment due to both practical and philosophical difficulties. But I did look at the macaque data in the previously ref'ed paper (Table 1. in http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1192171/pdf/jphysiol00526-0162.pdf) and believe that it indicates a small up-blip for red-cone sensitivity in the far blue meaning that there may be significant differences in the 3-space signals (combined with green response) to either side of "pure" blue. Still would be nice to see the human-Nature paper, but who am I to ask...

Some of this discussion may be off-point due to conflation of magenta and violet -- and whatever happened to "indigo" which was previously touted to be out there beyond violet? Magenta is a _non-spectral_ color and is produced by exciting blue and red cones near both their peaks. Violet is spectral and does -- in theory -- have a specific wavelength somewhat below 400nm. One of my philosophical difficulties with experimenting was that I have no idea how to point to the actual BLUE stripe from a prism in order to say that there is something I see on the other side. I suppose that could be solved with a spectrometer...but I can't get that at the library either.

I also noticed -- finally -- that the two response graphs on the wiki pages (color vision, color space) have different vertical scales. The vision one mumbles something about being linear and seems to be normalized, whereas the space one (which shows the red-blip) is clearly log. So maybe we've been talking about the same thing all along.

As to seeing color change into the IR, the tabular data and wiki graphs both indicate that green and blue sensors level out as the red tails off. So I presume all one sees is a gradual dimming of red rather than a color shift.