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Old 09-21-2016, 05:40 AM
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Violet and purple are totally different?

I had always thought that purple and violet were very similar, just slightly different shades of each other. But Wikipedia says differently:

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Originally Posted by Wikipedia
Purple is a color intermediate between red and blue. It is similar to violet, but unlike violet, which is a spectral color with its own wavelength on the visible spectrum of light, purple is a composite color made by combining red and blue.
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Originally Posted by Wikipedia
Violet and purple look very similar; but violet is a true color, with its own set of wavelengths on the spectrum of visible light, while purple is a composite color, made by combining blue and red.
I'm having a lot of trouble understanding the difference between a true spectral color, vs. a composite color.

Wikipedia also says that green and orange are spectral colors. Is there a composite color analogous to purple (i.e., not green) that is made by combining blue and yellow? Is there a composite color (not orange) that is made by combining red and yellow?
Old 09-21-2016, 05:53 AM
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Don't even get me started on Indigo.

Last edited by jtur88; 09-21-2016 at 05:54 AM.
Old 09-21-2016, 05:54 AM
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Crayola says they are the same color. The crayon is labeled Violet (Purple).
Old 09-21-2016, 05:56 AM
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It's important to note this from the wikipedia article on purple:
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While the scientific definitions of violet and purple are clear, the cultural definitions are more varied.
Old 09-21-2016, 06:00 AM
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It depends on what you mean by "color". You can have two completely different spectra, which will look different through a spectroscope, but which look identical to the human eye. For instance, if you have a pure red light (from a neon tube, say) and mix it with a pure green light (I'm not sure precisely what works for this, but there are sources), you'll get something that looks to the eye just like a pure yellow light (say, from a sodium vapor lamp).

Mix red and blue together, and you get something that looks just like if you took a single wavelength somewhat shorter than blue. This situation is somewhat unusual, in that violet is not in between red and blue, like most apparent color combinations are. But they still look the same.
Old 09-21-2016, 06:03 AM
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And an important difference between purple/violet and green/green, orange/orange as mixes or spectral colours is that those latter mixtures give you the sensation of a color with a wavelength between the two colours mixed, whereas purple is the mixture of opposite ends of the visible spectrum, giving you the sensation of the extreme end of "blue".
Old 09-21-2016, 06:18 AM
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The article on purple also mentions this difference between the spectral colour violet and the mix purple:
Quote:
One curious psychophysical difference between purple and violet is their appearance with an increase in luminance (apparent brightness). Violet, as it brightens, looks more and more blue. The same effect does not happen with purple. This is the result of what is known as the Bezold–Brücke shift.
Now the Bezold-Brücke shift is described as
Quote:
As intensity increases, spectral colors shift more towards blue (if below 500 nm) or yellow (if above 500 nm). At lower intensities, the red/green axis dominates.
So a mixed orange should stay orange, while a spectral orange sensed to be the same hue should shift towards yellow.

It'd be fun to experiment with this, but it would require a rather advanced system of wavelength-adjustable light sources ... Or just a lot of lamps and filters.
Old 09-21-2016, 06:42 AM
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Originally Posted by naita View Post
And an important difference between purple/violet and green/green, orange/orange as mixes or spectral colours is that those latter mixtures give you the sensation of a color with a wavelength between the two colours mixed, whereas purple is the mixture of opposite ends of the visible spectrum, giving you the sensation of the extreme end of "blue".
Thanks! I keep forgetting that violet is at the far end of the spectrum, and is NOT located between red and blue. The reason I keep forgetting that fact is probably because on a circular color wheel, it does end up between them.

So I guess now the question shifts to our perception of these colors: Why do we think of violet as being reddish, when in fact red is at the other end of the spectrum?
Old 09-21-2016, 06:48 AM
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Quote:
Originally Posted by Keeve View Post
I had always thought that purple and violet were very similar, just slightly different shades of each other. But Wikipedia says differently:





I'm having a lot of trouble understanding the difference between a true spectral color, vs. a composite color.

Wikipedia also says that green and orange are spectral colors. Is there a composite color analogous to purple (i.e., not green) that is made by combining blue and yellow? Is there a composite color (not orange) that is made by combining red and yellow?
it means it's possible for something to emit pure violet light. Violet light corresponds to a wavelength of roughly 400 nm. there are LEDs available which do this (Indium-gallium-nitride.)

purple is a range of colors, produced by mixing red and blue.
Old 09-21-2016, 07:39 AM
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Originally Posted by Keeve View Post
Thanks! I keep forgetting that violet is at the far end of the spectrum, and is NOT located between red and blue. The reason I keep forgetting that fact is probably because on a circular color wheel, it does end up between them.
I'm trying to wrap my head around this one.
Old 09-21-2016, 08:27 AM
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Spectral colors are the ones you find in the rainbow. You'll notice that there's nothing there that looks purple. The really intense blue at the end of the rainbow is sometimes named "violet" (hence "ultraviolet") but it's not very close to purple.

The white light of the sun consists of (almost) all wavelengths, and a rainbow or spectrum shows them from long to short. So each spectral color consists of a single wavelength of light. Non-spectral colors need multiple wavelengths. This includes all pastels such as pink or powder blue, those add white. Purple is is also a non-spectracl color, it consists of red and blue but no (or little) green.

Orange is a spectral color but you can get pretty good orange by mixing red and green, the same way you can get all colors by mixing the three primary colors are eyes are sensitive to, red, green and blue. (Actually it's a bit more complex than that.)
Old 09-21-2016, 08:28 AM
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Quote:
Originally Posted by Keeve View Post
I had always thought that purple and violet were very similar, just slightly different shades of each other. But Wikipedia says differently:





I'm having a lot of trouble understanding the difference between a true spectral color, vs. a composite color.

Wikipedia also says that green and orange are spectral colors. Is there a composite color analogous to purple (i.e., not green) that is made by combining blue and yellow? Is there a composite color (not orange) that is made by combining red and yellow?
What you've found is quite correct -- purple is a composite color, and not spectral -- that is, you can't separate out white light with a prism or diffraction grating and find Pure Purple. But you can do this with violet.

To answer your last question, about combining two spectral colors to get something not a spectral color. well, speaking technically, it happens all the time.

Color science is a weird thing, partly because it involves the human senses and brain, which form a weird mix. They spent a lot of time screwing around with trying to figure out how color works. In 1931 an international convention, the International Committee on Illumination (CIE, from its French name, Commission internationale de l'éclairage) took a lot of work that had been done and formally recognized it and published it. There have been other meetings and ramifications since, but the basics established then pretty much hold.

Using methods I'm not going to go into now, they represented color on a two-dimensional graph, in which the x and y axis are "normalized" colors. A roughly horseshoe-shaped curve on this represents the "spectral locus" of pure spectral colors. The line joining the ends is "purple". Al;l colors that can be observed fall inside the horseshoe-plus-line diagram: https://en.wikipedia.org/wiki/Chromaticity
https://en.wikipedia.org/wiki/Intern...n_Illumination

White is somewhere around (0.3, 0.3), the exact coordinates depending upon how you define "white". If you have two different colors represented by points on this diagram, you can generate any color on the line joining them. Thuis all putples can be generated by the red at one end and the blue at the other.

Since only points on the actual spectral locus are true spectral colors, the combination of any two spectral colors willgenerate a point NOT on the spectral locus, and therefore ANY combination of spectral colors meets your description.

The problem is that this is only technically true, because people's definitions of colors are kinda wobbly. On a 12931 CIE Chromaticity diagram, you can't tell apart colors falling within an ellipse (called "Macadam ellipses", after the guy who discovered them. His office used to be down the hall from mine). Besides, people are forgiving in their definitions of colors, and even outside the ellipses they'll still use the same names to describe them. (For what it's worth, we pretty much use the same name for any color along a line from that "white" center to the spectral locus. We just say that, as you get close to "white", the color gets lighter and more "washed out")

So, unfortunately, although you can generate new colors by mixing pure spectral colors, you're not going to create some whacky new color that you'll have to come up with a new name for, like mixing pure blue and pure yellow to make "gormwatz". What you'll make, unsurprisingly, is some kind of green, slightly lighter than the corresponding pure spectral green.



By the way, if you mix colors made by THREE colors represented by points on the CIE diagram, you can make anything inside that triangle. This is what happens with color monitors, which use three phosphors to make color images. There's a vast literature on this, which you can lose yourself in iif you want. Try here: http://eizo.com/library/basics/l...r_color_gamut/ or https://en.wikipedia.org/wiki/SRGB

The point is that, even if your three basis colors are on the spectral locus (and they aren't for most monitors, but I suppose you could do it with three lasers), yoiuur mixtures won't be, and you can't duplicate the spectrum. This is why the CIE diagrams you see on your monitor look mkind of wonky and not right.
Old 09-21-2016, 08:31 AM
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Originally Posted by Keeve View Post
Thanks! I keep forgetting that violet is at the far end of the spectrum, and is NOT located between red and blue. The reason I keep forgetting that fact is probably because on a circular color wheel, it does end up between them.
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Originally Posted by Channing Idaho Banks View Post
I'm trying to wrap my head around this one.
As I recall a color wheel relates to pigments and mixtures thereof. Mixing lights, as in stage lighting, follows a different path, and dealing with wavelengths as mentioned above may be different yet.
Old 09-21-2016, 08:58 AM
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Equal rights for indigo ...
Old 09-21-2016, 09:28 AM
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The point is that, even if your three basis colors are on the spectral locus (and they aren't for most monitors, but I suppose you could do it with three lasers), yoiuur mixtures won't be, and you can't duplicate the spectrum. This is why the CIE diagrams you see on your monitor look mkind of wonky and not right.
Ok, but couldn't you have violet paint that absorbs all the other visible spectral colors? Or a violet filter that only passes spectral violet from a true white light source?
Old 09-21-2016, 10:26 AM
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A related fact is that the color pink does not exist at least as part of the light spectrum. This article explains why and also has relevance to the purple/violet question.

http://techinsider.io/pink-light...y-exist-2015-8
Old 09-21-2016, 10:40 AM
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Here's a swatch of what I consider violet and purple to be, respectively (ignore magenta, it just looks like pink).

I note that most Minnesota Vikings regalia tends towards the violet rather than purple. That pleases me for reasons I don't fully understand.
Old 09-21-2016, 10:55 AM
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Originally Posted by iljitsch View Post
Spectral colors are the ones you find in the rainbow. You'll notice that there's nothing there that looks purple. The really intense blue at the end of the rainbow is sometimes named "violet" (hence "ultraviolet") but it's not very close to purple.
See, to me, and no doubt I'm influenced by Crayola and my alma mater's school colors (Northwestern), that's definitely a color I would also call "purple." I mean, look at the rainbow here. That's not purple at the end? Or look at the Northwestern logo. That's called "purple" by everybody connected to the school.

Colloquially, purple and violet are pretty much interchangeable in my dialect. I will tend not to call reddish purples "violet," but bluish purples can go either way.

Last edited by pulykamell; 09-21-2016 at 10:56 AM.
Old 09-21-2016, 11:36 AM
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See, to me, and no doubt I'm influenced by Crayola and my alma mater's school colors (Northwestern), that's definitely a color I would also call "purple." I mean, look at the rainbow here. That's not purple at the end? Or look at the Northwestern logo. That's called "purple" by everybody connected to the school.

Colloquially, purple and violet are pretty much interchangeable in my dialect. I will tend not to call reddish purples "violet," but bluish purples can go either way.
This is a good point, indeed if we wanted to reach as many people as possible we probably would want to use the word "purple". However, if we say the ozone layer absorbs ultrapurple light at a scientific conference, maybe we'd lose a bit of credibility.

Besides, sometimes violets are yellow
Old 09-21-2016, 11:58 AM
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Bravo for CalMeacham's post. I was going to post something on this, but he totally nailed it better than I could hope to. Really, it is all there. Go back and read it if you are still puzzled.

Important point. You cannot post links to pictures of colours and talk about whether there is purple or violet in them. Your screen is only capable or reproducing those colours that live within triangle on the CIE diagram the three basic colours it uses defines. If your screen's colour gamut does not include violet (and it won't) what you see on the screen is the three colour approximation your screen provides that does live within that triangle. And, guess what? It will be a mix of your screen's red, green and blue. Probably not much green, so - a mix of red and blue. Better known as purple. Your screens inability to actually deliver colours out of its gamut will fold down both true spectrally pure and mixes into that gamut. This renders discussion on the actual colour pointless.

A complicating factor in the question of purple versus violet is that if you look at the sensitivities of the eye's receptors, there is a glitch in the far blue spectrum, where the effective sensitivity to red is non-zero. So violet is perceived as a mix of red and blue, but due to an artefact of the sensor's spectral response. However, you can't reproduce the exact stimulus by actually mixing red and blue light, as this will inevitably result in some green stimulus as well, and you won't get exactly the same values presented to your brain. So violet manages to remain not purple, but still close. (Note, there are not such things are red green and blue sensors, just three different sensors with slightly different spectral responses and some tricky neural processing at the back of the retina that creates an encoding of the colour and brightness to send to the brain. That coding isn't RGB either.)
Old 09-21-2016, 12:04 PM
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This is related to a question I asked a while ago, and never really got a good answer to: why does the colour wheel "wrap around"? There seems to be no logical reason why the colours at the top and bottom ends of the visible spectrum should blend together so well. Is it just a quirk of human perception that red plus blue makes a very similar colour to violet?
Old 09-21-2016, 12:12 PM
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First of all, Look at an image of the CIE chromaticity diagram. Calling this shape (the full color gamut*) a "rounded triangle," you can see the spectral colors beginning at one corner (770 nm red), and proceeding around the rounded color, eventually to the third corner (380 nm violet). A mixture of spectral colors will appear inside the gamut as the weighted average (linear sum) of the spectral colors.

For example, if you combine 770 nm (red) with 520 nm (green) you'll end up very close to 568 nm (spectral yellow). This red+green yellow won't appear exactly the same as spectral yellow because the triangle leg connecting red to green isn't perfectly straight.

The third leg of the triangle (purple, reddish purple, purplish red) are colors not present in the spectrum (i.e., they are not colors in the rainbow) — they are produced by combining indigo and red.

(* - NOT the white triangle shown inside the full gamut: it depicts the gamut of colors that can be rendered on a RGB monitor.)

Note that what this CIE diagram shows is NOT intrinsic to colored light. It depicts the perceptions formed by the cones in human retina.

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Originally Posted by Keeve View Post
Thanks! I keep forgetting that violet is at the far end of the spectrum, and is NOT located between red and blue. The reason I keep forgetting that fact is probably because on a circular color wheel, it does end up between them.
Is it correct that Isaac Newton was the first to note that hues could be configured into a circular wheel, by introducing purple between red and indigo?

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Originally Posted by Keeve View Post
So I guess now the question shifts to our perception of these colors: Why do we think of violet as being reddish, when in fact red is at the other end of the spectrum?
Before leaving retina, information from the cones is transformed into three signals, in order of decreasing importance: Light vs Dark, Red vs Green, Yellow vs Blue. You're asking: Why doesn't the Yellow vs Blue signal "kick in" to make a big perceptual difference between purple and reddish purple? I have no answer except to note
(a) the CIE diagram is NOT intended to have equal perceptual differences reflected in equal distances — that goal would be contrary to the linear additivity property mentioned above.
(b) colors are discussed relative to an "average human"; different subjects may perceive colors differently.

Last edited by septimus; 09-21-2016 at 12:14 PM.
Old 09-21-2016, 12:31 PM
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It is interesting to note that Sir Isaac Newton, when first playing with the prism in the mid 1600s, would have been aware of the difference between purple and violet. According to the Ngram viewer, the word 'violet' was rarely used in the body of English literature, while the word 'purple' was commonplace. So Newton's choice of 'violet' to describe the spectral band is revealing.

https://books.google.com/ngrams/grap...violet%3B%2Cc0
Old 09-21-2016, 01:12 PM
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It is interesting to note that Sir Isaac Newton, when first playing with the prism in the mid 1600s, would have been aware of the difference between purple and violet. According to the Ngram viewer, the word 'violet' was rarely used in the body of English literature, while the word 'purple' was commonplace. So Newton's choice of 'violet' to describe the spectral band is revealing.
The sounds nice because everyone loves Isaac Newton however pretty much everybody who lived before Isaac Newton had seen a rainbow, and it's the same thing.
Old 09-21-2016, 01:16 PM
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I think of violet and purple in relation to the midpoint between red and blue: Violet is immediately to the blue side of that point and purple immediately to the red side. In other words, essentially the same color, but with diverging tendencies. As for precisely the midpoint between red and blue, what to call it? Purple, violet, eggplant, amethyst, lavender, wisteria? Looking around, you find different sources using these names for different shades in the neighborhood of that point... meaning it isn't amenable to exact, mathematically defined science.

Purple is the favorite color of both my GF and me. She and I differ in our views on the question, to an extent. Influenced by reading too many Wikipedia color articles, I conceptualize violet as next to blue and purple as next to red, while remaining aware that they're both approximately midway. She argues that rather violet is exactly the same as purple, and purple is the proper name for it. I think either of us is as right as the other, and the actual (versus theoretical) difference between our perspectives is infinitesimal.

I always used to wonder why the books I read always called it "violet," while all the other kids at Gesù Elementary would only say "purple." Our generation was coeval with Harold and the Purple Crayon, after all. It just felt more natural to say purple, it seemed to roll out of the mouth more easily.

Now I get it, having read the Wikipedia explanation. Violet is defined as a range of specific wavelengths of light. Also, a violet is a flower, a thing, tangible. What's a purple? Purple is... well, just purple! Or snail juice, when you get right down to it, but while violets were an everyday sight in my home, none of us had ever seen any snail juice, nor would we want to, so the literal meaning of purple was lost on us and it became a concept unto itself. Most of the books I read growing up were science or science fiction, and that accounts for all the insistence in print that "violet" is the correct name. My brain makes room for both purple and violet, together and opposed, mostly together.

Last edited by Johanna; 09-21-2016 at 01:18 PM.
Old 09-21-2016, 01:31 PM
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Why a circle?

The thing is that neither the eye nor brain know anything about spectral wavelengths. The eye has three sorts of sensors. If it had two there would be a line segment of colour, but with three you define a section of a plane. As septimus notes above - the eye does not send RB to the brain - it sends luminance, and two axes of colour. Red to green, and blue to yellow. Ie, location on a plane of colour. The colour wheel is simply representing that plane. The colours on the wheel being those on the perimeter of the plane.

The world we see is for the most part actually devoid of spectrally pure colours. Rainbows and those colours build with diffraction are as close as you will get. Almost everything we see is a blend of wavelengths. There is no reason for our to actually want or need to resolve spectral colours explicitly. We see a simple averaging of three wavelength bands. Three makes it a wheel.

The little glitch in the red channel sensitivity up in the far blue means that there is a natural wrap from violet into the blue-red mix. It isn't impossible that this has been selected for to make the colour perception work better/smoother. Or it might be a coincidence, and had it not been there we would have simply seen violet as even deeper blue, and only mixed colours as purples.
Old 09-21-2016, 02:21 PM
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Equal rights for indigo ...
Indigo, like Pluto, has been cast into the outer darkness where there is much wailing and gnashing of teeth. Except that no one really cares that much about indigo, so very little wailing etc.
Old 09-21-2016, 02:59 PM
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Indigo, like Pluto, has been cast into the outer darkness where there is much wailing and gnashing of teeth. Except that no one really cares that much about indigo, so very little wailing etc.
What is the deal with "indigo," anyway? Am I right in remembering that it was sort of shoehorned into the spectrum to make it seven colors or something like that? It almost seems to me the "BIV" part should really be "CBV" (cyan, blue, violet), as there is a distinct color between green and blue that is cyan or turquoise. I mean, just look at a spectrum. There is a very obvious band (at least to me) at around 500nm that seems to me like it should get its own name as a rainbow color instead of the "indigo" transition between blue and violet. But I suppose it's all kind of arbitrary. Or is there a possibility that the "blue" and "indigo" in the original naming of the spectral colors (I assume by Newton) really corresponds to what we might call "cyan" and "blue" now?

Last edited by pulykamell; 09-21-2016 at 03:01 PM.
Old 09-21-2016, 04:30 PM
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tl;dr
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Originally Posted by pulykamell View Post
What is the deal with "indigo," anyway?
CalMeacham has written a longish explanation here, post #25

Tl;dr It's all Isaac Newton's fault. He was trying to make light work like a musical scale and needed 7 colors to do that. He had red, orange, yellow, green, blue and violet and needed another, so he grabbed the name of a blue dye and used that.
Old 09-21-2016, 04:40 PM
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tl;dr

CalMeacham has written a longish explanation here, post #25

Tl;dr It's all Isaac Newton's fault. He was trying to make light work like a musical scale and needed 7 colors to do that. He had red, orange, yellow, green, blue and violet and needed another, so he grabbed the name of a blue dye and used that.
Huh. And I made a very similar comment in that thread, too, about ROY G CBV. And also mention the violet vs purple thing. And Pleonast mentions the blue=cyan, indigo=blue bit, too. Weird. I don't remember that.

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Old 09-21-2016, 05:14 PM
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Huh. And I made a very similar comment in that thread, too, about ROY G CBV. And also mention the violet vs purple thing. And Pleonast mentions the blue=cyan, indigo=blue bit, too. Weird. I don't remember that.
There were also people saying "No Pluto! No Indigo" in that thread, somewhat like my comment.
Old 09-21-2016, 05:31 PM
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The sounds nice because everyone loves Isaac Newton however pretty much everybody who lived before Isaac Newton had seen a rainbow, and it's the same thing.
My point was Newton;s choice of a word for the color everyone saw. Purple was in wide parlance, but Newton chose Violet, which was a rarely used term in his day.
Old 09-21-2016, 06:28 PM
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Actually, you never see spectrally-pure colors in a rainbow, either. An entire rainbow, from red through violet, is about 2 degrees wide. But because the Sun is half a degree wide, the band produced by any single spectral color in a rainbow is also half a degree wide. So at any point along the width of a rainbow, you're actually getting a mix of a range of colors that's a significant chunk of the entire visible spectrum.
Old 09-21-2016, 08:12 PM
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I came across this image that Wikipedia claims is by Newton's own hand.

See, indigo, who's gonna argue with Sir Issac Newton?

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Old 09-22-2016, 01:23 AM
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As a fan of Indigo Girls, I'm biased, that's given. But I find indigo useful as a name for a color intermediate between violet/purple and blue. Even though I know that literal indigo plant dye is blue, really just blue, and that our color terminology is complicated because Isaac Newton played fast-and-loose with words for the high-frequency end of the spectrum. Granted, all that. I still want a name for that blue-violet tertiary, and indigo fits well enough.

In my palette I'm accustomed to thinking in terms of a twelve-part color wheel with 3 primaries, 3 secondaries, and 6 tertiaries. At this point I'm already getting away from analyzing electromagnetic spectral frequencies anyway while I'm mixing paints. The one-twelfth between blue and violet, if I don't call it indigo, what am I gonna call it?
Old 09-22-2016, 01:39 AM
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At this point I'm already getting away from analyzing electromagnetic spectral frequencies anyway while I'm mixing paints.
As you should. The movement that though that the spectral colours were the "pure" natural ones were misguided. They sort of are if you see using a spectrometer. But we don't. For subtractive mixing, spectral colours are even less useful. Tricks using small dots of colours next to one another are of course a way of getting additive mixing. Still not spectral colours, but you get a different gamut. Colour theory is gloriously complicated
Old 09-22-2016, 04:18 AM
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For one of my color theory classes in art school, I decided to make a comprehensive color wheel, i.e. one that included shades, tints and saturations. It turned out to be a cylindrical in shape.

The default layer was a traditional circular color wheel, with the colors around the edge. The center was gray, and as you moved away from the center, the gray acquired color in 10% increments, and the colors increased in saturation.

The next layer upward consisted of 90% tints of all the colors in the default layer, i.e. each color was 10% lighter. Each subsequent layer consisted of colors that were 10 additional percent lighter than the previous layer. The top layer was pure white.

The next layer downward from the default mixed 10% black with everything on the base layer, followed by a 20% layer, etc., and culminating in a final layer that was pure black.

The virtue of this configuration was that you could easily see how colors that aren’t shown in traditional color wheels relate to colors that are; e.g. the browns, or what happens when you mix grays with various amounts of pigment.
Old 09-22-2016, 04:18 AM
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This is from the original poster:
Quote:
Originally Posted by Keeve View Post
Thanks! I keep forgetting that violet is at the far end of the spectrum, and is NOT located between red and blue.
Does this really happen? How often have you forgotten this? Is it even possible to forget the order of the colors?
Quote:
The reason I keep forgetting that fact is probably because on a circular color wheel, it does end up between them.
I don't believe this. Are you the only person who has never seen a rainbow?
Old 09-22-2016, 05:07 AM
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Quote:
Originally Posted by Channing Idaho Banks View Post
This is from the original poster: Does this really happen? How often have you forgotten this? Is it even possible to forget the order of the colors?
I don't believe this. Are you the only person who has never seen a rainbow?
I don't see how that proves or disproves anything, but speaking for myself over the course of my life, I have seen colors on a color wheel far more often than I have seen a rainbow. And I have never seen a full-spectrum rainbow that clearly shows all the colors from violet to red. Usually it's just a few smudges of colors near the middle.
Old 09-22-2016, 07:45 AM
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Originally Posted by Channing Idaho Banks View Post
This is from the original poster: Does this really happen? How often have you forgotten this? Is it even possible to forget the order of the colors?
I don't believe this. Are you the only person who has never seen a rainbow?
The order of the spectral colours is determined by ordering them by wavelength. That covers a small part of the colours that a human perceives. There are many colours that you cannot find in a rainbow that the human eye/brain perceives. The most obvious ones are those that are a mix of red and blue - so purples. But not just purples. In reality the vast majority of colours we see are not in a rainbow or even a high quality spectrum (say from a prism or diffraction grating fed from a thin slit).

As I noted above, because the eye/brain system encodes colour with two (nearly) orthogonal signals, one coding for green versus red content, and the other coding for blue versus yellow, you don't actually see a line of colour, you see a plane of colours, and the colour wheel is simply those colours that run around the edge of that plane. There are colours over the entire area of the plane as well, and when you add the luminance component, you actually get a three dimensional solid of colours.

Ordering just the monochromatic spectral colours by wavelength is a very trivial approximation to the reality of colours.
Old 09-22-2016, 08:43 AM
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Originally Posted by watchwolf49 View Post
I came across this image that Wikipedia claims is by Newton's own hand.

See, indigo, who's gonna argue with Sir Issac Newton?
That image comes from an 1855 book, and shows purple as a prismatic color.

Opticks, 4th edition by Sir Isaac Newton, Knt. is available on-line. It is pretty clear on Newton's use of purple
Quote:
If red and violet be mingled, there will be generated according to their various Proportions various Purples, such as are not like in appearance to the Colour of any homogeneal Light, and of these Purples mix'd with yellow and blue may be made other new Colours.
Old 09-22-2016, 02:25 PM
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Originally Posted by Acsenray View Post
And I have never seen a full-spectrum rainbow that clearly shows all the colors from violet to red. Usually it's just a few smudges of colors near the middle.
You must not get very bright rainbows where you live. The brighter they are, the more colors you can see. I've seen rainbows where I could pick out several distinct shades in between green and yellow and between yellow and orange.
Old 09-22-2016, 03:26 PM
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Originally Posted by septimus View Post
That image comes from an 1855 book, and shows purple as a prismatic color.

Opticks, 4th edition by Sir Isaac Newton, Knt. is available on-line. It is pretty clear on Newton's use of purple
"The originall or primary colours are Red, yellow, Green, Blew, & a violet purple; together with Orang, Indico, & an indefinite varietie of intermediate gradations."

With spelling like that, you trust Ike?
Old 09-22-2016, 03:55 PM
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A spectral color is a single wavelength of light that stimulates one or more of the cone cells in the retina. A composite color is a mixture of two or more wavelengths striking the retina together, giving the impression of a single color.

To use a musical analogy, violet (a spectral color) is a pure note, and purple (a composite color) is a chord.

Yellow is a spectral color that stimulates the red- and green-sensitive cones. However, a mixture of red and green light can also stimulate those cones in the same way, creating a perception of "yellow" in the brain even though no yellow light is present. Red and green spotlights are combined into yellow lighting in theatre productions, and red and green pixels create the illusion yellow on video screen. So yellow can also be a composite color.
Old 09-22-2016, 06:55 PM
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Originally Posted by Colophon View Post
"The originall or primary colours are Red, yellow, Green, Blew, & a violet purple; together with Orang, Indico, & an indefinite varietie of intermediate gradations."

With spelling like that, you trust Ike?
Opticks 4th edition, published posthumously 1730, contains no such quote (the word "gradation" appears not at all). Your quote comes from a 1672 Draft of 'A Theory Concerning Light and Colors,' fifty years earlier.

He was mixing colors in 1672 but may not have stumbled onto red+violet=purple so didn't need to use 'purple' in a special sense then. Setting up prisms in just the right way was tedious — even for this genius, perspiration may have been key to inspiration.

Quote:
Originally Posted by Isaac Newton in his 1672 draft

The same colours in Specie with these primary ones may be also produced by composition: ffor a mixture of yellow & blew makes green, of red & yellow makes Orang of Orang & yellowish green makes yellow. And in generall if any two colours be mixed, which in the series of those generated by the Prism are not too far distant one from another, they by their mutuall alloy compound that colour which in the said series appeareth in the midway between them. But those which are situated at too great a distance doe not soe. Orang & Indico produce not the intermediate green, nor Scarlet & green the intermediate yellow. [my emphasis]
(I'll guess that his Orang-Indico mix gave him a grayish brown he deemed uninteresting — he hadn't yet discovered purple.)

Some of the confusion about "purple" might go away if magenta were used as the name for red+violet. Perhaps Doper cmyk should weigh in!

Last edited by septimus; 09-22-2016 at 06:57 PM.
Old 09-22-2016, 07:08 PM
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Originally Posted by Channing Idaho Banks View Post
Is it even possible to forget the order of the colors?
I find this question baffling. Are you claiming that the spectral order of colors is common knowledge? I'd be surprised if one person in ten among the general population could accurately name them in order. Why would the ROY G BIV mnemonic be necessary if everyone knew this?

--Mark
Old 09-22-2016, 09:15 PM
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I've asked the question about why red+blue results in the sensation of a more engergetic color than you started with. I didn't quite get a definitive answer, but this is what I took away from it:

Spectral violet actually somewhat activates the red cones in our eyes. The best guess for why is an "octave effect." An octave is a doubling of the wavelength of sound. And sounding a higher octave, will cause a string of the lower octave to sympathetically vibrate. Something similar may happen with the eyes, as violet correponds to almost double the frequency (half the wavelength) of red.

Of course, we can simulate this sympathetic vibration by adding a little red to something that is blue. And that is how systems such as RGB produce purple.

You'd think that this would have to be built into cameras, so that they would convert violet to purple. And the CIE chart, with its extra red hump towards the deepest violets, seems to indicate that this is the case.

Last edited by BigT; 09-22-2016 at 09:15 PM.
Old 09-22-2016, 10:52 PM
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I doubt it is an octave effect - although it is a neat thought. It isn't on an octave, and for the most part optics doesn't really work that way.

I don't think I have seen this built into cameras - although it certainly could be. We survived for many years with colour film, and that has all sorts of issues.

One thing is - as we have discussed earlier - there is very little actual violet out there. You would need to find a light source that was close to monochromatic violet. Apart from a rainbow you are not going to find one outside of a lab (and as Chronos notes, even the rainbow is significantly mixed.) So cameras cheerfully just deliver purple when they see purple, and very pure blue in the incredibly rare times they see pure violet.

Indeed, any sort of monochromatic light almost always causes cameras to fail in their colour representation. The best known examples are white light sources that are made from a set of line spectra. Cameras have a terrible time trying to get the colours of things illuminated with such sources to match what you eye perceives.

The thing about the CIE chart is that it represents colours as we see them. Colour is an eye/brain artefact. There is really no such thing as an intrinsic colour outside of this.
Old 09-22-2016, 11:14 PM
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Originally Posted by Francis Vaughan View Post
As I noted above, because the eye/brain system encodes colour with two (nearly) orthogonal signals, one coding for green versus red content, and the other coding for blue versus yellow
And white vs. black or luminance, which happens to rely on the "red" AND "green" content, but not so much the blue.
Quote:
Originally Posted by Johanna View Post
As a fan of Indigo Girls, I'm biased, that's given. But I find indigo useful as a name for a color intermediate between violet/purple and blue. Even though I know that literal indigo plant dye is blue, really just blue, and that our color terminology is complicated because Isaac Newton played fast-and-loose with words for the high-frequency end of the spectrum. Granted, all that. I still want a name for that blue-violet tertiary, and indigo fits well enough.
I'm certain that I could not identify an indigo plant, but I like the word because using it makes me feel like a pirate. We have a shipment of rum and indigo, matey!
Quote:
Originally Posted by panache45 View Post
For one of my color theory classes in art school, I decided to make a comprehensive color wheel, i.e. one that included shades, tints and saturations. It turned out to be a cylindrical in shape.
Snip. Some color systems are cyndrilical, and it sounds like you stumbled across that empirically. For example, HSV/HSL (subtle difference) puts Hue on the circular part, Saturation is out from the center (as if cutting it pizza-style), how mixed it is with gray, and cut down the cylinder and its Value or Lightness, how bright it is. If you go into a drawing program, even the basic MS Paint, you can pick colors in either RGB or HSL, the latter is useful because you can say, keep brightness constant and change the hue (at least if your monitor was calibrated to do so), while with RGB it isn't intuitive how to do this.
Quote:
Originally Posted by BigT View Post
I've asked the question about why red+blue results in the sensation of a more engergetic color than you started with. I didn't quite get a definitive answer, but this is what I took away from it:

Spectral violet actually somewhat activates the red cones in our eyes. The best guess for why is an "octave effect." An octave is a doubling of the wavelength of sound. And sounding a higher octave, will cause a string of the lower octave to sympathetically vibrate. Something similar may happen with the eyes, as violet correponds to almost double the frequency (half the wavelength) of red.

Of course, we can simulate this sympathetic vibration by adding a little red to something that is blue. And that is how systems such as RGB produce purple.

You'd think that this would have to be built into cameras, so that they would convert violet to purple. And the CIE chart, with its extra red hump towards the deepest violets, seems to indicate that this is the case.
Just checking: are you referring to this: https://en.wikipedia.org/wiki/CIE_19..._Functions.svg ?

These don't directly model your retina's response, but represent an experiment from the 1930s where observers had to mix three lights to match the range of wavelengths. So they had to add a little red to the blue in order to reproduce violet. The cones' responses are rather Gaussian, with a little heaviness and asymmetry in the tails, but there isn't an extra hump. But beyond the very first few layers of the retina, color is encoded as Francis Vaughn describes.
Old 09-22-2016, 11:28 PM
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Oh, and red, green, blue are shorthand, but really the peak response of the cones is respectively yellowish, yellow-green, and violet, so in color science they're normally called the L, M, and S cones (long, medium, short wavelengths). And the opponent color system doesn't respond to red vs. green and blue vs. yellow as we normally understand them. I characterize the B vs. Y as purple vs. baby puke green (left figure).
Quote:
Originally Posted by CalMeacham View Post
By the way, if you mix colors made by THREE colors represented by points on the CIE diagram, you can make anything inside that triangle. This is what happens with color monitors, which use three phosphors to make color images. There's a vast literature on this, which you can lose yourself in iif you want. Try here: http://eizo.com/library/basics/l...r_color_gamut/ or https://en.wikipedia.org/wiki/SRGB
Sharp was claiming for awhile that they introduced a fourth phosphor. We had a thread about it and didn't really solve it, but I understand it was mostly a marketing gimmick. If you were to add a "yellow" as they claim, it wouldn't add much to the gamut if you look at where yellow is in the CIE charts vs. the sRGB triangle.
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