How do we see the color violet?

  • #26
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I wasn't aware that Newton was very well informed about how modern colorimetry works. Did he have knowledge of the tristumulus basis of colour vision?.
Well, Newton called them colors and he was there before you.
 
  • #27
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The first reply mentioned yellow, which is different; yellow is neither a pure nor composite signal from the three types of cones' responses...

The signal for yellow is synthetically comprised at a higher later stage in visual processing. Yellow is the only color that is done differently like this in the human visual system, and somehow (...perhaps because its generation is "closer" to the later stages of processing? ...or perhaps it incurs a slight additional processing delay with respect to the signals from the cones?) this makes yellow a subtly more "interesting" color subjectively - generally a top choice for danger and warning signs, barricade and police line tape, road lane lines, fast food restaurant and consumer food packaging, classic Post-it notes, etc.
 
  • #28
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Well, Newton called them colors and he was there before you.
How is that relevant? Newton got many things wrong - we later found. He can be excused for confusing colour with wavelength because he had only a limited experience to work on. He probably never considered applying his theory to the colour Magenta.
 
  • #29
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The first reply mentioned yellow, which is different; yellow is neither a pure nor composite signal from the three types of cones' responses...

The signal for yellow is synthetically comprised at a higher later stage in visual processing. Yellow is the only color that is done differently like this in the human visual system, and somehow (...perhaps because its generation is "closer" to the later stages of processing? ...or perhaps it incurs a slight additional processing delay with respect to the signals from the cones?) this makes yellow a subtly more "interesting" color subjectively - generally a top choice for danger and warning signs, barricade and police line tape, road lane lines, fast food restaurant and consumer food packaging, classic Post-it notes, etc.
That's an interesting response. Are you using the term "yellow" as a general description for what are often referred to as 'skin tones'? Humans are very much fine tuned to discriminating between different skin tones because they are good signals of emotion. Do you have an evolutionary 'reason' (or advantage) for this special treatment of Yellow? There almost certainly has to be one, which warning signs are deliberately exploiting - snakes and wasps, perhaps?
I really think this thread should be discussing non-spectral colours more. After all, we never (not too strong a word, I would say) actually see spectral colours - certainly not in everyday life. You have to try really hard to find examples of totally saturated Yellow or any other 'rainbow' colours.
 
  • #30
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Yellow might relate to current skin tone perception but I imagine lighter skin tones appeared long after the peculiar human processing for the color yellow.

Visual processing is one of those things that is just increasingly amazing the closer one looks; here are some other things to consider is this general discussion...

Some creatures have more than three types of cone response (fish, amphibians, reptiles, and birds have four) and some less (dogs only have two). The retinal density of some bird's cones is 25x that of the human eye, providing profound resolution.

In humans the red-green trichromatic channel is involved in the perception of both color and form. The blue-yellow channel is not involved in form, only color.

There are various types of "horizontal" layer processing in the retina (which has ten layers); one of these has two forms responding to color opponents red-green or blue-yellow, another codes only for brightness (no opponent color response between cones).

The processing streams of visual information split into four different functional / physical pathways in which form, color, "dynamic form" (location), and motion are each processed separately as attributes. There are five subsequent locations of processing in the streams for motion and location, six for color, and seven for form. That is to say, when you play with a tennis ball, the shape, color, location, and motion of the ball that you see as one thing are being individually processed in different regions of the brain, yet the appearance of these attributes is integrated so seamlessly well you can toss and catch the ball easily.

The "wiring" from the optic nerve to the rods and cones in the retina is not behind the retina but to the front of the retina, inside the eye, and these connections lie all over the retinal surface. The "blind spot" is where these connections pass through the retina to become the optic nerve behind the eye (in a sense, your retina is installed "backwards", but so is the image upside down and left right reversed... and the focal "plane" projected into the inside of a sphere...).

There is a complicated system that conspires to prevent eye movement (both using the muscles around the eyeball to rotate the eyes' orientation with respect to the head, and gross rotations of the head) from presenting a disorienting "whooshing by" of the visual field... this system integrates complexly with the suppression of motion detection processing to stabilize the apparent visual field.

The brain's surface (cortex) has ten physical layers of functional processing... as does the retina. Evolutionarily, it is as if the brain has managed to migrate a bit of its surface out to interface with the external world...!
 
  • #31
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Yellow might relate to current skin tone perception but I imagine lighter skin tones appeared long after the peculiar human processing for the color yellow.
Aamof, there is very little difference between the chromaticity values for skin, with or without the dark pigment (which, I suppose, must be or less neutral). It is easy to experiment with RGB values on an area of 'yellow' on a TV display and you will find that keeping the ratio of R/G/B much the same, you can produce yellow and brown areas. My point was that evolution does not produce a characteristic without it being some sort of advantage (or a spin-off from some other characteristic). There has to be a 'reason' for this special situation with yellowish colours.

You are absolutely correct to point out that perception and the spatial awareness of our world is incredibly involved. It is so easy to look at a scene and assume that's all you are doing. The internal model is so convincing (of course) that the temptation is just to take the whole process for granted. In fact it can be disturbing / creepy to dwell too much on what is actually going on in our heads that allows us to see that keyboard, screen, pot of flowers on the shelf as 'really there', even when we are watching a film or looking in a mirror.
It is amazing how trite the attitude of Science teaching is, when the 'inversion' of the image on the retina is stressed (the bloomin' obvious) as if it's the most relevant thing in our vision system. The brain can cope with so much more than that!
 
  • #32
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How is that relevant? Newton got many things wrong - we later found. He can be excused for confusing colour with wavelength because he had only a limited experience to work on. He probably never considered applying his theory to the colour Magenta.
You are really underestimating Newton. In one of Newton's famous experiments, he split white light into the full spectrum and then recombined them into white light. So I assume that splitting Magenta into two colors seemed trivial to him. From http://www.biotele.com/magenta.html: "Sir Isaac Newton noticed that magenta did not exist in the spectrum of colors from white light when he played with prisms. But when he superimposed the red end of the spectrum on to the blue end, he saw the color magenta."

The science and math of considering any periodic function as a Fourier series of pure frequencies has been well understood for 200 years. That is a basis for the science of any wave-like function, including light.
 
  • #33
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You are really underestimating Newton. In one of Newton's famous experiments, he split white light into the full spectrum and then recombined them into white light. So I assume that splitting Magenta into two colors seemed trivial to him. From http://www.biotele.com/magenta.html: "Sir Isaac Newton noticed that magenta did not exist in the spectrum of colors from white light when he played with prisms. But when he superimposed the red end of the spectrum on to the blue end, he saw the color magenta."

The science and math of considering any periodic function as a Fourier series of pure frequencies has been well understood for 200 years. That is a basis for the science of any wave-like function, including light.
Colourimetry has nothing directly to do with Fourier analysis. Our eyes are not directly sensitive to the time variations of light - they work on a photo chemical effect.
I'm afraid that link, which talks about "Colours of the Spectrum", is just 'too darn sloppy' for me. Interesting as it is, I don't see that it's very relevant to this argument. Colours are what we are subjectively aware of. Wavelengths are what a spectrum is composed of. She talks about Magenta not being a colour; by that argument, neither are all the other colours that occupy CIE colour space, except the ones on the peripheral curve. She is being too fanciful, imo, and reading what she has to say could easily confuse people.
I have no objection to an 'arty' and subjective discussion of colours but, Scientifically, it is a bit of a dead end (or it takes you onto a divergent path of phycho physics, which is not really the brief of PF General Science Forum) Additive and Subtractive mixing of colours to produce other colours, nowadays, is based on the CIE work and does not need to involve spectral primaries (it seldom does, actually). Subtractive mixing cannot, by definition, use 'spectral' filters or dyes because that would result in black every time.
I was wrong about Newton's work on Magenta (I sort of remember about it now - I think) but he did not get into the serious business of modern colorimetry because he lacked the technology - and he was not contemplating colour TV. Colour mixing in those days was subtractive and not easy to do quantitatively. Painters knew how to get the effects they wanted but the system was pretty ad hoc, albeit stunningly successful at times.
 
  • #34
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to put it simply the receptors are not limited to the specific colors they each perceive. since the brain processes all three primaries and the mixes which overlap between them.
if we were limited to the three primes, browns, black, white would be left out too. the brain has its own tricks to increase the accuracy of color detection because we don't only have one group of receptors at one location the disposition of the receptors increases the amount of available differentiation.
 
  • #35
Thanks for sharing that link. It does look very interesting and will take me some time to read and digest all the information carefully. I also want to set up my spectrometers again and do some careful experiments with it. So I won't comment more here until I have done more research. Thanks again for all the responses and links. Cheers

All three sensors have sensitivity that stretches over the visible spectrum, albeit with low sensitivity 'out of band'. The brain uses all three signals and compares the relative amplitudes (the ratios of the signals), which allows it to fine tune its response over the range of each sensor. Without the other two sensors, all you would know would be the value of the luminance arriving. It's the overlap that is the clever bit. The response curves in this link show what I mean and, in particular, what happens at the blue end gives worse discrimination than anywhere else in the visible spectrum. The two longer wavelength sensors have pretty well run out by 'the blues' so working out their contributions is harder for the brain - producing uncertainty and even conflicting colour sensations e.g. 'spectral' violet and 'looks violet'.
@my2cts: you put it briefly but I wouldn't disagree with that concise statement.
r l
 
  • #36
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Think about what you "see." The eye connects thousands of rods and cones to the brain and the brain translates those many thousands signals into what we "see." We do not directly "see" the objects we look at, we "see" the image that our brain creates. When the brain receives signals from the red, blue, and green sensors in a particular area of the eye, it creates an image that has the characteristics we call white. Differing amounts of signals translate to different hues and intensities.
Our brains are some rather fantastic computing machines. All this is done not only unconsciously, but we have no way of "looking" or "sensing" inside our brains to detect what is going on. Absolutely incredible.
 
  • #37
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I really think this thread should be discussing non-spectral colours more. After all, we never (not too strong a word, I would say) actually see spectral colours - certainly not in everyday life.

I don't think it's making too fine of a point to say, if you hold true your definition of colour to be a subjective perception, then this is a redundant statement. What is spectral or non-spectral colour if it is not tied to a spectrum of wavelengths/frequencies? If colour is connected to a spectrum, then how?
 

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