What is really green ?

  1. bobie

    bobie 682
    Gold Member

    What is really "green"?

    I read many articles but still I do not know what are colours like green:
    Is it
    -a subjective quality that is made up in human brain, or
    -the result of sum of yellow and blue, or
    -the presence of radiations of two frequencies in the same source, or just
    -the quality of a radiation of a single frequency, as wiki says, in the range between 570 and 590 nm?

    Thanks for your help
     
  2. jcsd
  3. sophiecentaur

    sophiecentaur 13,785
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    This is a frequent question. Your list of ideas all have some relevance to Colour.
    The concept of Colour is entirely subjective and the sensation of colour involves the outputs from the three sets of receptors in your eye and what your brain makes of them. (So your first statement is correct). The receptors are an extremely crude form of spectrometer; the three types have a very broad response to the longer wavelengths (encompassing all the red/orange/ yellow spectral colours), the middle wavelengths ( yellow / green etc. spectral colours) and short wavelengths (greens and blues). There is a massive overlap in the responses so you can see all wavelengths in the visible range with just three receptors. If you could just 'see' narrow bands of wavelengths (pure spectral red, pure spectral green, pure spectral blue) there would be many wavelengths that you couldn't see - pretty useless! Your brain looks at the three signals it gets from the receptors and remembers the relative and absolute levels for any particular combination. It can then recognise when it sees a similar combination and call that (privately) the same 'colour'. Of course, two different people will usually agree (almost) on when two different objects 'look the same' and they use a word to describe this - these are the names of the colours we use. There can be a big spread in people's memory and assessment of colours. 'Matches' between colours of fabrics can be the source of many arguments in the clothes shop and the art of fabric dying is big business. I would say that colour is an essentially private sensation. Many 'colour blind' people get along fine and their disability hardly shows until they take a colour vision test.
    It is well known that you can produce a given 'colour' with many different combinations of wavelengths. When you add the right amounts of spectral red light and spectral green light, you can get the same colour sensation as with spectral yellow. Note the difference between that and your statement about green - but what you say is not totally incorrect; it's just that most colour mixing in 'displays' uses additive primaries of R G and B. You are referring to subtractive mixing, using pigments. (Google additive and subtractive colour mixing)

    The CIE colour chart in this link shows how all the colours we see can be represented as co ordinates on a plane diagram. This link shows how three primaries can be added in weighted amounts to produce a 'metemeric' match with any colour within the triangle of the three primaries. (Note the huge number of colours that a typical display cannot produce!)

    I would say it is the 'sensation' rather than the quality. It would be wrong to equate wavelength exclusively with the sensation of colour. Colour involves three variables where wavelength involves just one.
     
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  4. bobie

    bobie 682
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    Thanks for your deep analysis and precious links, but ,please tell me: if we have a beam of photons all of 580 nm wavelength, what colour do we see?
     
  5. Nugatory

    Staff: Mentor

    A friend of mine sees the same color as a carrot (yes, really - I'm not making that up!) while I see the same color as the leaves of a healthy plant, as will most of the people reading this thread.

    Because only one in a few million people have his genetic mutation, we say that he has rare form of color-blindness and the light is "really" green... But we're being sloppy, because the light is really 580 nm and "green" is a label that we've attached to that wavelength.
     
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  6. sophiecentaur

    sophiecentaur 13,785
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    You will call the colour you see 'green'. It will be similar to (and nearly match with) a lot of other 'greens', like grass, the 'green' part of the rainbow (which is very desaturated by the addition of the blue{isn} sky) and the 'green that you can mix with blue and yellow paints.
    The word "really" doesn't come into it - as with most of Science, aamof. Going down that route is more philosophical than scientific.
     
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  7. sophiecentaur

    sophiecentaur 13,785
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    . . . . . and a lot of other green things.
     
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  8. Drakkith

    Staff: Mentor

    My dad is red-green colorblind, meaning that he cannot separate wavelengths in that area of the spectrum into colors as well as the average person. Its so bad that he cannot tell the difference between green and red stoplights. They are both the same color to him. This is why color is termed "subjective" and why we use wavelength and frequency in science since they are direct measurements and correspond to what light waves actually do.
     
  9. AlephZero

    AlephZero 7,298
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    Homework Helper

    What is "really" green depends on language (or culture) as well as biology. If you spoke a language that only had color words for white, red, and black, you might not even have the concept of "green". Japanese and Mandarin use "blue" and "green" differently from English. For example traffic lights are the objectively the same colors, but are described as blue, not green. And one language describes green as "grass yellow."
    http://en.wikipedia.org/wiki/Color_term
     
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  10. D H

    Staff: Mentor

    I'm surprised no one has caught this yet. You would see yellow, not green. 580 nm is smack dab in the middle of the spectral yellows (570 nm to 590 nm). Green (spectral green) is light between 520 and 570 nm.

    @bobie, notice that I said spectral yellow and spectral green. Imagine a screen that is white except for a colored circle in the middle. Imagine that that colored circle emits light of one frequency only, and that that single frequency can be made to slowly vary from the deepest red to the deepest violet. You will see the colors of the rainbow as the frequency gradually changes from red to orange to yellow to green to blue and finally to violet. Those are the spectral colors.

    Most people have three kinds of cones in their eyes. The behavior of those cones is dictated by chemistry and physics. Some cones respond strongest to reddish light, others to greenish light, and yet others to blue/violet. The response curves of these three kinds of cones overlap. Both the red and green cones will respond fairly strongly to that 580 nm laser light; the blue cones will not respond much at all. We see the different colors of the rainbow because different frequencies elicit different combinations of responses from those three kinds of cones.

    Is that all there is to color? Absolutely not. There are very, very few pure light sources in nature. Almost all of the colored objects we see have a broad spectrum of light coming from them into our eyes. Going back to that colored circle on white background, let's change that central circle so that emits multiple frequencies of light at once. Two key results show up.

    1. Metamers.
    Tune the frequencies produced just right and you'll get a response from the cones that is indistinguishable from a pure spectral color. The manufacturers of computer screens take full advantage of this effect. Your computer screen does not produce yellow light. So how does it produce this?
    [​IMG]
    If you aren't colorblind you should see that central block as yellow, even though there is no yellow light coming from your screen. The light coming into your eyes is a combination of red and green light that elicits the same response as would spectral yellow. So you see yellow.

    2. Non-spectral colors.
    We see more colors than just the colors of the rainbow. That white background, for example. There is no such thing as spectral white. White is a non-spectral color. Another even more interesting one is purple. Our eyes do something rather funky with the spectrum. The full spectrum of electromagnetic radiation goes from radio to gamma. Visible light is but a tiny, tiny part of that full spectrum. In terms of the spectrum, there is no such thing as the color wheel. That violet turns into purple, then magenta, and then red: That's something our vision system does that is distinct from frequency.


    That's just biophysics. There's a lot more to color perception than this low level, biophysical description of how you see color.
     
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  11. sophiecentaur

    sophiecentaur 13,785
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    It's important to note that the colours of the rainbow are not 'spectral' colours. When you look at a rainbow you see spectral colours PLUS all the pale blue of the sky / white of the clouds, behind. The rainbow is not pure. (If we want to be accurate in our discussion of such things).
    The yellow you can see on a monitor, in the centre of the square will not be a match for spectral yellow because spectral yellow does not lie on a line between the display primaries. They do a fair job but it's not exact - spectral yellow is an 'illegal colour' and outside the gamut of the primaries.

    I would say that there are, in fact, 'no' pure light sources in nature, unless you include some of the gas discharges that you can see in the Aurora. It's only since Newton that anyone has actually seen pure spectral colours and we certainly didn't evolve to take them into account in our assessment of colours.

    I would be happy if people would just stop equating colours to wavelength - that would be a good start.
     
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  12. sophiecentaur

    sophiecentaur 13,785
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    OH yes - and well spotted about the wavelength / colour thing at the beginning. I didn't bother to check!
     
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  13. D H

    Staff: Mentor

    You took what I wrote far too literally. The best way to see the "colors of the rainbow", the spectral colors, is to do what Newton did: Sit in a dark room, let a small beam of light in to hit a prism, and see the results on a sheet of white paper.
     
  14. Nugatory

    Staff: Mentor

    Nor me :redface:
     
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  15. Yellow is a little different from the other colors; the retina has ten neural functional layers of feature detection processing (there are ten layers of the cortex, too - almost as if the surface of the brain made an extension through the optic nerve to the back of the eye), and the processing layer in the retina that extracts yellow is further in the processing sequence than that used for sourcing the other colors.

    The extraction of yellow is a little "more synthetic" than the other colors; may be why it is used so much for food advertising, warning and danger signs, other things meant to "catch our eye".
     
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  16. Some background reading in order for people to tease apart the physical/physiological/mental aspects of color: Stanford Encyclopedia of Philosophy: Color

    bobie: whether or not one believe colors exist outside of perception changes how one would answer your question. I like Descartes formulation, "It is clear then that when we say we perceive colors in objects, it is really just the same as saying that we perceived in objects something as to whose nature we are ignorant but which produces in us a very clear and vivid sensation, what we call the sensation of color." I think that any statements about color in the context of physical sciences are really statements about photons, and statements about color in the context of perception are actually statements about the contents of particular mental states.
     
    Last edited: Feb 25, 2014
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  17. sophiecentaur

    sophiecentaur 13,785
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    I was just reading to back to you as it would be read by the majority of people, who believe that the rainbow consists of pure colours. It is no surprise that an ancient expression like "colours of the rainbow" is mis-applied so often. Perhaps 'Hues of the Rainbow' would be an alternative. :wink:
    Doing what Newton did, initially, will blow the socks off anyone who is not aware of real spectral colours.
     
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  18. bobie

    bobie 682
    Gold Member

    Thanks for your wonderful response. I made a mistake in the OP, I meant sort of 540 nm.
    My question was not philosophical and did not refer to sensations, btw, I thought that the debate between Newton and Leibniz had been settled
    I always read that green is not a basic colour, but, from what you say that is wrong: if a beam of photons of 500 or 550 nm reaches a human eye most people will recognize the frequency or "colour" emitted by any leaf.
    If that is right, I do not understand the need for further speculations.

    Do you know of any site where I can digit a frequency and get a "colour"?
    Thanks for your invaluable help!
     
    Last edited: Feb 25, 2014
  19. sophiecentaur

    sophiecentaur 13,785
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    Firstly, you should explain what you mean by a "basic colour". No colour is any more basic than any other - your eye responds in the same way to all colours, and it's not a measuring instrument. There is no colour that will elicit a single non zero value from just one of your receptors.

    There are Primaries, which are colours that are chosen so that you can get a good range of colours by mixing them. Paints and pigments use subtractive mixing and the best primaries are Yellow, Cyan and Magenta (look at the inks in your printer - and look up Subtractive Mixing of Colours). For adding light sources (as with TV displays), the best primaries are basically Red Green and Blue - but none of the primaries used in displays are monochromatic (spectral) because you just can't get enough light out of such sources.

    A leaf reflects light of many wavelengths. There is no single wavelength that can represent the perceived colour of a leaf (or any normal non-luminous object for that matter).

    You should try to read this Hyperphysics link before you pursue this any further.

    I have not managed to find a reference to any calculator that will give CIE co ordinates of the spectra colours but it would not take you long to assemble a look-up table from the figure on the Hyperphysics link. Anything you get would be quite accurate enough for any application you may have. How would you want to use the numbers you obtain?
     
  20. bobie

    bobie 682
    Gold Member

    Thanks, sophiecentaur, that link answered all my questions, cleared all doubts, that is all I was looking for: "green" is as basic as any other colour.
    But I think I understand, also, that human mind makes a sort of average between frequencies so that a frequency of yellow and one of blue are perceived as intermediate " green"
    Is that correct?: whilst sound frequencies merge into a sum and reache the ear as a single curve, two light frequencies travel side by side and are blended in the mind.
     
  21. sophiecentaur

    sophiecentaur 13,785
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    As written earlier, the "human mind" has only three signals to work on - the outputs from the three sensors. You really must look at more web pages about tristimulus colour vision and they you will get a better grasp of the 'mechanics' of it.

    But, when you get down to it, our appreciation of the light that enters our eyes and the way we 'categorise' the sensation is much the same as we categorise tastes, music, the feel of things etc. We have a very 'internal' memory / model of these things and we communicate a very restricted version to other people.
    It always amazes me how people treat 'Colour' as a special part of experience. Would we ever ask the question about how "really soft" is a cat's fur? Humans are strange, complex things.
     
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