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Author Topic: Are primary colours a matter of opinion?  (Read 2784 times)

Offline bizerl

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Are primary colours a matter of opinion?
« on: 23/07/2012 05:26:56 »
I'm not sure where to post so general science seemed safest.

Is there anything fundamental about primary colours and their properties in combining to make up the entire spectrum of colours we see? Or is it an artefact of the types of receptors (cones or rods? I can never remember) we have in our retinas?

Or, in other terms, are primary colours purely an arbitrary reference based on our ocular physiology, or would their qualities exist without our retinas?

I'm reminded that the metric system makes a lot of sense and seems intuitively natural, but holds no connection to mathematic fundamentals. It only appears natural to us because we decided that a base 10 counting system was right. The metric system would be ridiculous if we had a base 7 counting system!

Regards,
Luke


 

Offline David Cooper

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Re: Are primary colours a matter of opinion?
« Reply #1 on: 23/07/2012 21:15:42 »
The colours of the rainbow are all genuine colours in their own right, with an infinite range. What our eyes do is use three different types of colour receptors to cover that whole range of colours, but other species do things differently, some having eight different kinds of colour receptors, so some of them would not think our televisions are at all good at representing the way the world looks - the technology we use to record and display images is probably only suited to human eyes.

Because we don't have specific yellow receptors, we rely instead on seeing pure yellow light by having red and green receptors which are also triggered by yellow light, so that allow us both to see yellow light and to distinguish it from red or green light (each of which would only trigger one type of receptor. A television creates the illusion of yellow light by emitting both red and green light, so that can trick us into seeing yellow light even though there isn't any yellow light there at all. A species with yellow receptors in its eyes would not be fooled by this trick as it would see pure yellow as a different colour from a mixture of red and green. How it would actually see a combination of green and red is a question that we can't answer, but it would certainly not want to see is as yellow as that would negate the entire advantage of having yellow receptors.

By the way, a combination of red and green light which looks yellow to us, a pure yellow with no red and green in it, or a combination of all three are called metamers of yellow - they all look exactly the same to us, but are in reality different.

Note that the colours of a rainbow continue on from either end with colours which are invisible to our eyes - infrared being beyond the red end and ultraviolet (which is filtered out by the lens - some people with faulty contact lenses can see ultraviolet light because the filter layer is missing) beyond the violet, so they do not wrap around. However, our red and blue receptors are both sensitive to violet, so that allows us to see violet without needing violet receptors.

When a pixel on a computer screen has the blue and red pixels set to maximum brightness while the green one is completely dark, we see magenta, but I'm not sure this is a real colour that can be produced as a single frequency of light - it may only be possible as a mixture of red and blue.

I've tried to find metamers of yellow in the real world, looking for objects which reflect yellow light and others which don't but which do reflect both red and green. I have not managed to distinguish between any, and the reason for that is partly because I only have access to receptors and sensors which behave the same way as our eyes - all our cameras are unable to distinguish between yellow and red-green. However, it occurs to me that an object which only reflected pure yellow would be a lot darker than a red-green object as the latter would be reflecting a lot more light, so it's only really with emitted light that pure yellow is going to look the same as red-green. Anyway, it's difficult to examine things properly without access to the right tools. If you have a range of lights which emit pure frequencies, such a lasers (though they're probably too bright to work with) or LEDs (which I think emit pure colours), you might  be able to do the experiments that I currently can't. I've only got red LEDs and white ones which must be mixing red, green and blue (I'm guessing), so that makes it difficult to explore the actual colours of anything.
 

Offline evan_au

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Re: Are primary colours a matter of opinion?
« Reply #2 on: 25/07/2012 10:48:52 »
Humans have cones that are sensitive to 3 bands of colour, and these are addressed by the three primary colours.
http://en.wikipedia.org/wiki/Color_vision

Note that there are two sets of Primary Colours which are both tuned to the human visual system:
  • Red, Green and Blue, which are called "Additive" (used on a TV or movie screen)
  • Cyan, Magenta and Yellow, which are called "Subtractive" (used in paint and printed books)

Some people considered to have "normal" colour vision have a slightly different genetic variant of their colour receptors, so they would perceive the primary colours slightly differently to other people.
 

Offline cheryl j

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Re: Are primary colours a matter of opinion?
« Reply #3 on: 28/07/2012 02:07:25 »
wow, David, that's really interesting. Thanks.
 

Offline David Cooper

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Re: Are primary colours a matter of opinion?
« Reply #4 on: 28/07/2012 21:45:58 »
I've just remembered a forgotten detail - if you use a red LED bicycle light (and if I'm right in assuming that they put out a single, pure frequency of light) then testing yellow objects with that will tell you quite a lot about what kind of light they reflect - if a yellow object looks black under this red light, it would be reflecting pure yellow rather than red-green, whereas if it looks red, it's clearly reflecting red light (and green too to balance it), and when I was doing experiments of this kind a few years ago I couldn't find any yellow objects that didn't reflect red light (though I obviously didn't have the means to determine how much of a wide varieties of frequencies from green to red was being reflected). That is why I suspect that you will only find pure yellow light when it's emitted by something like a yellow LED or laser rather than reflected off a yellow object under a white light source.
 

Offline Atomic-S

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Re: Are primary colours a matter of opinion?
« Reply #5 on: 29/07/2012 07:21:37 »
Compare the light given off by a yellow incandescent "Bug Lite" and the light given off by a low-pressure sodium lamp. The two lights look quite similar to the eye, but if you place objects under one or the other, they may look quite different. That is because the actual spectra differ markedly.
 

Offline Atomic-S

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Re: Are primary colours a matter of opinion?
« Reply #6 on: 29/07/2012 07:25:37 »
Finding an ordinary object that reflects a narrow yellow spectrum (or any other narrow spectrum) may be difficult. That type of thing probably would have to come from a scientific supply house.
 

Offline Atomic-S

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Re: Are primary colours a matter of opinion?
« Reply #7 on: 29/07/2012 07:27:00 »
Which is not to say that such objects are impossible. Narrow band filters are known, but most that I am  aware of are designed to transmit, not reflect, the desired spectrum. However, making a reflective version is well within the reach of known technology.
 

Offline David Cooper

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Re: Are primary colours a matter of opinion?
« Reply #8 on: 29/07/2012 19:50:52 »
Maybe butterflies would be worth a closer look, and fluorescent pens.
 

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Re: Are primary colours a matter of opinion?
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