additive colour mixing

There are – broadly speaking – two types of colour mixing: additive colour mixing and subtractive colour mixing. Subtractive colour mixing relates to how inks, paints, dyes etc add together to form different colours; additive colour mixing refers to how light-emissive colour devices create colours. So we’re talking about how computer monitors work or how phone displays work.

The essential principle behind additive colour mixing is that we can mix together three colours – called colour primaries – and create a surprising range of colours. See my earlier post – http://colourware.wordpress.com/2009/07/08/what-is-a-colour-primary/ – for further details about colour primaries. The additive primaries are red, green and blue. Is there anything special about these three colours that justifies their use as the primaries? No, apart from the fact that if you use red, green and blue as the additive primaries you get a large gamut (range of colours that can be produced).  There is no reason why you couldn’t use orange, purple and turqiose as the additive primaries – it’s just the range of colours that could be created would be unsatisfactorily small. And nobody would like that!

So, we have red, green and blue as the additive primaries. The figure below illustrates how additive colour mixing works. Imagine that we have three projection lamps at the back of a hall – one has a red filter and so produces a beam of red light, and the other two use filters to produce green and blue beams. We project these onto a white screen and get three circles of light (one, red, one green and one blue). We then move the angles of the projectors so that the circles of light overlap. We get something that looks rather like this:

additivemixing_b

Where the red and green light overlap we get yellow. We get magenta and cyan for the other two binary mixtures. So,

red + green = yellow

red + blue = magenta

green + blue = cyan

And if we mix all three primaries we can achieve white (or other neutral colours). The primaries could be single wavelengths of light – so we could use a primary at, say, 700 nm (for the red) and one at 450 nm (blue) and one at 530 nm (green). In practice, most devices (CRTs, LCDs etc) don’t use single-wavelength primaries since it would be hard to create bright screens (gamuts are 3-D not just 2-D) but in principle could do so. It’s also important to note that different devices and different manufacturers use slightly different primaries.

But let’s imagine for a second that the three primaries used in the pictuire above are at 450 nm, 530 nm and 700 nm. Green light (530 nm) and red light (700 nm) additively mix together and generate yellow. When this happens what is being mixed and where does this mixing take place? Take a few moments to consider this before reading on.

Notice I said that they additively mix to generate yellow – I specifically avoided saying that they mix to generate yellow light. If we look at the part of the screen that is yellow we would see that we have some light at 700 nm and some at 530 nm. The wavelengths are not mixed; they don’t mix together to generate some third wavelength of light such as 575 nm (I choose this wavelength since monochromatic yellow light is about 575 nm). So no physical mixing takes place other than – I suppose one could argue – that the red and green lights are mixed in the sense that they are spatially coincident. But that’s not really mixing, for me, and certainly doesn’t even begin to explain why we have the sensation of yellow when we look at these wavelengths together.

So when we say that the red and green lights are mixed together to create yellow we should be aware that no phsyical mixing takes place. Indeed, one could argue that mixing is really the wrong word to use. Though as I write this I am struggling to think of a better one – suggestions on a postcard please.

When we look at the mixture of red and green light we see yellow – but the eye is still receiving the indivual wavelengths of red and green light. However, the visual response to this is that yellow is perceived. Indeed, a carefully composed mixture of red and green light could produce a yellow that is visually indistinguishable from yellow monochromatic light; but physically the mixture would still consist of light at 530 nm and light at 700 nm. If mixing occurs at all in any real sense it is in the perceptual mechanisms of the visual system. Indeed, at the heart of this matter is the way in which our visual pigments respond to light …. more about that another time.

11 thoughts on “additive colour mixing

    1. Hi Sunil,

      I assume you are talking about your display. It could be that you have quite an old system and it is broken. Or it could be that the settings are unusual – you can set a gamma for each channel, for example, and it could be that the blue gamma setting is odd. The red coming out orange seems strange though. It’s hard to be more specific without seeing it.

  1. “a carefully composed mixture of red and green light could produce a yellow that is visually indistinguishable from yellow monochromatic light; but physically the mixture would still consist of light at 530 nm and light at 700 nm.”

    Such a misture will give you something very close, but not the exact same colour as the yellow “monochromatic” light.

    For the word suggestion, I suggest optical or mental “average”: the resulting colour is the average of the two primaries (one being a monochromatic 530nm and the other 700nm)

    A

  2. I do believe all the ideas you have offered to your post.
    They are very convincing and will certainly work.

    Nonetheless, the posts are too quick for newbies.

    May you please prolong them a bit from subsequent time?
    Thank you for the post.

  3. How can we know if the color is from monochromatic light or a mix?
    How can we know that the colors other than RGB that we see in a rainbow are monochromatic colors and not ‘mixes’ giving a perception of that color in our brain? Or are they actually mixes?

    1. The only way to know if a light is monochromatic or contains a mixture of wavelengths is to measure it. We would do this with a spectroradiometer, for example. This would easily distinguish monochromatic yellow light from a mixture of green and red wavelengths that looks yellow. The spectroradiometer would record the amount of energy present at each wavelength interval.

      How could we know the difference between a monochromatic yellow and a mixture of red and green wavelengths just by looking: The answer is, we just cannot. That is the point. Because we sense colour via three types of cones in the eye that have broadband and somewhat overlapping spectral sensitivity the world presents itself to us as metamers – that is physically different stimuli that are perceptually indistinguishable.

      However, it is not all bad news. this property of colour vision allows colour imaging to work. To give an example, we can have a computer or TV screen that consists of additive mixtures of three lights (red, green and blue) and yet we have the perception of a full range of colours including yellow, orange, white and brown. If we have eye that could detect light wavelength by wavelength (like a spectroradiometer) then life and colour technology would be very different.

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