I have worked in colour pretty much all my life. In 1980 I started learning at the University of Leeds where I was enrolled on BSc Colour Chemistry. That was 44 years ago!
Since 1980 I have been learning, working, researching and teaching colour for almost all of those years. I have learned a lot and I am sure I still have a lot to learn. For example, only a few weeks ago learned that the colour name magenta is named after an actual town in North Italy. I discovered this when doing some research about colour names for the following video.
And I recently I have been learning so many new things from a book called The History of Colour by Neil Parkinson. I can’t tell you how much I am enjoying reading it. But more about that later.
I often say to people that the most important thing about colour that I ever learned is called the principle of univariance. I read about it in Brian Wandell’s book Foundations of Vision in the 1990s. It was discovered by someone called William Rushton in the 1960s. It is about how the human cones operate and it is so fundamental to explaining how colour vision works. It explains how we can discriminate between different wavelengths of light despite only have three types of light-sensitive cells that each have broad-band spectral sensitivity.
It explains why we have metamerism – which is where, for example, two spectrally dissimilar objects can look the same colour when viewed under one light source but then be a mismatch when viewed under a different light source.
It explains why additive mixing occurs. Why we can additively mix red and green light to get yellow. And it even explains subtractive colour mixing if you think deeply about it.
So the video How does colour vision work?, is really about how cones work and the principles of univariance.
I really like this nice awareness video about colour blindness in sport – in football (or soccer, as you may know it) in particular. It is very well done and makes the point that it is not just about the spectators but about the players themselves – and even the referees!
I posted a few other comments recently about some recent activities in this area which you can see here and here.
However, despite the brilliance of the video about inclusivity in sport I want to make one point about the cone responses. The video states that the cones in the visual system are sensitive to red, green and blue light. There is a level of approximation where it is not unreasonable to say this – and to be honest, for this video it is probably fine. However, when we simplify things it can lead to misunderstandings which are then repeated when they really shouldn’t be.
The graph above shows the spectral sensitivity of the long-, medium- and short-wavelength sensitive cones. Note that neither is sensitive to just a few wavelengths; each has quite broad wavelength sensitivity. However, the L cone, in particular, is not even maximally sensitive to red. I took the table (below) from wikipedia but I think it is a reasonably summary of the colours that we normally associate with various wavelengths in the spectrum.
From this you can see that the L cone (the one that is sometimes referred to as the red cone) is maximally sensitive to light that we typically see as being yellow or even yellow-green.
One common assumption is that the cones are only sensitive to light that we see as being red, green and blue and it leads to people saying things such as – the visual system only sees in red, green and blue and then generates all the other colours form these. And this is a gross misrepresentation of course.
I gave a talk at the Leeds Philosophical and Literacy Society a week or so ago and it was recorded. In this talk I referred to this and related issues. If you would like to see it then you can do so here.
Earlier today I posted something on quora about who many colours there are. It’s quite a long post. You can read it here. However, if you want the short cut the answer is 3-5 million. 🙂
However, I also linked to the post on LinkedIn and someone asked me a question about the relationship between colour-matching functions and cone sensitivities so I thought I would make a new post today about that topic. I have used my message on LinkedIn as the basis for this post but modified it a little to make it suitable for general consumption.
Here are two diagrams:
This shows the actual spectral sensitivities of the MLS cones in the human eyeThe 1931 CIE XYZ colour-matching functions
It’s another common misconception that people get these two diagrams confused. The cone spectral sensitivities are the actual sensitivities of the cones in the eye. Although people often say that the eye responds mainly to red, green and blue light, it’s not so simple. In 1931 the CIE measured the colour matching functions. One of the reasons that they did this was that in 1931 we didn’t actually know what the cone spectral sensitivities were; these were only known for sure in 1964. So in 1931 the CIE measured the amounts of three primary lights that an observer would mix together (additively) in order to match a single wavelength of light. And they did this for each wavelength. The second of the diagrams above shows the amounts of each of the primaries needed to match each wavelength on the spectrum. Originally, the CIE used three lights (these were RGB) or primaries. However, they mathematically transformed their RGB colour matching functions to create the XYZ colour matching functions. These are sometimes also known as the CIE colour matching functions or the CIE standard observer.
These are the original CIE RGB colour-matching functions
The point of these (XYZ) colour matching functions are that they allow us to calculate the CIE tristimulus values XYZ of an object if we know the spectral reflectance of the object and the light it is viewed in. The XYZ values are the amounts of the three XYZ primaries that an observer would, on average, use to match that object viewed in that light source. If two samples have the same XYZ values then they are a visual match; because an observer would, on average, use the same amounts of the XYZ primaries to match each. And this was the whole point of the CIE system; to determine when two colour stimuli are a visual match. Had we known the cone spectral sensitivities in 1931 it’s possible that history would have taken a different course and that instead of having CIE XYZ we would simply calculate the cone responses LMS. And we could say that if two samples have the same cone responses they are a visual match. But I guess we’ll never know.
Now, if two samples have the same XYZ values then they will have the same cone responses. This is a bit technical but this is true because the cone spectral sensitivities are a linear transform of the CIE XYZ colour matching functions. They are also a linear transform of the CIE RGB colour-matching functions.
The colour-matching functions depend upon which primaries are used whereas the cone spectral sensitivities are more fundamental. Doesn’t this make the colour-matching functions arbitrary? Not really. Although the actual shapes of the colour-matching functions depend upon the actual primaries used, the matching condition does not. If two samples generate the same cone responses then the observer would match them with the same amounts of the XYZ primaries and the same amounts of the RGB primaries.
Human colour vision under normal lighting levels is mediated by three cones (light-sensitive cells) in the retina. Each class of cone has peak sensitivity at a different wavelength and thus the cones are known as L (long-wavelength sensitive), M (medium-wavelength sensitive) and S (short-wavelength sensitive) cones or (sometimes) as red, green and blue cones. Both colour and luminance are captured by the same cone mechanism. The L and M cone responses are combined to give luminance and various cone responses are compared to give rise to hue and chroma. Interestingly, the distribution of L, M and S cones in the retina is not uniform but is random.
A recent paper – http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0008992 – by scientists at Washington University School of Medicine (St Louis, USA) reveals that chickens have five types of cone. Interestingly, one of these types of cones (so-called double cones) seems to encode luminance, whereas the other four cones (red, green, blue and violet) give rise to tetrachromatic vision. The cones are very regulary spaced in the retina.
The spacing of cones in the human retina may result from a compromise – the same cones need to encode colour and luminance. The avian colour vision system seems to be more sophisticated. One can only wonder at what benefit was bestowed in avians by separating the processing of colour and luminance information.
