Tag Archives: wavelength

inclusivity in sport

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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.

non-visual effects of light

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Most people know that the ear system has two functions: hearing and balance. It is less well known that the visual system also has two functions. The first is seeing. The second is a set of non-visual functions including circadian rhythm. Mechanisms are being discovered that are particularly sensitive to blue light. So short-wavelength, or blue, light inhibits melatonin which is a chemical that makes you drowsy. So looking at bright lights late at night, especially blue ones, can contribute to a poor night’s sleep. So put your smart tablet away now and go to sleep!

In all seriousness though, I knew there was a reason why I do not like watching Chelsea on Match of the Day.

colour

Studying these functional effects of colour and how they can be used in design is a major theme of the research I lead at the University of Leeds in the School of Design. If you have interest in these areas please contact me.

how colour vision works

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yellow

Really super article by Ana Swanson in the Washington Post about colour vision and how it works. As she explains, it is not really correct to think of the long wavelength visible light as being red. It is better, as Newton knew of course, to say that the long-wavelength light has the ability to cause the sensation of redness in us. She gives a nice visual example of how the spectrum looks to a dog, something (by coincidence) that I was only talking about in a lecture last week. As she says:

Is what I see as “blue” really the same thing as what you see as “blue”? Or have we both learned the same name for something that looks different to each of us?

Her article is really worth reading.

There is just one thing I take issue with. It may be ‘nit picking’. But she says “A green leaf, for example, reflects green wavelengths of light and absorbs everything else.”

My image, at the top of this post, shows the reflectance of a typical yellow object. At each wavelength the reflectance is between 0 and 100 per cent. But notice that it is not zero at any wavelength in the range shown (400-700nm). That means that the object reflects light at every wavelength. And it is not 100 at any wavelength meaning that it also absorbs to some extent at every wavelength. It’s just it absorbs more at the shorter wavelengths than at the longer wavelengths and it reflects more at the longer wavelengths than at the shorter ones. But notice one other remarkable thing – the yellow object reflects more light at 700nm (a wavelength we would normally associate with red) than it does at 580nm (a wavelength we might normally associate with yellow).

Yes, the reflected light does look yellow. But, the notion that a “A yellows object reflects yellow wavelengths of light” is misleading. It suggests that the yellow object only reflects, for example, the wavelengths in the spectrum we would normally think of as yellow (around 580nm) and absorbs the rest. This is just not how things are.

On CIE colour-matching functions

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In 1931 the CIE used colour-matching experiments by Wright and Guild to recommend the CIE Standard Observer which is a set of colour-matching functions. These are shown below for standard red, green and blue primaries. These show the amounts – known as tristimulus values – of the three primaries (RGB) that on average an observer would use to match one unit of light at each wavelength in the spectrum. Why are these so important? Because they allow the calculation of tristimulus values for any stimulus (that is, any object viewed under any light as long as we know the spectral reflectance factors of the surface and the spectral power of the light).

650px-CIE1931_RGBCMF.svg

I gave a lecture this week about these and so they are fresh on my mind. I wanted to use this blog post to explain two things about the colour-matching functions that may be puzzling you. The first was stimulated after the lecture when one of the students came up to me with a question. You will note that for some of the shorter wavelengths the red tristimulus value is negative. Hopefully you are aware that no matter how carefully we choose the three primaries we cannot match all colours using mixtures of those three in the normal sense. What we have to do is to add one of the primaries to the thing we are trying to match and then match that with an additive mixture of the other two primaries. The question from the student was, wouldn’t that change the colour of the thing that is being matched? The answer is that it would of course. But it’s ok.

We normally represent this matching with an equation:

S ≡ R[R] + G[G] + B[B]

which simply means that the stimulus S is matched by (that is the symbol ≡) R amounts of the R primary, G amounts of the G primary, and B amounts of the B primary. The values R, G and B are the tristimulus values. I put square brackets around the primaries themselves to distinguish them from the amounts or tristimulus values of the primaries being used in the match.

Now when we add one of the primaries to the stimulus (the thing we are matching) itself, we can write this equation:

S + R[R] ≡ G[G] + B[B]

The new colour, S + R[R], can now be matched by an additive mixture of the other two. Hmmmmmm? You may ask. How does that work? Well, we can rearrange this equation to make:

S ≡ -R[R] + G[G] + B[B]

In other words, matching the additive mixture of the original stimulus S and some red with some green and blue, means that – if it were possible – we could match the original stimulus S with the same amount of green and blue and a negative amount of the red. I appreciate that this is mathematical but I hope that it is maths that anyone could understand. It’s not rocket science. Just simple adding and subtracting. This is how we arrive at the colour-matching functions above. No matter what RGB primaries we use one of them will have to be used in negative amounts to match some of the wavelengths. In practice, this is done by adding it to the stimulus as described above. Of course, you may also know that the RGB colour-matching functions were transformed to XYZ colour-matching functions. These are the XYZ values everyone is familiar with. But that is another story I will devote another post to one day.

The second question though, is isn’t this just arbitrary? If we used a different set of RGB primaries wouldn’t we get a different set of colour-matching functions? Again, the answer is yes, but again it doesn’t matter. The whole point about the CIE system was to work out when two different stimuli would match. If two stimuli are matched by using the same amounts of RGB then by definition those two stimuli must themselves match. If we used different RGB primaries the amounts of those tristimulus values would change, of course, but the matching condition would not. Two stimuli that match would also require the same RGB values as each other to match them, not matter what the primaries were (as long as they were fixed of course). So the key achievement of the CIE system was to define when two stimuli would match. However, it was also useful for colour specification or communication but that does indeed depend upon the choice of primaries and requries standardisation.

I hope people find this post useful. Post any questions or comments below.

dog vision

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I just read an article in The Daily Mail that says that most people think dogs do not have colour vision. The article then goes on to say that Russian scientists have proved that dogs do have colour vision. It seems to me quite accepted that dogs are dichromats – that is they have two types of light-sensitive cells that contribute to colour vision in their eyes. We – humans – are trichromats because we have three such cells. It turns out that the one that is missing – in dogs – is such that dogs’ colour vision is rather like that of a human who has red-green colour blindness. The image below shows how the spectrum looks to a trichromatic human and a dichromatic dog.

dog_vision

As you can see, dogs can bee blues and yellow but have difficulty discriminating between colours in the red-green part of the spectrum. So I am not sure what the fuss is about with the Daily Mail article. After all, everything in the Daily Mail is true!! See http://www.youtube.com/watch?v=5eBT6OSr1TI if you don’t believe me.

Where is colour mixing?

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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:

ColourMixing

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

This is called additive colour mixing as I am sure you know. 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). So 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. When I sat down with a couple of students last week and asked then what they though they said that the red and green light mixed together to create yellow light and when I pressed them, they went further to say that the yellow light was at about 575 nm.

visible-a

If we measure 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. 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. It also makes me think that additive colour mixing, if it can be said to occur anywhere in particular, occurs in the eye. And I do mean eye, not brain.

is there such a thing as visible light?

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I would argue that there is no such thing as visible light – or at least that the term visible light is a meaningless one.

Light is part of the electromagnetic spectrum which is describes electromagnetic radiation by its wavelength. An electromagnetic wave has both electric and magnetic field components. What is really very interesting is that depending upon the wavelength of the field the electromagnetic radiation has very different properties and we give it a different name.

electromagnetic-spectrum

When the wavelength is very long, the radiation is radio waves or micro waves. When the wavelength is very short, the radiation is x-rays or gamma rays. There is a narrow range of wavelengths (from about 360 nm to about 780 nm – a nm is 0.000000001 of a metre) to which our eyes are sensitive. Because we can literally see this radiation we call it light. I still find it amazing that light, x-rays, radio waves, and microwaves are all essentially the same thing (electromagnetic radiation) with just a change in the wavelength!! However, my point for today is that light is radiation that is visible – to talk about visible light would be bizarre since by its very definition light is visible. Technically, visible light is a pleonasm; pleonasm is a word derived from the Greek word “pleon” meaning excessive. Other examples of pleonasms – easily confused with oxymora – include the phrases end result and invited guests.

special females

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Our colour vision results from the fact that our eyes contain three types of light-sensitive cells or cones that have different wavelength sensitivity. Some people (called anomalous trichromats) are colour blind and this is usually because one of their cones is mutated and has a different wavelength sensitivity compared with those in so-called normal observers. Colour-blind is a misnomer really because anomalous trichromats can still see colour; they just have less ability to discriminate between colours than normals. Some people are missing one of the cone classes altogether and are referred to as dichromats; they have even poorer colour discrimination but can still see colour. Only monochromats are really colour blind and they are extremely rare.

For a long time I have known that some females have four cones classes (this makes them tetrachromats). Dr Gabriele Jordan, a researcher at the Institute of Neuroscience (Newcastle University) has spent the last 20 years working on human colour vision. She has discovered that tetrachromatic females exist and that although this gives them the potential for colour discrimination much better than normal trichromats in practice most have normal colour discrimination. However, in a recent report she has found a tetrachromat who really does have enhanced colour discrimination. This is really exciting news!

The report in the Daily Mail suggest that a functional tetrachromat could be able to see 99 million more hues than the average person. Personally I am skeptical of this claim even if, as I suspect, it means 99 million more hues than the average person. The number of colours that an average person can see is debatable but I think may be about 10 million (see my previous blog post).