Tag Archives: colour vision

how colour vision works


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.

Do women use more colour names than men?

I just came across this funny cartoon about the difference between men and women in terms of colour names.


But on the same page I found the results from an actual colour survey where over five million colours were named across 222,500 user sessions. One aspect of the results is shown below:


It does seem that there is some evidence that women use more colour names than men – though generally there was agreement between how the names were used. For further details see the original article.

colour and language

One of the things that #TheDress controversy has highlighted is that colour is not as fixed as the majority of people believe. We tend to think that objects have a single colour and that we all see that colour the same way. However, in the image below you can see two central grey patches that are physically identical but probably look different in colour to you. My experience is that the majority of people would explain this as the two grey patches being the same colour but looking different in colour because of the background. An illusion.


I don’t agree with this way of thinking however. The colours we see when we look at something do depend upon the other colours around it but this is not a a special case. It’s not unusual, as Tom Jones would say. It’s how colour works. If it is an illusion then it’s happening all of the time, almost whenever you are looking at colour. So what is the real colour of something? Is it even sensible talk about an object having a single fixed real colour?

There is a body of research emerging that suggests that the language that we use influences how we see things. Jules Davidoff, a Professor at Goldsmiths University, went to Namibia where he conducted an experiment with the Himba tribe, who speak a language that has no word for blue or distinction between blue and green. When shown a circle with 11 green squares and one blue, they couldn’t pick out which one was different from the others. But the Himba have more words for types of green than we do in English. When looking at a circle of green squares with only one slightly different shade, they could immediately spot the different one, even when the difference was so small that we would find it very difficult to see the odd one out. See below for an example.


In the image above – a screenshot from one of Davidoff’s experiments – the Himba tribe can easily see that the green patch at about 1 o’clock is different from the others.

In fact, some people even think that in ancient times we could not see blue at all because we had no word for it. In the Odyssey, Homer famously describes the “wine-dark sea.” But why “wine-dark” and not deep blue or green? It turns out that most ancient languages (including Greek, Chinese, Japanese and Hebrew) did not have a word for blue. Does this mean that they didn’t see blue? Is blue a relatively modern phenomenon? There is a thought-provoking article about this by Kevin Loria at Business Insider. Read more here.


I was asked to comment on the radio today about a dress which is topping the trends of social media in the USA in particular today.


The dress has sparked controversy because different people say that it is different colours. There is a group who say it is blue and black and another group who say that it is white and gold. What do you think?

I will give my explanation but it is not simple so …

Now, about 1 in 12 of all men in the world are colour blind. But if we consider the rest of the population you may be surprised to know that there is variability in our colour vision. This is mainly due to the colour receptors in our eyes. Put simply, some people have more red receptors and some people have more green receptors, for example. So we know that we don’t all see colour in the same way.

There is a second complexity and that is just because we use different names for a colour doesn’t mean we see it differently. This most often happens with brownish colours where some people will refer to it as more of a green and others will be adamant that it is definitely a brown. So words – colour names in particular – are not always very precise. We can see at least 3 million colours in the world and how many names do we have? A few hundred at least.

There is a third complexity which is that people think the camera never lies – that is, that they take an image of something using their phone and put it on the internet and everyone is seeing a faithful reproduction of the thing they took a picture of. Sadly, the camera does lie. Variability in the light that is used to capture the image, the settings on your display (whether you have a warm white or a cool white, for example) and how bright the light is in the room when you look at your screen – these can all dramatically affect the colour. Take a look at the picture below:


This is the manufacturer’s photo of the dress. Taken professionally, I think most people would see it as blue and black. But the image that is on the internet is very different. I suspect it was taken in a very bright light and the colours are consequently a bit washed out.

So, in summary, the camera does lie. I think the lighting conditions under which the photo was taken were far from ideal and have changed the colours from how they would have appeared if you had been there. However, that is only half the story. Since people looking at the same image on the same screen are disagreeing with the colours. To fully explain what is going on you need to invoke the knowledge that we can sometimes see colours differently (because of variability from one person to the next) and even if we see the colour the same we might give it a different name (because colour names are crude ways to communicate colour).

Of course, fundamental to this is the idea that things are not coloured at all but your brain constructs a colour from the signals it receives in the eye. This allows us, for example, to discount changes in colour that may occur when the light source changes (this is known as colour constancy). We have evolved to discount the effect of light being bluer or yellower, for example, so that we normally see the colours that the object would have in neutral daylight. In the case of the dress image it may be that people are using different processing strategies and discounting the effect of the light source in different ways.

Which all goes to show that colour is complex. But if you have been reading my blog you already know that, don’t you?

MRes Colour Communication

colour communication

We’re starting a new programme at Leeds University next September. It’s MRes Colour Communication. This is a one-year Masters programme by research but with a twist. There is a taught component in the first semester to get everyone up to speed to make sure they understand the basics of colour communication. They then explore one aspect of this in their research project and submit a dissertation at the end of the year. Please contact me at my University email of s.westland@leeds.ac.uk for further information or visit http://www.design.leeds.ac.uk/pg/research-degrees/.

dog vision

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.


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.

eyes change colour?


I didn’t realise how sophisticated reindeers are. It turns out they have two layers of fur to help them keep warm, are able to shrink the pads on their hooves to give then better grip, and can detect ultraviolet light which enables them too see in very dim light. And it also turns out that their eyes can change colour in winter so that their vision is more sensitive. Reindeers, like cats, have a reflective layer behind the retina (which is the inside of the eye ball where all the light-sensitive cells are) that helps them to see in dim light. This is why, if you see a cat at night, you might see the eyes shining; you are seeing light being reflected back at you from the cat’s tapetum lucidum (which is the technical term for the layer behind the retina). The light that shines back in most animals with this layer is golden but in reindeer it apparently shifts to blue in the winter. The shift to blue allows more light to be scattered and improves the vision of the animal.

The full paper can be read in the Proceedings of the Royal Society.

Where is colour mixing?

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:


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.


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?

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.


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.

chicken colour vision

Most humans are trichromatic; that is, our colour vision is mediated by three types of light receptor in our eyes. These receptors are known as cones and the three types have peak sensitivity in different parts of the colour spectrum. We sometimes refer to these as LMS cones because of their peak sensitivity at long-, medium- and short-wavelengths light.

Some people (men, in the main) are colour blind and this is because they are anomalous trichromats (they have three cones but the spectral sensitivities are less optimal than they should be or they are dichromats (they are missing the L, M or S cone types). But what about other species?

Most mammals are dichromats including dogs and cats. However, many fish and birds have better colour vision than do we humans. I just came across an article that reports that chicknes have five cones compared with our three. The research has been conducted the Washington University School of Medicine in St. Louis (USA). It is suggested that birds often have more cones than we do because they descended directly from dinosaurs and never spent any part of their evolutionary past living in the dark.