Tag Archives: color

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.

Do women use more colour names than men?

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

doghouse_color_wheel_altered

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:

doghouse_analysis

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.

Press coverage of #TheDress

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Whatever anyone thinks about the colour of dress and the attention it is received there is one undeniable fact – this story had received huge attention from the public and from the media. That in itself is probably more interesting than the debate itself.

The Daily Mirror story covered the angle that we are all right whatever we see because colour exists only in our heads. According to Dr Paul Knox, a reader from the University of Liverpool’s department of Eye and Vision Science, “Colour isn’t something that exists in the world. Different wavelengths of light exist and can be observed but colour is something we make up inside our heads.”

ITV also took the view that the explanation is that colour doesn’t exist. I broadly agree with this view, but the interesting thing is that that doesn’t explain why there was so much disagreement about the colour in this particular case whilst normally we barely notice any disagreement. If it is simply that colour doesn’t exist then why do we ever agree about colour at all?

On the other hand, in the Guardian an article by Bevil Conway considers cognitive processes in our colour vision and visual strategies that may vary from one person to the next. Of course, Bevil Conway is a super scientist and I agree with almost everything he says. Certainly, cognitive strategies could have something to do with this phenomenon. However, when he says that “By accident or design, the dress is a carefully created composition of orange and blue that confounds our visual systems,” I have to disagree. If you look at a properly taken photograph of the dress or the dress itself in real life what you see is shown below:

dress_original

The dress is not a carefully crafted composition of orange and blue – the dress is blue and black. However, Bevil is probably talking about the image that was circulated not the one shown above. To understand this phenomenon you need to understand colour imaging and the fact that colour images are sometimes not faithful reproductions. One of the reasons why this story has run and run is that there is no simple answer, no 10-second soundbite that can put the story to bed. It is a complicated phenomenon.

Extraordinary facts relating to the vision of colours

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In 1794 John Dalton presented a lecture to the Manchester Literary and Philosophical Society about colour vision. The first two sentences are shown below:

It has been observed, that our ideas of colours, sounds, tastes, etc. excited by the same object may be very different in themselves, without our being aware of it; and that we may nevertheless converse intelligibly concerning such objects, as if we were certain the impressions made by them on our minds were exactly similar. All, indeed, that is required for this purpose, is, that the same object should uniformly make the same impression on each mind; and that objects that appear different to one should be equally so to others.

It is interesting to reread this sentence again in the light of the recent controversy about the blue and black dress.

Dalton

colour and language

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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.

90

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.

davidoff

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.

#TheDress

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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.

2622C22600000578-0-image-a-32_1425001827044

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:

dress_original

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?

guess what – red is sexy

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red is sexy
Guess what? Another article that concludes that women wearing red are more likely to attract a mate. Scientist claims women are reflecting their sexual intentions ‘from the beginning’ by wearing bright red clothing. It’s a shocker!!! Who would have thought it!

It must be true because I read it in the Daily Mail.

Colour and Manchester United

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van gaal

It’s not often I get to write about two of my favourite things at the same time. So I couldn’t resist remarking on a story today in the Daily Mirror about a colour code that Manchester United manager Louis van Gaal uses to describe different players in his squad. Apparently:

Blue:
“A blue player is intellectual and is always looking for structure and security in his job on the pitch.”

Red:
“A red player is creative, full of power, will want to work and is always focusing on the future.”

Green:
“A green player is very emotional, sensitive for different emotions or a different atmosphere in the squad.”

See the original article for the colours that the newspaper thinks that different players should be be allocated.

It couldn’t get much blacker

opinions/rants, technology, , ,

black
A few weeks ago I was taking my son to a birthday party and a journalist from The Independent phoned me to ask my opinion on Vantablack. This is the blackest material ever made. Whereas most black materials reflect about 4% of the light (or more) at all wavelengths, this new nano-material has really really low reflectance. It only reflects about 0.035% of the light. I gave a few comments and an article appeared in The Independent which was nice. I used to really like The Independent, back in the days when I read newspapers. The original article by Ian Johnston was very good imho.

However, a few days later the story was all around the world and I was often cited, all based on that one phone interview with Ian. The thing was that it was not even that big news. That is, yes, it is the blackest material ever made, but the truth is it is an incremental improvement in blackness beating the previous blackest material from a few years ago. My name even appeared in the Daily Mail. Most embarrassingly, I was interviewed on an American radio show. The reason I say it was embarrassing was that this new development actually had nothing to do with me and I didn’t want people thinking I was trying to claim credit. So when I agreed to do the radio show I told the researcher that they needed to be clear that this was nothing to do with me. I didn’t invent it. Imagine my surprise when John Hockenberry (that was his name, I believe) asked me, “So Dr Westland, what have you stumbled upon?”. Arghhhhh!!! Luckily, it was not a live interview because it actually got worse. A lot worse. So bad, that I could barely summon up strength to listen to it when it went out the next day. But actually, the editors did a good job and the final cut is not too bad. You can hear it here.

It would be nice to talk about my own work. I work in the area of blackness. One of the things I do is to ask people to rank different black samples in order of least black to most black. This allows me to discover, for example, that women prefer reddish blacks and men prefer bluish blacks. Also, asians prefer reddish blacks and caucasians prefer bluish blacks. I am developing a blackness index; a way to measure a sample and say how black it is or whether one sample is blacker than another. Why? Well, one application is for manufacturers of black ink for printers (which may be made from coloured inks). Different recipes produce different blacks. What if one recipe is chromatically neutral but another recipe is less neural (it has a slight hue) but is darker – which one is blacker?