Indigo – a colour of the rainbow?

knowledge, , , ,

From time to time I come across web pages and groups of people who get irrate about indigo being in the rainbow. There is even a facebook group called “Get Indigo out of the rainbow”. It was Newton who suggested that the rainbow contains seven colours: red, orange, yellow, green, blue, indigo and violet. It has been suggested that, at the time, Newton was trying make some anology with the musical scale and the octave (with its seven intervals) and hence was keen to identify seven colours in the rainbow or visible spectrum. Many modern commentators claim that only six distinct colours can be observed in the rainbow.

Interestingly, the facebook group referred to above would like to eject indigo from the spectrum on the basis that it is not a primary or secondary colour but rather a tertiary colour. The group shows the following colour wheel:

colour wheel

In this so-called painters’ wheel the primary colours are red, yellow and blue and the secondary colours are orange, green and violet. It is argued that since six of the colours in the rainbow are primary or secondary colours in the colour wheel and indigo is not, then indigo has no right to be there. This is wrong on so many levels it is hard to know where to start.

The first thing I would have to say is that this argument seems to ignore the difference between additive and subtractive mixing. Additive mixing – http://colourware.wordpress.com/2009/07/13/additive-colour-mixing/ – describes how light is mixed and the additive primaries are red, green and blue. The additive secondaries are cyan, magenta and yellow. Orange is not in sight – and yet surely if we are to make an argument for inclusion in the spectrum based on primaries (and/or secondaries) then it is the additive system that we should be using since the spectrum is emitted light.  

The optimal subtractive system primaries are cyan, magenta and yellow (with the secondaries being red, green and blue) though the artists’ colour wheel (which is like the painters’ wheel above) has red, blue and yellow as the primaries. 

In my opinion there is nothing special about the colours that we see in the spectrum. Indeed, orange is clearly a mixture of red and yellow and does not seem to me to be a particularly pure colour. I just do not think that arguments to exclude indigo from the spectrum based upon colour wheels or primary colours is valid. That said, I have already mentioned that many people believe that indigo cannot be seen in the spectrum as a separate colour; but this is a phenomenological observation not dogma. I am one of those who believe that indigo and violet cannot be distinguished in the spectrum and therefore I agree with the aims of the facebook group even if I do not agree with their arguments.

The really interesting question is why we see six (or even seven) distinct colour bands in the spectrum when the wavelengths of the spectrum vary smoothly and continuously? I have postulated some possible reasons for this in an earlier post – http://colourware.wordpress.com/2009/07/20/colour-names-affect-consumer-buying/ – but it is far from a complete and convincing explanation. It may explain why we see distinct colours in the rainbow, but why six and why those six in particular. Comments on this would be very very welcome.

Colour names affect consumer buying

design, ,

Have you ever wondered why, when you look at a rainbow, you see distinct bands of colour? You may see red, orange, yellow, green, blue, indigo and violet (though more likely you will not be able to distinguish between indigo and violet). We know that the wavelength of light in the visible spectrum – http://colourware.wordpress.com/2009/06/29/colour-101/ – varies smoothly and continuously, so why don’t we see a smooth and continuous colour spectrum? Why do we see distinct colour bands?

myspectrum

In my opinion the reason we see bands is because of something called categorical perception. We tend to want to group things that we perceive together into one class or another. But this grouping is not just a matter of putting things into boxes; it has an impact on how we perceive those things. We see categorical perception everywhere – indeed, I have often wondered whether even the periodic table of chemical elements is a true and accurate representation of how the world is or whether it stems from our categorical perception.

A recent study by Skorinko at the University of Virginia and colleagues at Rice University (published in Psychology and Marketing, 2006) finds that consumers have a more positive reaction to products whose colours are given rather exotic and flashy names such as mocha compared with the same products that are given plainer and genric names such as brown.

 untitled

 And how is that linked to the early statements I hear you ask (even though you asked very quietly)? Well, the authors hypothesise that the reason for the improved consumer reaction to the fancy colours is …. categorical percetption. The fancy names stimulate a more positive category than their plainer alternatives. It is also suggested that more ambiguous descriptions (mocha as opposed to brown, for example) yield higher consumer acceptance and safisfaction. I cannot resist finishing this blog with the last line from the paper by Skorinko et al. (2006) who write:

Indeed, the judgement of “that we call a rose” seems to be influenced by its name (Shakespeare, 1595). 

CIE system of colorimetry

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For about 100 years there has been an international system for colour specification – it’s called the CIE system. The acronym comes from Commission Internationale de L’Eclairage.

This system is based on the notion of additive colour mixing – http://colourware.wordpress.com/2009/07/13/additive-colour-mixing/

Since it is possible to mix together three primary lights and make a wide gamut of colours (though not, of course, all colours) the principle is that the amounts of these primaries that an observer would use to mix togther to match a colour is a useful specification of that colour. We refer to these amounts as tristimulus values. One could imagine a visual colorimeter whereby an observer would try to match a colour that is to be specified by adjusting the intensities of three primary lights that are mixed together – once a match is obtained then the tristimulus values would define or specify the colour. All that would be necessary would be to able to decide on a set of primaries and manufacture the visual colorimeters so that they are very consistent from one device to the next. It would be a little clumsy though to have to use one of these visual colorimeters. But in principle it could work.

Fortunately the CIE does not require the use of such visual colorimeters since in 1931 the CIE measured the trismumulus values that observers made when matching various colours. These were averaged to create the so-called CIE standard observer.  And here’s the really clever bit. Having defined the CIE standard observer it is possible to calculate the tristimulus values (the amounts of the three primaries that an observer would use to match a colour) without any further observations. All that is required is that we know the amount of light at each wavelength reflected by a sample or (in some cases) emitted from a device such as computer display and then – by using our knowledge of the CIE standard observer – it is possible to calculate the tristimulus values.

So what were the primaries. If you have read my previous post, What is a colour primary – http://colourware.wordpress.com/2009/07/08/what-is-a-colour-primary/ – you’ll know that the choice of colour primaries is somewhat arbitrary. Well, in fact the original determination of the standard observer what carried out in England using red, green and blue primaries. But the data obtained were later modified to refer to a different set of primaries known as X, Y and Z. It was necessary to make this adjustment because using any set of real primaries it was impossible to match any colour with mixtures of the primaries; using RGB meant many colours could be matched, but not all. So a set of so-called imaginary primaries was conceived which could – in theory – be used to match all colours. So the tristimulus values of the CIE system are known as X, Y and Z. 

In fact, it didn’t really matter which set of primaries was used; the CIE system was concerned with colour matching. If two samples have the same tristimulus values then they would be a visual colour match no matter which set of primaries was used. So the choice of primaries really was not critical.

Today many instruments are commercially available – colorimeters, reflectance spectrophotometers, radiometers) – that, with the use of software, allow the CIE XYZ values to be measured; these instruments are extremely valuable in many industrial and commercial applications. The CIE system is still very much alive today, though many users often prefer to use one of the more advanced colour spaces – such as the CIELAB colour space – which was defined by the CIE in 1976 and whose values are very easily calculated from the CIE XYZ values.  For further information about the CIE please visit their web site – http://www.colour.org/

colour and brand loyalty

design,

I’m on the way to a dental conference in Houston to speak about tooth whitening. So with a few hours to kill in Philadelphia airport I am taking the time to read Martin Lindstrom’s Buyology – http://www.amazon.com/Buyology-Truth-Lies-About-Why/dp/0385523882

buyology

He describes an experiment that he conducted where he invited 600 women into a room and presented each of them with a blue Tiffany’s box. Their heart rates were being measured and they went up by 20% when they received the box. The interesting thing is, the women never even saw the logo. Just seeing the colour – and the presumed association of that blue colour with the Tiffany brand – was sufficient to excite them. Indeed, the book describes a study by Seoul International Color Expo that showed that colour increases brand recognition by upto 80%. Interesting ….

Orange wedding

news,

Tomorrow I’m flying to Houston to present at a meeting of the Society of Color and Appearance in Dentistry; http://www.scadent.org/. So it was a strange coincidence that I came across a news story today that there is a trend in Houston (of all places) for people getting married – possibly brides, though I wouldn’t be so sexist as to suggest that – to use orange as a key colour in their wedding decorations. Interesting to see that orange is still a fashionable and contemporary colour. I knew I was right using orange as a main theme for the appearance of this blog! For the full story visit http://www.examiner.com/x-11875-Houston-Bridal-Scene-Examiner~y2009m7d12-Orange-you-glad-your-wedding-colors-reflect-you

Rose-tinted lenses

news,

In a previous post – http://colourware.wordpress.com/2009/07/04/colour-blindness-news/ – I expressed sceptisism about whether coloured contact lenses or spectacles can improve colour discrimination in colour-blind observers. Today I came across a related story about the use of coloured lenses to improve reading in some people suffering from migraines and/or learning difficulties. It seems to be news in Australia – http://www.theage.com.au/lifestyle/wellbeing/tinted-lenses-bring-words-into-living-colour-20090712-dhfe.html – but I have been aware of research that coloured transparencies can help with reading for many years now. Apparently certain people respond better with certain colours – but I wonder why?

additive colour mixing

knowledge, ,

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.

Society of Dyers and Colourists

professional

I joined the Society of Dyers and Colourists in about 1983 when I was undertaking my doctorate at Leeds University. I mainly joined for the fun to be honest, but as a serious young student I think I thought it may have some positive effect on my career. Little did I know that I would be still a member some 25 years later and that during that time the SDC would have been a very important part of my professional life. Through the SDC I have gained technical knowledge (through my membership of its technical committees such as the Colour Measurement Committee) but also met many kind and knowledgeable people in the world of colour. And I’d like to think I have put something back too through my role on the editorial board of the journal Coloration Technology and, more recently through my role as founding academic editor of the SDC online journal Colour: Design and Creativity.

I would encourage anyone with an interest in colour technology or colour design to have a look at the SDC. They are based in Bradford but have an international presence and hold meetings all over the world.

Visit their web site http://www.sdc.org.uk/ 

The SDC even has its own blog – http://sdc-colourblog.blogspot.com/

What is a colour primary?

knowledge, , ,

I try to keep my blogs short. But this is going to be difficult!

Let’s start with what a primary is not. The primaries are often quoted as being red, yellow and blue. And on the BBC website – of all places – it says “Primary colours are three key colours – Red, Yellow and Blue. They cannot be made from any other colour”. For the BBC site visit http://www.bbc.co.uk/homes/design/colour_wheel.shtml.

The statement that primary colours cannot be made from any other colour is simply not true. Further, it gives the mistaken impression that there is something special about these primaries (red, yellow and blue) that makes them stand apart from the other colours. Another false statement that one often sees even in quite scholarly work is that any colour can be made by the mixture of three primary colours.

What is true is that if we select three appropriate colours and mix them together we can make a suprising range of colours. These colours – let’s call them primaries – can combine to make a huge range of different colours but unfortunately there are no three primaries that can be selected such that in mixture they can create any other colour. Depending upon the choice of primaries the range (or gamut) of colours that can be created in mixture is larger or smaller. What makes a good set of primaries? Well, I think it is reasonable to say that a good set of primaries is one where the gamut of colours that can be created is large; indeed, I would argue that the optimal primaries are those that create the largest possible gamut.

There are certain sets of primaries that can easily be predicted to give a small gamut. For example, if any of the primaries are dull and desaturated then the gamut will not be very big. Also, if the three primaries are similar to each other then the primaries are not likely to be a good set. And finally, if it is possible to combine two of the primaries together to make the other one then the gamut will be tiny. This is where – I believe – the misleading statement on the BBC website comes from; for a good set of primaries it is important that the primaries are independent (that two cannot be mixed to match the other one) but this is a long way from the BBC statement. Once we havs selected three suitable priamries then it’s true that they cannot then be made by any other colours that the primary system can make – but to argue that this is why they are primaries is clearly a circular argument.

There is nothing special about red, yellow and blue. In fact, they are not even the optimal primaries! To say what the optimal primaries are we need to specify the type of mixing: additive or subtractive. Additive mixing describes the behaviour of light-emissive systems such as computer displays, subtractive mixing describes how paints and inks mix. For subtractive mixing the primaries are often quoted as red, yellow and blue, as in the BBC article. However, a larger subtractive gamut is obtained if we use cyan (instead of blue) and magenta (instead of red) – see figure below.

primary 

One of my current research interests is to understand why red, blue and yellow became known as the artists’ primaries when a larger gamut is obtained if a bluish red is used (something closer to a magenta) and if a greenish blue is used (something closer to cyan). One only has to look at the primary colours used in inkjet printers for example (where the manufacturers have a vested interest in being able to create a large gamut) to realise that cyan, magenta and yellow are the optimal subtractive primaries.

For additive colour mixing the optimal primaries are red, green and blue. The additive and subtractive primaries have an interesting relationship – but that’s for another blog, another day.

Is black a colour?

knowledge, opinions/rants

This is one of the most frequently asked questions I come across. 

For me there is no doubt that black is a colour whether argued from an objective or subjective position (see http://colourware.wordpress.com/2009/07/03/does-colour-exist/).

If we take an objective position and argue that colour results from pigments or dyes in the world then we should note that black most commonly occurs with the pigment carbon black, an efficient absorber of light at most visible wavelengths. I see no categorical distinction between carbon black and other pigments (e.g.  prussian blue, malachite green, titanium doixide). Those objectivists who associate colour with light and hence argue that black occurs in the absence of light and is therefore not a colour, can be countered with my observation that in over 25 years I have never seen an object that absorbs all of the light that falls upon it;  though it is my understanding that no light ‘escapes’ from black holes.  But I have seen lots of black things. In fact, most black objects reflect at least 5% of the light that falls upon them.  If you look at an old CRT screen turned off the screen would look grey (not black). However, when turned on, black objects in the CRT image would look far blacker than the screen when it was turned off though they obviously cannot be emitting less light. The enhanced black is caused, of course, by contrast. Modern LCD screens are less prone to this effect and I believe this is the result of more sophisticated coatings that mean that the screen reflects less of the ambient illumination. For a nice example of contrast, see the image below; the four small grey rectangles are all physically the same but appear to be lighter as we move from right to left.

contrast1

This leads us to the subjective view of colour. Colour is a perception. In this sense I see no reason to distinguish between black, white, grey, red and blue. That is not to say that there are no differences between these colours. White, black and grey are, as we know, achromatic whereas red and blue are chromatic colours of a specific hue. To argue that black is not a colour in this sense, however, would lead one to question whether white and grey are colours. Of course, some people take this position. There is a tradition in visual neuroscience to separate so-called colour processing from lightness processing in the visual system because they are believed to result from distinct neurophysiological processes. However, a modern understanding of colour perception is that colour is a three-dimensional percept; the dimensions being hue, colourfulness and brightness.  I would therefore argue that black is not the absence of light since black often (in fact, usually) occurs when light is being reflected or emitted; however, black is an achromatic colour like white and grey.