Some Observations on Colour Vision

Fred Howett

The evening was split into two parts. The first part, up to the tea break, covered use of computers in art related guild activities, and the second part presented an insight into the operation of colour vision

Before the tea-break Fred posed the following questions.

After the tea-break the audience gave their answers. There was a concensus that there were three primary colours and a general agreement that they were red, blue and yellow. Only one member of the audience gave an answer to the third question.

The scientific bit.

Electromagnetic waves are a fundamental feature of the universe and transfer energy even in a vacuum. Mechanical waves on the other hand require the presence of a material medium to transport their energy from one location to another. Sound waves are examples of mechanical waves while light waves are examples of electromagnetic waves.

Radio waves, X-rays, microwaves and visible light are all lectromagnetic waves. The eye is only sensitive to a tiny portion of the electromagnetic spectrum as shown in the diagram above. This is known as the visible spectrum.

Isaac Newton in 1672 demonstrated (by passing a narrow beam of sunlight through a glass prism) that white light consisted of a spectrum of different colours, which could be recombined to produce white light. He also demonstrated by passing an individual colour through another prism that no further splitting of the colours occurred. In other words the individual colours of the spectrum were pure.

End of the scientific bit!

Newton was the first person to give the spectral colours names; red, orange, yellow, green, blue, indigo, violet - names which are still used today. By arranging the colours from 'warmest' to 'coolest' (a subjective rather than scientific process) he came up with the concept of the colour wheel. His colour wheel only contained the seven colours that he had identified in the visible spectrum, so red was placed next to violet (i.e. no purple). He also came up with the theory that red, yellow and blue were the primary colors from which all other colors are derived.

Shown on the left is an early colour wheel by Frenchman Claude Boutet from 1708

Below is a modern version of the colour wheel

Below is an early alternative to the colour wheel from 1775 which attempts to show the secondary colours formed by mixing the primary colours

I have often wondered why colours can be arranged in a continuous seamless manner around a colour wheel, when, as shown in the slide of the electromagnetic spectrum above, the red and violet are at opposite ends of the visible spectrum More on this later.

Back to the question on the primary colours. Most of the audience said the primary colours were red, blue and yellow. I then asked them how they could see yellow on the projected images in the demonstration or on their television screens when there was no yellow colour being transmitted. In fact only the colours red, blue and green were being transmitted. The diagram below is a section of a modern high resolution television screen and consists only of red, green and blue pixels.

If one were to take 3 lamps of red, green and blue and shine them on a white background so the colours partially overlap, the answer to the above question becomes clear.

Where the red and green lights overlap, the colour yellow is perceived yet there is no yellow light present. Similarly the overlapping red and blue appears as magenta, and the overlapping green and blue appears as cyan. Where the red, green and blue overlap, white is seen. Obviously there is a perceptive process going on in our brains. (Just as well there is otherwise colour television would not work!)

So what is going on? To answer this question we need to understand what is going on inside our eyes.

It is an interesting observation that the chickens that wander about outside our studio window have better colour vision than the artists on the inside. Also bulls have no colour vision so the expression 'red rag to a bull' has no truth in it.

The reason we perceive three primary colours is due to the fact that the human eye has three different types of colour receptor which correspond approximately to red, green and blue as shown in the diagram below. The outputs from these receptors are fed by the optic nerve to the brain.

It can be seen in the diagram above that there is a big overlap between the red and green receptors so if the eye receives some yellow light it is picked up by both the red and green receptors and fed to the brain which calculates the strengths of the red and green and interprets it as YELLOW. But if the eye receives red and green lights only, the red and green receptors send the same signals to the brain so it is also interpreted as YELLOW.

During the early years of the 20th century much experimental work was carried out internationally on human perception of colour and how combinations of different colours are perceived by the brain. This was codified in 1931 by the Commission Internationale d’Eclairage (International Commission on Illumination) to describe all colors visible to the human eye. These results are presented in the diagram below.

At first glance this is very similar to the well established artists colour wheels. One major difference is that it is horseshoe shaped rather than circular. The curved part of the diagram represents the visible spectrum that lies between the infra-red and ultra-violet portion of the electromagnetic spectrum shown in the earlier slide. The straight line acrosss the bottom of the horseshoe shape is known as the line of purples. The colours 'seen' along this line do not exist in nature and cannot be created in the physical world. They are created within our brains, purely 'in the eye of the beholder'.

An important concept to appreciate when trying to understand how colour works is the difference between the additive process and the secondary process. The additive process refers when sources of light of different colours are added together. This is the process invoked when watching television, your computer screen, and increasingly your smartphone screen. The subtractive process occurs when objects are lit by white light and reflect back different colours. It is important to realize that paints do not give out light - if viewed in a darkened room, they will appear dark. A red paint for example when lit by white light absorbs the blue and green light but reflects back the red part of the white light thus 'appearing' red to the observer. This is shown in the slide below.

Here we can see that for the additive process the 3 primary colours are red, blue and green, whereas for the secondary process they are cyan, magenta and yellow. Interestingly the primary colours of the subtractive process are the same as paired colours of the additive process. So in fact the primary colours used by artists should be cyan, magenta and yellow instead or the commonly believed red, blue and yellow. This has been well understood by the makers of colour printers who invariably use cyan, magenta, yellow and black inks.

To test this out I thought I would try to do a painting using just cyan, magenta, yellow and black paints. Below is my first attempt.

When putting this lecture together I thought just a single example would not be particularly convincing so a couple of days before this talk, I spent an afternoon producing the examples below using just cyan, magents, yellow and black acrylic paints.

So when you are next out sketching and need to travel light, remember you only need to take cyan, magenta and yellow paints with you!

It is very interesting to realize how much the brain is involved in what we 'see'. We have already shown that regarding colour vision, the brain can 'see' yellow when there is only red and green present, and can invent purples which do not exist in nature. The brain is also not perfect when resolving complex geometry or assessing tone values as shown in the following examples.

In this example the top bar is actually horizontal and parallel to the lower block.

In this example there are four concentric circles with no overlap.

In this example the two large grey areas are exactly the same colour and tone value.

In this example, the squares A and B are exactly the same colour and tone value.

This seems so hard to believe, that in order to prove it, I loaded it into the image editor, picked up the colour of square A and brushed in a line to square B as shown in the slide below.

A famous artist who used the brain's confusion with handling complex geometry to great effect was Dutchman M.C.Escher as typified in the animated image below.

The human brain can be 'tricked' audibly as well as visually. Have a listen to this.

The rising scale repeats itself without in fact getting anywhere, very much like moving round the colour circle.

Some photos of the bored fascinated audience!

I hope you found this evening interesting and has given you some food for thought.