Monthly Archives: September 2009

red-blue flicker causes epilepsy

In 1997, more than seven hundred children in Japan suffered an epileptic attack while watching an episode of Pokemon cartoon. This was later diagnosed as a case of photosensitive epilepsy (a kind of epilepsy caused by visual stimulus) triggered by a specific segment of the cartoon containing a colourful flickering stimulus. However, despite the ubiquitous presence of colourful displays and materials, very little is known about the relationship between colour-combinations (chromaticity) and photosensitivity. A new study has recently been published that suggests that certain colour combinations, for example, red-blue flickering stimuli, cause larger cortical excitation than other colour combinations such red-green or blue-green stimuli –

solid ink printers in UK

As an alternative to laser or inkjet technology, Xerox offers solid-ink technology printers. A solid-ink printer uses solid sticks or blocks of ink instead of toner or inkjet cartridges. Xerox claims that solid-ink printers offer better colour consistency, are less expensive per sheet of printing and are more environmentaly friendly, producing less waste than lasers and inkjets – The technology uses solid inks that are melted and sprayed onto the page as droplets, in a manner not dissimilar from inkjet printing. But because the inks are solid at room temperature, they can be supplied in blocks that are simply dropped into the printer, rather than being contained in a cartridge.

Now solid-ink technology is available in the UK with the launch of three Xerox ColorQube printers. However, they are out of the reach of the home user, costing around £13K each.

yellow transport

In the UK, official taxis run by the council tend to be black. However, Derby – a city in the UK – introduced a new regulation in 2001 that all the official taxis should be yellow. Presumably this was to make them more distinctive so that members of the public would be more certain that they were getting into an official taxi rather than an illegal one. One of the taxi drivers – John Kirkham – painted his car yellow to comply but was then shocked to have his application for the renewal of his licence refused because he had used the wrong shade of yellow!


The council backed down after adverse publicity. If only they had specified the colour tolerance as part of their specification they would have had a much stronger case.

Another reason for choosing yellow could be safety. Many school buses in USA are yellow – presumably for reasons of safety. A number of regions in the UK have recently been trialling the use of yellow buses. Getting more school children to travel to School by bus is seen as being good for the environment and would free up the congested roads. In the USA more than half of all chidren travel to School by yellow buses according to the BBC –

yellow bus 

The use of yellow for school buses in the USA was introduced in 1939. The colour – National School Bus Chrome Yellow – was introduced for reasons of consistency and also for safety because of its high visibility. Cost was also a consideration since manufacturers charged extra for special colours. In addition the black lettering was easily visible by contrast on the yellow background (as you can see in the picture below).

yellow bus usa

The approximate colour can be obtained by #FFD800 or [255 216 0] in RGB values.

What is a colour space?

In my job I probably use the phrase “colour space” every day and have done for the last 20 years. So imagine my surprise when I was talking with a colleague recently and after a few minutes he said “Can I stop you for a second there Steve – when you say colour space, what exactly do you mean?”.

A colour space is like a map. A map of New York, for example, shows the location of various landmarks with reference to the xy coordinates (the position in horizontal x and vertical y units on the map). A colour space or colour map does the same thing with colours. Perhaps the simplest colour space is the spectrum, see below:

As we look from right to left on the spectrum the wavelengths changes from around 700nm on the far left to about 400nm on the far right. So this map shows colour with reference to wavelength. Although it is a commonly used colour space it is limited because it only really describes how hue changes with wavelength. Hue is only one of three ways in which colour can change or vary.

The most well-known really useful colour space then is the CIE chromaticity diagram – see below.


The CIE chromaticity diagram shows colours arranged on a 2-D plane. We can easily refer to any colour by how far from the left it is (the x coordinate) and how far from the bottom it is (the y coordinate). This space only shows two of the dimensions of colour; the hues are arranged in a somewhat circular way and the colourfulness increases as we move outwards from the white point (a position near to the centre of the diagram). However, we can also consider the third component of colour (brightness) if we imagine a dimension coming out of the page towards you ( The CIE defines several different colour spaces; the CIELAB colour space, for example, is another 3-D space that defines a colour by its L*, a* and b* values.

It is useful to think of an image-display device as also having a colour space. Consider the display on which you are probably reading this blog. The display shows colour by changing the amount of the red, green and blue light emitted at each point on the screen. The diagram below is a representation of what the RGB colour space of your display device may look like.



In the RGB cube, black is in the bottom left. As the RGB values increase colours are created and white results from each of the RGB primaries at full strength. So the RGB colour space defines the relationship between RGB values and colour. However, here’s the really interesting thing: The colour space for different display devices is very different. Even if we take a single device – such as the one that you are reading this blog on – then as we change settings (the brightness, the contrast, the gamma, the colour temperature, etc.) then the colour space changes. That is, the relationship between RGB and colour changes as you change those settings. This is a huge problem. Imagine if there were many maps of New York and each showed the position of, say, the Empire State Building to be in a different position. How confusing would that be? Well, that’s the problem with colour-display technology. If we didn’t do anything about this problem then every time we looked at a colour image on a different display device the colours could change markedly. This is why we need colour management. Colour management can make compensations to the RGB values that are sent to each display device so that the colours always appear the same (well, nearly the same). To make this compensation the colour management software (which is embedded in your Windows or Apple operating system) needs to know about the colour space of each device connected to the computer. Each device needs to have a profile that describes the relationship of its own colour space with respect to some standard colour space. 

How good is colour management? Well, that depends upon many factors. Most printers, cameras, scanners, and screens (LCD, CRT, etc.) come with a driver that includes a crude colour profile. This ensures that there is a basic level of colour management and for a great majority of users this is more than adequate. However, if you want better performance then you need to think about making some measurements that will allow a more accurate colour profile to be built. In a recent blog I described a new device that you can buy to enable you to do this – There are many such devices on the market. I highly recommend Andrew Rodney’s book Color Management for Photographers which is both clear and accurate (though the edition I have works on Adobe’s CS2 package whereas the latest package is CS4).


However, no matter how hard you try, colour management is never likely to be perfect. This is because different devices have different colour gamuts; a printer is likely to be able to display some colours that your display physically cannot and vice versa.

cure for colour blindness

One of the reasons I enjoy travelling by train is that it gives me an opportunity to read a newspaper from front to back (something I very much enjoy but rarely have time to do). Yesterday I was travelling to Bristol where I was delivering a lecture at the IMPACT6 Printmaking conference on colour management and took the train from Leeds to Bristol during which I was able to read The Times. I couldn’t fail to notice the story about a potential cure for colour blindness –

Congenital red-green colour blindness occurs when either the L- or M-cone class is either missing (making the sufferer a dicromat) or shifted in terms of peak wavelength of sensitivity (resulting in anomalous trichromacy) – see

Scientists working at the universities of Seattle and Florida have restored normal colour vision to two colour-blind monkeys by injecting a virus with a modified gene (called L opsin) that is known to be responsible for red-green colour blindness. The success of this work is remarkable in that it suggests that the brain is able to rewire itself to take advantage of the new receptors. 24 weeks after the injection the monkeys were able to correctly distinguish patterns of grey, green and red dots that they had previously been unable to distinguish.


Jay Neitz, professor ophthalmology at the University of Washington, is now looking to start work that could lead to a similar treatment for humans.

The work has just been published in Nature –