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Capturing the image: Colour space

How would you describe the colour red? You might say that it is the colour of blood or that it approaches the colour seen at the least refracted end of the spectrum, but neither description offers great precision. This is a problem when it comes to digital photography. Digital processes need numbers to crunch. To see how the problem has been solved, we need to start at the beginning.

How we see colour

Incandescent objects, such as the sun, an electric light bulb or a candle, emit radiation. This radiation extends over a wide electromagnetic spectrum and can include radio waves, microwaves, infrared and ultraviolet radiation, x-rays and gamma rays. Within this wide spectrum is a very narrow band of what we call light. Light is simply radiation that is visible to the human eye.

The human eye is composed of many parts, but to understand colour we need to look at the retina. This is the lining at the back of the eye that receives light from the lens at the front of the eye. The retina contains light-sensitive ‘rods’ and ‘cones’. Rods are distributed mostly around the edges of the retina and respond to the intensity, or brightness, of light. Cones are concentrated in the centre of the retina and respond to the frequency, or wavelength, of the radiation. It is estimated that there are around 120 millions rods in the human eye, but only around six to seven million cones.

There are three different types of cones, each responding to a different band of frequencies. One type of cone handles the higher frequencies of visible radiation, responding to the light we call blue. Another cone looks after the lower frequencies, responding to the light we call red. And the third cone covers the middle range, responding to the light we call green.

Very little light is sufficiently pure to stimulate just one type of cone. Most light will stimulate two, or all three, types of cones to different degrees. At the same time, the light will stimulate the rods. All this information from the rods and cones passes along the optic nerve to the brain. With just these colour and brightness readings from a myriad of points around the retina of each eye, the brain is able to create a clear, finely-graduated virtual image.

Stimulation of a rod or cone produces a tiny electrical charge. All the charges from all the rods and cones are passed to the human brain for processing. Although it is our eyes that collect the data, it is the brain that creates the image. A digital camera operates in a similar way, with the sensor unit producing data that is passed to a microcomputer inside the camera for processing.

For the most part, we see objects because they reflect light. White objects reflect the full spectrum of light - roughly equal amounts of red, green and blue. Coloured objects only reflect some of the light and absorb the rest. For example, a red object absorbs green and blue light and reflects red. The wide range of colours we see in objects occurs because every object absorbs and reflects different amounts and wavelengths of light. Black objects do not reflect any light.

The colour-sensitive cones in the human eye are not very effective in low light. The rods, on the other hand, work well in low light, but have no colour sensitivity. That’s why colours appear very muted in low light. In fact, when walking outside on a dark night you are probably seeing almost entirely in black-and-white.

It is worth keeping in mind that what we see is not an objective view, but the brain’s subjective interpretation of the scene. There is no guarantee that you see the same view as another person.

Colour blindness test

Are you colour blind? Estimates vary, but it is probable that up to 10% of the male population is colour blind, but only a small percentage of the female population. True colour blindness - seeing the world in black-and-white - is extremely rare. Red-green colour blindness is the most common form. Here, the red receptors (cones) are weak or missing and red can appear as dark grey or even black. You can check your colour vision using the Ishihara test. This asks you to identify numbers within circles of coloured dots. You can take this test at a number of websites, including:

Creating colour values

In the mid-19th century, Scottish physicist James Clerk Maxwell demonstrated that different intensities of red, green and blue light could be mixed to create any other colour. All three colours together at the same intensity created white light.

In 1931, the Commission Internationale de l’Eclairage (CIE) used this principle to define colours. They undertook a large number of experiments with what have become known as ‘standard observers’. Each observer, or person, was asked to mix red, green and blue light in different intensities to match a monochromatic (pure) light source. The results from all the observers were averaged to create a table of values. For the first time, any colour could be accurately defined in terms of its red, green and blue components.

Well, actually, no it could not. It was found that there were many colours that could not be created using the three primary colours, red, green and blue. This was overcome by creating three theoretical primary colours - X, Y and Z. All the colours that are seen by the human eye could now be defined in terms of their CIE X, Y and Z components.

If beams of pure red, green and blue light are projected onto a white screen, the area where all three beams overlap will be white. Where only two of the beams overlap, you will see cyan, magenta or yellow.


The CIE system was revised in 1976. It uses three variables - luminance (L), plus colour values on a red-green axis (a) and a blue-yellow axis (b). This gives us the CIE LAB system. It is not a practical system for digital photography as it contains many more colours than can be captured or reproduced by digital devices. However, it is invaluable as an independent reference system for colour management.

This is a visual representation of the CIE colour system. Pure colours are found around the outside of the shape. In the middle is a white point. A straight line from this white point to a point on the edge defines colours with the same hue but different saturation. When this shape is placed on a grid with x- and y-axes, it is possible to identify every hue and saturation visible to the human eye with just two grid references. If you draw a shape, such as a triangle, within the area of colours you can select a range of colours smaller than the total - this range is called a colour space.

Managing colour spaces

The theory of colour is interesting, but what does it have to do with the practicalities of digital photography? The answer is, quite a lot, as many photographers are beginning to discover.

One of the big advantages of digital photography is that it puts you in control. You can be involved in every stage of the process, from the exposure to the printing. Unfortunately, this can also be one of the big disadvantages of digital.

Think about film photography for a moment. The only real choice you make here is the make and type of film. After exposure you give the film to a processing laboratory and they send back the finished photographs, either prints or slides.

Digital is different. For each exposure you can change the ISO speed, contrast, sharpness, saturation and colour tone. You can choose to shoot RAW or JPEG images, and the JPEG images can be at different resolutions and compressions.

Once the exposure has been made, the image file can be transferred to a computer and opened in image processing software. Here you can make further changes to the sharpness and colour. Then you print out your images, and there may be another range of options available.

It’s not that film photography is simpler. It’s just that you leave most of the decisions to the experts - film manufacturers and processing laboratories. With digital, you need to be the expert.

When you first start shooting with a digital camera, you may well be disappointed with the results. Don’t blame the camera. The chances are that you have not worked out how to match the camera to your computer and printer. To do this, you need to know all about the colour spaces used by each piece of equipment.

RGB colour models

Like the human eye, digital cameras and computers use the RGB (red, green, blue) colour model.

The CMOS or CCD sensor inside your camera is made up of millions of tiny pixels, each sensitive to red, green or blue light. After exposure, the data from these pixels is processed by a microcomputer inside the camera and saved as a file to the storagemedia in your camera. There are obvious analogies here to the rods and cones in the human eye, and the human brain.

Computers take the image file and convert it back to a colour image via a monitor. The way the monitor works depends on whether it is a CRT (cathode ray tube) or flat screen, but the end result is that red, green and blue pixels on the screen are activated to give the appearance of a full colour image. So although the RGB image might seem limiting, the screen is actually presenting the colour information in the way our eyes perceive it.

Colour spaces

While digital cameras and computers use the same RGB colour model, they may use different colour spaces. Colour models represent the full spectrum of colour; a colour space is a gamut, or range of colours within that model.

No digital camera or computer can reproduce the full range of colours seen by the human eye (neither can film). In some cases, the colour space is the maximum range of colours that can be handled by the device. However, you can also set some devices to a colour space that matches the capabilities of another device.

It can be a problem if different colour devices use different colour spaces. Imagine viewing the same colour file on different colour monitors and seeing very different results. You would not know if the image needed any correction before printing.

The sRGB colour space was developed specifically to overcome this problem. The ‘s’ is short for ‘standard’. Most computer monitors can be calibrated to sRGB.

By default, EOS cameras shoot with sRGB colour space. This means that the range of colours captured by the camera matches the range of colour that can be displayed by most computer monitors.

However, the monitor can only display a relatively limited range of colours - by no means the full range that can be seen by the human eye. Shooting with sRGB is a little like shooting with a contrasty colour film - the results can look good, but they lack the depth and range of more subtle film emulsions.

By their nature, sRGB images are ideal for displaying on a computer monitor, which means they are the first choice for images shot for use on websites. They also give good results when output to ink jet printers. If you are shooting for publication in books or magazines, however, it helps to have a wider range of colours available.

It is for this reason that Adobe - the leading publisher of software for commercial photographers, designers and printers - has introduced an alternative RGB colour space. Called, not surprisingly, Adobe RGB, it has a wider gamut than sRGB, though still does not cover the entire range of colours visible to the human eye.

Printer devices

RGB is excellent for capturing digital images, and for displaying them on computer screens. It fails miserably, however, when you want to output your images to paper. This is because images on paper are created with inks or dyes.

Mix red, green and blue light of equal brightness and you get white light. Mix equal amounts of red, green and blue ink and you get a muddy brown colour. With RGB light, the absence of colour gives black; with paper, the absence of colour leaves white.

This means that a totally different colour system is needed for printing. It is provided by cyan, magenta and yellow inks. These are actually the complimentary (opposite) colours to red, green and blue. Cyan and magenta form blue, magenta and yellow form red, cyan and yellow form green. Mix all three inks together and you obtain black.

Well, almost. The three colour inks together do not provide a pure black, so most printing processes also include a black ink to make up for this deficiency. This gives the cyan, magenta, yellow, black colour system - or CMYK. (‘K’ is used for black, because ‘B’ would be confused with blue. Why ‘K’? It is probably short for ‘key’, referring to main, or key, black plate on printing presses, to which plates for the other colours must be registered.)

In theory, full-colour images can be printed using just three inks - cyan, magenta and yellow (CMY). In practice, the black created by mixing these three colours is a dark brown, rather than a true black. A black ink (K) is usually added to compensate for this, giving the four-colour CMYK printing process.

Most print output uses the CMYK system. Cyan, magenta, yellow and black inks or dyes create a wide gamut of colours, producing a fair representation of the original scene. However, CMYK is the exact opposite of the RGB system used to capture digital images, and this causes a few problems. Blue, for example, which is a primary colour in the RGB system, is only a secondary colour in the CMYK system. It is created by mixing cyan and magenta inks, but this does not produce really rich blues. You will find it difficult to match the blue you see on a computer screen to the blue reproduced on an ink jet print. There are also problems reproducing other colours accurately, which is why some ink jet printers now use six or more inks: light magenta, light cyan and others in addition to the standard colours. This introduces subtle shades and gradations missing from four-colour printing.

Colour management

It is possible to use three colour spaces in the progress of a digital image - Adobe RGB in the camera, sRGB on the computer monitor and CMYK for the printer. This can give problems. The range of colours captured by the camera is larger than the range of colour that can be displayed on the computer monitor. And how do you match the output to either when a totally different colour space is used for printing?

The answer is colour management. This is a system that produces consistent colours across different digital devices.

The management of colour and colour spaces is being made easier by software such as Exif Print. This adds tags to JPEG files, recording such information as the exposure time, light source, use of flash, white balance, exposure mode, subject distance, contrast, saturation and sharpness. This data removes much of the guesswork from the operation of the printing application. The resulting prints are usually much closer in colour to the original scene. An imaging application - Easy-PhotoPrint - provides Exif Print support to many Bubble Jet printers. Digital cameras with Exif Print support include the EOS-1Ds, 1Ds Mark II, 1D Mark II, 1D Mark II N, 5D, 10D, 20D, 30D, 300D, 350D, 400D and D60. In addition to handling Exif Print data, Easy-PhotoPrint opens image files in the same colour space as the camera originally stored the image. This can provide the printer with enhanced colour information, giving more life-like colours.