Colour Space and Colour Gamut
As a species, we have pretty good colour vision. Although it doesn’t extend far into the infra-red or ultra-violet spectra, it’s good enough to navigate the world we live in. The range of colours we can see, technically called the colour gamut of our vision, is quite a lot bigger than the gamut a printer can produce and, to a lesser extent, bigger than the gamut a monitor screen can display.
The way colours are represented differs between monitors and printers. With a colour monitor, colours are made up from various levels of Red, Green and Blue light – hence the term RGB – in an ‘additive’ model. This means that as you add more of each colour, the colour gets lighter and lighter until, when you have full levels of red green and blue, the screen should show white.
With a printer, though, things are rather different. Here you have three colour inks: Cyan, Magenta and Yellow, which work in a ‘subtractive’ or reflective way. The light that falls on a printed pages is absorbed or reflected in different amounts by these three inks. If you have fully-saturated prints of all three colours you get black, not white – printers rely on the white of the paper for light colours.
In fact, printer inks are not pure cyan, magenta and yellow, because it’s very hard to achieve these theoretical colours. If you mix them all you tend to get a dark, muddy brown, which is why a true black is added to the colour trio, to get the full range of colours – CMYK. K is used as an abbeviation for blacK to avoid confusion with B for Blue. In a similar way, six-colour photo printers include light cyan and light magenta inks, to improve the range of light colours they can reproduce – to extend the colour gamut for these colours.
Calibration
As you may imagine, mapping all the colours that show on a monitor screen to their equivalents on a printout isn’t an easy job, which is why you may have noticed that a photo or page design looks different – usually not as bright – when printed.
You can improve things by calibrating you monitor to your printer. Some more expensive printers come with calibration software and some high-end graphics applications, like CorelDraw do, too. They’re usually automated systems, where you select the closest matches for a series of printed colours to their equivalents on the screen, or vice versa.
An ICC profile is a standardised form of look-up table of the colours a monitor, printer, scanner or other piece of colour equipment can produce or resolve. Software, such as photo editors and graphics design tools, can use an ICC profile to determine the different shades available when displaying, printing or scanning.
If you do a lot of colour work, it’s worth running a calibration routine, so it’s less of a surprise when the colours your select from an on-screen palette are printed out. Utilities like Monitor Calibration Wizard (www.hex2bit.com) and Calibrize (www.coolsw.intel.com) will do the job of setting up your monitor (for free).
Colour spaces
If you study the rather daunting-looking diagram on this page, you can see how colour spaces work. The whole of the colourful horseshoe-shaped area represents the colours the human eye can see. You’ll notice that all three of the coloured triangles laid over this ‘colour space’ have much smaller areas. This is because neither a monitor nor a printer can get close to the full gamut of colours we can see.
The standard Red Green Blue (sRGB) triangle, with a dark blue border, is a standard colour space defined jointly by HP and Microsoft in the 1990s, to provide a standard set of colours that could be used on devices working with the Internet. Using sRGB, software and hardware developers can design their products and be reasonably sure that the colours they are using are the colours people using their software or hardware will see.
The Adobe RGB triangle, with the orange border, defines a larger colour space needed by the kind of graphics professional its products are designed for. Both these colour spaces are for monitors and use the additive colour model.
The third triangle-ish shape, bordered in yellow, is a typical CMYK colour space for an inkjet printer. As you can see, although it’s bigger than the sRGB colour space, it doesn’t extend as far into the green space as Adobe RGB.
So what happens if you try and print a photo or other colour document which uses colours that fall outside the inkjet CMYK colour space. Clearly the printer won’t be able to reproduce them accurately. It’ll have to choose a colour within its own colour space which most closely approximates to the one you’ve chosen.
This is where ICC profiles come in again, as software can compare the colours its working with, with those the profile says are available on the printer it’s using and make the best choice for mapping one to the other.
Some printer colour gamut basics
It’s no accident that the majority of photos are printed out on inkjet printers. Apart from the fact that inkjets are still quite a bit cheaper than colour laser printers, they also do a much better job of reproducing colours. Partly due to the different printing techniques, where inkjets use liquid ink, sprayed onto the paper and laser printers use powdered ink, melted onto its surface with heat, inkjet print has a much larger colour gamut than laser print.
If you compare prints of an identical photograph printed on both types of printer, you’ll notice the laser print looks exaggerated, with far less variation in some of its colours. This is particularly noticeable with things like sky and sea scenes, which involve a lot of subtle shades of blue and green. Colours that the inkjet printer can render quite naturally are much harder for a laser printer, which has fewer shades of colour to pick from.
It’s not all bad news for the colour laser, though, as its main use is in the office, where good, bold primaries are the order of the day. Even with a smaller colour gamut, these colours can be well reproduced, producing vibrant business graphics and promotional materials at much lower cost per page than from an inkjet.
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