Stumbling Through The Jargon... Part Five


The last couple of articles have looked at image and scanning resolutions, this month it's time to move on to the various colour models, such as RGB, HSB, and CMYK.

Just what is a colour model? Well, it's the way colour can be mathematically defined. The most common are Red, Green, Blue (RGB), Hue, Saturation and Brightness (HSB), Cyan, Magenta, Yellow, Back (CMYK) and CIE L*A*B.

With the HSB model all colours are described in terms of three fundamental characteristics, Hue, Saturation and Brightness. Hue is the wavelength of light reflected or transmitted from an object, although more commonly, hue is know as the actual colour, such as red, yellow, or blue. Hue is measured as a position on the standard colour wheel, and is described as an angle in degrees, between 0 to 360.

Saturation is the amount or strength of the colour (or hue). It is measured as a percentage from 0 to 100 percent. At 0% the colour would contain no hue, and would be grey, at 100%, the colour is fully saturated.

Brightness is the lightness or darkness of the colour, again measured as a percentage between 0 to 100%. If any hue has a brightness of 0%, it becomes black, with 100% it becomes fully light.

A large number of the visible colour spectrum can be represented by mixing the three primary colours, Red, Green and Blue, known as the RGB colour model. This model is the one used by most computer monitors, TV screens, graphics cards and lighting effects. Mixing different amounts of each of the red, green or blue, creates different colours, and each can be measured from 0 to 255. If all red, green and blue are set to 0, the colour is black, is all are set to 255, the colour is white.

The CMYK colour model is based on printing and ink absorbing into paper. To gain the greatest number of printable colours, from the fewest number of inks, CMYK colour printing is used. By using varying amounts of Cyan, Magenta, Yellow and Black, a great number of colours can be printed. Most full colour printed materials, magazines, posters, packaging, are printed using just the 4 CMYK inks. Here the level of ink is measured from 0% to 100%. As an example, orange would be represented by 0% Cyan 50% Magenta 100% Yellow and 0% Black.

Whereas most 'on screen' colour is based on the RGB model, most full colour printing is based on the CMYK model. One of the main principles to understand is that some colours can only be represented by RGB, and cannot be replicated by the CMYK process. Images may look bright, with full saturation on screen, however the colour may be out of the range of the CMYK process. This is why images may look far different when on screen to when printed.

So when would I use each model? All 'on screen' images are RGB, whether it's for web pages, multimedia or games. CMYK is usually only used for full colour printing on a printing press, or possibly for printing to a desktop colour printer.

An image can be converted from RGB to CMYK using a program such as Adobe Photoshop, then re-saved as a CMYK version, or it can be done via a desktop printer at the time of printing. An RGB image printed to a desktop CMYK colour printer will be converted to CMYK by the printer, at the time of printing, to allow it to print.

There are many colours that cannot be created using CMYK, but may need to be printed. In such cases, for printing press work, a single ink known as a 'Spot Colour' is used. There are many families of spot colours, one of the most common is the Pantone Matching System. A Pantone Swatch book contains samples of the colours available, indicated by a colour number, and when a colour is decided upon, the Pantone number can be entered into DTP programs such as PageMaker or Quark Xpress. Then, when the document is printed on the printing press, the same Pantone ink is used as specified in the DTP program.

Spot inks can be used for colours that cannot be created with CMYK, and also when only a single colour is needed, and the cost of CMYK is too high. For instance a document may be created using, say, two inks, black and Pantone 280 (a blue), rather than using the 4 CMYK inks to create only a single black and blue.

Other spot inks include metallic silver and gold, and the bright fluorescent colours. In some cases, a combination of the CMYK process and spot colours can be use, although this really pushes up the cost. A recent trend for magazines, has featured full colour covers (CMYK) with additional metallic silver and fluorescent colours, both spot inks, totalling 6 inks.

A recent development in printing, is the HexaChrome process. This is an extension of the CMYK process, but with an additional two inks, to allow for further colour range. However, most printers and most full colour work is still achieved with only the 4 process CMYK inks.

To predict how images will print with CMYK inks, programs such as Adobe Photoshop will display the image based on the type of inks and printing press to be used. Varying the inks set-up, will vary the display, giving an approximation of how the image will finally print. However, there can be a big difference between how a colour or image may look on screen compared to the final printed version.

It's highly recommended that a colour swatch book, such as a Pantone Guide or Trumatch Swatch book, or any of the CMYK process guides are used. These guides are ideal for choosing colours, far more reliable then choosing 'off screen', a colour is picked from the guide, and it's CMYK ink values or spot colour number is entered into the DTP package. The colour on screen may look different to the printed guide, but at least you'll know it'll print correctly.

Next month it'll be time to look at just how full colour printing is achieved with just the 4 CMYK inks, and the whole process of printing, from computer screen to printed page. In the meantime, please feel free to email me with any comments or queries you may have, and happy DPT'ing.


Home Andy's Columns

Copyright © 1995-1998, SVCG. All rights reserved.