Epsonow Magazine

Page 6 - Colour Management

In the last issue of Epsonow, we talked about some of the different colour proofs used in the printing process. For people in business who commission the design and print of paper-based documents and marketing material, the temptation sometimes exists to approve material based on what they see on the designer's screen. Unfortunately, when they take receipt of the final print, giving in to that initial temptation often results in an expensive error when they discover the colours that were so appealing on the screen are nothing like those that have been printed.

A little bit of explanation is required. Firstly, it should be understood that colour devices, such as monitors and printers, simulate the range of colours visible to the human eye. The two main methods used to create colours in the digital environment are additive and subtractive.

Shedding Light on Additive

As the name implies, simulating colours under the additive model is based on combining, or adding, varying amounts (0 - 255) of primary colours. In this case, the primary colours are the familiar Red, Green and Blue (RGB). An RGB value of 0, 0, 0 results in the absence of any primary colours - black. At the other end of the scale, 255, 255, 255 will generate a simulation of white.

A familiar system that employs additive colour is the theatrical lighting system, where a single set of lights consists of three independent lights - red, green and blue. In order to cast a white light upon the stage all three lights are set to converge on a single point. Similarly, to create a yellow light, the red and green lights are combined. Using different light intensities and combinations, this simple yet highly effective additive model can create an enormous range of colours.

In colour printing, colour photographic prints, the subtractive colour model is employed. This model is based upon the fact that different pigments absorb different light wavelengths. Those wavelengths not absorbed are reflected and picked up by the human eye. Printers such as the Epson PRO Graphics 10000/CF simulate colours by using varying amounts of inks. In this particular case, rather than the traditional four colours (Cyan, Magenta, Yellow and Black - CMYK) an additional two colours (Light Cyan and Light Magenta - Lc and Lm) are used.

So, we have a situation where a document being prepared in a Desktop Publishing (DTP) environment such as QuarkXPress™ contains colour data that will have been derived from a range of different sources - scanners, digital cameras and illustration software. A great deal of this colour data will be stored as RGB data. The problem is getting the printer to reproduce accurately that RGB data in the CMYK or CMYK Lc Lm colour space, and ensuring that the colours chosen by the designer are actually printed.

To achieve this control in the digital environment, the solution is a Colour Management System (CMS). At a top level, a CMS manages the conversion from one colour space to another. It does this by maintaining device-specific profiles, containing information that details how each device deals with colour.

With the Epson Colour Management System, as with many others, the CIE (Commission Internationale de l'Eclairage) colour model is employed. This model is based on the CIE's 1931 definition of three standard primary colours, which can be combined to produce all visible colours. Essentially, by using the CIE colour model as a reference, RGB or CMYK data from a particular device can be converted to another device's colour handling profile.

The benefits of this method encompass also the problem mentioned earlier, where colour simulations differ from one program to another.

What it all boils down to is a case of understanding that what you see is not always going to be precisely what you get. Colour management in the digital workspace is reliant upon numerous sciences and is being constantly researched and refined. It also pays to bear in mind that throughout the process of concept-to-final print, there are usually four different types of proofs that may be presented for approval, these being:

  1. Concept Proof. This is an initial print to show the designer's concept or understanding of what is to be designed;
  2. Design Print. A complete design of the document, but may not include final text and images;
  3. Check Proof. This proof is used to check the accuracy of the text and positioning of images;
  4. Final/Contract Proof. This is the last proof prior to final printing and is usually created by the printing company. It is this proof that should be used as a reference for checking colours.

Printers such as the Epson PRO Graphics 10000/CF simulate colours by using varying amounts of inks (Cyan, Magenta, Yellow and Black - CMYK). An additional two colours (Light Cyan and Light Magenta - Lc and Lm) are used.

Red, Green and Blue (RGB).

An RGB value of 0, 0, 0 results in the absence of any primary colours - black. At the other end of the scale, 255, 255, 255 will generate a simulation of white.