Understanding Colour Management

CALIBRATION - CHARACTERISATION - CONVERSION (SEPARATION) - WORKING SPACE

Colour management is based on a few principles that must be carefully defined in order to understand the mechanisms for applying it.

Regardless of where you are in the graphics chain, there are a number of steps that needs to be taken into consideration. These steps are:

• Calibration
• Characterisation
• Conversion (Separation)

Calibration

To calibrating equipment in the graphics chain means to take it to a chosen operating state and ensuring that it remains in this state.

For example, calibrating a screen consists of using software and the correct measuring equipment (called a colorimeter) to set the screen’s physical parameters; the white point, the contrast and the brightness (see below in Creating monitor profiles). Calibrating an offset press consists in of choosing precise parameters for operating, and as well as monitoring these parameters to ensure that they remain within an acceptable range. Examples of such parameters are the colour readings for the solid colours on the chosen paper and the greyscale curve

 Characterisation  

 Once the equipment is calibrated, characterisation can take place, in other words, the colour identity file can be established. In this process it is determined how your equipment reproduces and presents colours under the present settings. The result of this characterisation is the ICC profile. The profile is a translation table that allows the digital values in your file to be transferred into those needed to obtain good colours on the selected equipment.

Conversion (separation)  

Conversion, whether done in Photoshop with an image by image approach, or as a batch in a more developed work flow, means that the colour values of your document are translated. The values from the workspace in which the documents were created are translated into values that allow the “right colours” to be obtained in the selected printing system.

Each conversion is a mathematical transformation of the files and the values that are changed or lost can no longer be recovered by reversing the process.

A conversion is done every time a file is transferred and processed from one type of equipment to another; e.g. while being transferred from the scanner or camera to your computer, when you open an image in Photoshop on your screen (virtual conversion carried out as a background task), or if you decide to convert your files yourself from RGB to CMYK.

All the other stages of working on the images and documents rely on the colour management but are not part of it. For example image correction and retouching, page layouts, creating pdf’s and preparing plates for offset printing all rely on colour management.  

An RGB working space is a Digital profile created for complete neutrality and good homogeneity when correcting and retouching images. In RGB there are three values R=G=B that ensure perfect neutrality in an image, provided it is described in a neutral and homogenous working space. This is the case in the following well-known spaces: Adobe98, sRGB, eciRGB for example.

The CMYK working space to be used in the colour parameters of your applications, must correspond to the one that you use most often for the jobs that you convert into CMYK and send to your printer. See below for the standard and personalised profiles.