Colour management systems are designed to address the Problem of device variabilitv, adiusting: the colour rela- ~ usually provide profiles for a wide range of devices from ‘ ,’ I ” tionihips between devices to ensure consistent colour throughout the publishing process. A CMS translates colours ~ which the can select. from the colour-gamut, or colour space, of one device into a ‘neutral’ device-independent colour space, and then fits that colour information to another device’s colour gamut by a process called colour mapping.
The CMS obtains the colour characteristics of each device from its device profile in one method, the relationship between colours is preserved as they are shifted into the device’s colour gamut. In another method, only the out-of-gamut colours are replaced by colours that the device can produce, without preserving the relationships between the colours. Profiles for the most popular devices are usually supplied with the CMS software and those matching the devices on the user’s system are installed at the same time as the CMS software.
Profiles for other devices are usually supplied with the device installation software. Manufacturer’s device profiles are based on a particular set of calibration settings for a given device. To use a colour management system effectively, devices first have to be calibrated to match the expected performance defined in the device profile.
The quality of the final result depends on how well the devices match their profiles. There are various methods and techniques used for calibration of devices, the necessary instructions and software often being bundled with a CMS or with individual application programs. The following provides an overview of the calibration process.
Since a scanner’s light detectors are affected by prolonged use, the RGB output signals will vary over time, affecting colour balance and linearity. This means that updating the scanner’s profile is needed, from time to time, by recalibration of the scanner. Scanner calibration requires a target – sheet of colour swatches and a data reference file, both supplied by a vendor.
The target sheet is I first scanned to produce a TIFF scanner’s output, and software then compares the values in the TIFF file with the values in the data reference file. Any differences which are detected are then used to update the profile for the scanner