Hard Copy Output Devices
CMYK inks are also subject to variations. The inks used in Europe are different than inks used in the Americas and different still from those in Asia.
Paper stocks vary greatly along with coatings. These further alter the printed colors.
Different desktop printers use their own proprietary ink pigments.
Digital Printers and Proofing Devices
A test target is printed and the colorimetric values are measured.
This defines the transformations from the device CMYK colorants to the CIE values.
The device profile is then calculated and saved.
The ink densities and dot gain need to be controlled.
The uncorrected dot gains may be very high (~30%), but usually will be accounted for in the profile.
SWOP and Eurostandard specify different hues.
Compare test image with original and target press.>
Imagesetters/Platesetters
Laser beam intensity along with processing (film and/or plate) variables need to be calibrated.
Need sufficiently high densities (~3-4) in solid regions along with linear tints (no dot gain).>
Printing Presses
Most printers work with SWOP or Eurostandard inks. These differentiate between coated and uncoated paper.
Dot gain can be controlled somewhat by adjusting ink viscosity.
Remaining dot gain can be compensated for by employing appropriate correction curves (in software) or transfer functions (in PostScript PPD).
Accurate and valid measurements are essential to profile creation and a single error in several hundred measurements can destroy the quality of the profile.
Thus, recalibration of instruments and reprofiling of output devices should be performed periodically.
Workflow Issues
A CMS controls conversion to CMYK in a transparent fashion.
It uses device profiles (obtained from above discussed calibration processes) defining transformations to and from CIE spaces and handles profile mismatches.
Input profiles may be embedded into image files such as TIFF and EPS.
The embedded profile passes the information to the CMM.
Recall that color conversion occurs at various stages of the process.
With ICC profiling some of the possibilities are:
1. The image is converted through a profile to profile or mode change using an image editing program.
2. The image is converted to a device-independent color space for storage.
3. The image is printed from an ICC-aware program such as PageMaker, InDesign or Photoshop, selecting the appropriate source and destination profiles in the print dialog box.
4. The image is sent to a RIP driving a platesetter or imagesetter, with the profile embedded in the file.
Profile Quality - Generally an average DE value around 1 is considered good.
