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9

Characterization of Printers

9.1 Introduction

Some general comments regarding characterization can be found in Chapter 7,
Section 7.1. Physical models tend to play a more important role in the
characterization of printers than they do with other imaging devices. One reason
for this is that the relationship between printer inputs and CIE tristimulus values
is usually extremely non-linear. In addition, however, there is a great deal of
theory that has been developed to predict the colour of printing inks from
colorant concentration values in a wider context. The Kubelka–Munk theory,
for example, has been used for more than half a century to predict spectral
reflectance from colorant concentration values. Artificial neural networks have
also been used quite widely to find mappings between vectors of colorant
concentration values and spectral reflectance values. Numerous technologies are
used in printers and this is another reason why different and specific models are
used to characterize the devices. Most printers use three or four primaries: cyan,
magenta, yellow and black. Note that the primaries of a subtractive colour-
mixing process are quite different from those (typically red, green and blue) for
an additive colour mixing process. For both additive and subtractive devices the
primaries are normally selected to enable the greatest gamut of colours to be
reproduced. In a subtractive process, the intensities of the red, green and blue
light in the print are indirectly controlled by the amount of the cyan, magenta,
and yellow ink deposited, respectively. Some printers – typically dye-sublimation
printers – operate by depositing a layer of ink where the thickness of the ink is
varied to control the colour of the print. Other printers, however, such as most
laser printers, use a half-tone process. For half-tone printers a fixed thickness of
ink is deposited in a pattern of dots, and tonal and colour variation is achieved
by varying either the size or the frequency of the dots. It is not unreasonable
therefore that different physical models are used for different printers depending
upon the technology that the printer uses. Nevertheless, the aim of printer

Computational Colour Science Using MATLAB. By Stephen Westland and Caterina Ripamonti.
& 2004 John Wiley & Sons, Ltd: ISBN 0 470 84562 7