Printing – Processes – Position or alignment
Reexamination Certificate
2000-10-10
2003-06-10
Barlow, John (Department: 2853)
Printing
Processes
Position or alignment
C400S074000
Reexamination Certificate
active
06575095
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of image rendering by means of printing devices, particularly multicolour output devices; the invention especially concerns calibration of these devices.
BACKGROUND OF THE INVENTION AND DEFINITION OF TERMS
A “colorant” designates in this document an independent variable with which a printing device can be addressed. A “colorant value”, denoted as c, is an independent value that can be used to control a colorant of the printing device. The colorants of an offset printing press, for example, are the offset printing inks. It is customary to express the range of physically achievable values for the colorants of a device in %, which means that usually the colorant values range from c=0% to c=100%. In graphic arts, colorant values are often called dot percentages. A “colorant hue” is a basic colour of the printing device; the colorant hues of a traditional offset printing press and of a CMYK printer are cyan, magenta, yellow and black (as is customary, in this document C represents cyan, M represents magenta, Y represents yellow, K represents black and W represents white). The “hue” of an object denotes whether its colour appears red, orange, yellow, green, blue, or purple (or some mixture of neighbouring pairs in this list). “Hue” is also discussed under the ‘definition of remaining terms’ further below. A printing device with n colorants, wherein n≧1, will also be called below a “printer” or an “n-ink process”. A printing device with colorants of at least two different colorant hues is called a “multicolour output device”. An example of a multicolour output device is a CMY printer.
A “colorant space” is an n-dimensional space wherein n is the number of independent variables that are used to address the printer. In the case of an offset printing press, the dimension of the colorant space corresponds to the number of inks of the press.
A “colour space” is a space that represents a number of quantities of an object that characterise its colour. In most practical situations, colours will be represented in a 3-dimensional space that reflects some characteristics of the human visual system, such as CIE XYZ space (see “The Reproduction of Colour in Photography, Printing & Television” by R. W. G. Hunt, Fountain Press, England, fourth edition, 1987, ISBN 0 85242 356 X, sections 8.4 and 8.5 for CIE XYZ; this book is referenced to below as [Hunt]). However, other characteristics can also be used, such as multispectral values that are determined by means of a set of colour filters; a typical example is an m-dimensional space of which the axes correspond to densities.
A “colorant gamut” or “colorant domain” is the delimited space in colorant space of the colorant combinations that are physically realisable by a given printer.
A “printer model” is a mathematical relation that expresses the printer's output colour values as a function of the input colorant values for a given printer. The input colorant values are denoted as c
1
, c
2
, . . . , c
n
, wherein n is the dimension of the colorant space.
Because of the close relationship between an n-ink process and the printer model, the operations that are typical for a printer model are also defined for the corresponding n-ink process. The transformation of an n-ink process to colour space is equivalent to the transformation of the corresponding colorant domain to colour space by making use of the printer model.
A “colour gamut” is the delimited region in colour space of the colours that are physically realisable by a given printer, while also taking into account possible extra limitations on colorant combinations. Take for example a CMY output device. A CMY process is a three-ink process. The colorant gamut
16
is a cube in the three-dimensional CMY colorant space
17
, as shown in FIG.
1
. The colorant combinations in this domain
16
are transformed to colour space
18
by the printer model. The range of this transformation is the colour gamut
19
of the three-ink process. This transformation is represented in FIG.
1
.
A “densitometer” is a photo-electric device that measures and computes how much of a known amount of light is reflected from—or transmitted through—an object, e.g. a receiving substrate such as paper or transparency film. A densitometer usually outputs a single value, i.e. a “density”. In most densitometers, a colour filter selected from a set of available filters can be put into the light path to limit the used light to the wavelengths that are relevant for the colour of which the density is to be measured (see e.g. “Offsetdrucktechnik” by Helmut Teschner, seventh edition, 1990, Fachschriften-Verlag, Fellbach, ISBN 3-921217-14-8, pages 542 to 549, for more information on densitometers and densities).
A “calorimeter” is an optical measurement instrument that responds to colour in a manner similar to the human visual system (i.e. the human eye): a calorimeter measures the amounts of red, green and blue light reflected from an object, as seen by the human eye. The numeric values of the colour of the object are then determined in a colour space, such as the X, Y, Z values of the object's colour in CIE XYZ space.
A “spectrophotometer” is an instrument that measures the characteristics of light reflected from or transmitted through an object, which is interpreted as spectral data. To compute spectral data, a spectrophotometer may examine a number of intervals along the wavelength axis, e.g. 31 intervals of 10 nm, and then may determine for each wavelength interval the reflectance (or transmission) intensity, i.e. what fraction of the light is reflected (or transmitted).
Device Calibration
In general, colour is specified in a colour space that reflects some characteristics of the human visual system. Typical examples are CIE XYZ and CIELAB, but many more spaces exist such as appearance models (e.g. CIECAMs). In CIELAB space, a colour is represented by its three-dimensional co-ordinates (L*, a*, b*). Printers, however, cannot interpret colours specified in these spaces and hence conversions have to be made from such a space to the colorant space of the corresponding printer, e.g. from CIELAB space to CMYK space. This involves characterisation of the printer; see also FIG.
4
.
“Characterisation” of a printer is concerned with modelling the printer so as to predict the printer's output colour values as a function of the input colorant values for the printer. The object of printer characterisation is not to change the device, but to describe how it works. Before a printer is characterised, it is first “calibrated”, which means that the printer is put in a standard state; this is discussed below. Then, a characterisation target is printed by the printer. A characterisation target consists of a number of colour patches that are usually defined in the colorant space of the printer; a typical example of a characterisation target for a CMYK process is the IT8.7/3 target. To print a specific colour patch, the (CMYK) colorant values that correspond to the specific colour patch are used to address the printer. The colour patches of the printed characterisation target are then measured to determine their colour values, e.g. their colour co-ordinates L*, a* and b* in CIELAB space. A measuring device such as a colorimeter or a spectrophotometer may be used to determine these colour values. Based on the input colorant values, used to address the printer, and the measured corresponding output colour values, a printer model is created that predicts colour as a function of colorant values. This printer model is inverted to generate a “characterisation transformation” that transforms colours from colour space to colorant space. The characterisation transformation may be implemented e.g. as a “characterisation function”, or as a “characterisation table” supplemented with interpolation techniques. As an example,
FIG. 4
shows a characterisation table
40
that transforms colours having co-ordinates (L*, a*, b*) in CIELAB space to colorant values
Hoof Dirk Van
Mahy Marc
Agfa-Gevaert
Do An H.
Merecki John A.
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