The primary aim of the upcoming International News-paper Color Quality Club 2002-2004, run jointly for the second time by Ifra, the Newspaper Association of America, and supported also by the Pacific Area Newspaper Publishers’ Association, is to raise day-to-day quality in reproduction and printing.

Andy Williams

In this respect, the importance of calibration cannot be overestimated. Calibration is the bridge that leads to predictable and consistent color reproduction. The keystone of calibration is measurement. So whether you’re interested in joining the INCQC or simply just concerned with color quality in general, the focus of this article — calibration and measurement procedures for prepress and press control — will hopefully be of some help.

Calibrate your instruments

Calibrate means standardizing the performance of the instrument and the units of measurement so that the results are repeatable. Alas, most users rarely calibrate instruments, though the process is quickly performed. (Note: Calibration is not pre-setting a densitometer or spectrophotometer so that paper white is read as zero.) For calibration, if the instrument does not have a built-in white reference and the process is not done automatically, you need a manufacturer’s calibration strip or plaque. This is a white reference area, occasionally accompanied by a black reference area.

In the case of a densitometer, printed ink patches of solid cyan, magenta, yellow and black are required to standardize the response from each color filter. The key action is to zero the instrument on the instrument manufacturer’s white reference element and then match the manufacturer’s specified target values for the four color patches. Follow the manufacturer’s instructions for this. Don’t skip these steps.

Spectrophotometers, in general, and densitometers fitted with narrow band, glass interference color filters, are relatively stable and need a calibration check once a month. However, densitometers fitted with the normal gelatine filters need more frequent calibration, say every two weeks.

Manufacturers’ printed calibration strips become dirty with use. How often they need replacing depends on the amount of use. In a good clean environment, they may need to be replaced every two years; in a production environment, every six months.

For the INCQC, we use a densitometer with glass narrow band filters and the following peak spectral response and bandwidth (standard: according to German Industrial Norm — DIN 16536): Peak: C (620 nm), M (530 nm), Y (430 nm). Bandwidth: C (10 nm), M (10 nm), Y (10 nm). After calibration, we zero the densitometer in an unprinted area of the newsprint.

The International Color Quality Club color hexagon graph (the printed ink gamut).

click to enlarge further

INCQC color hexagon graph

When we measure the print gamut (cyan, magenta, yellow, red, green, blue, black and paper colors shown in the picture above), the density measurements are not used to award points. We are guided by the spectrophotometric measurements of the solid ink patches. However, since it is impractical to manually control the inking on a press using a spectrophotometer, it is useful to find approximately equivalent density values so that simple density measurements can be used to control the press inking levels.

Contrary to what is commonly stated, densitometers are not “color blind.” Using, for example, the Color Quality Club 2000-2002 density and spectrophotometer measurements, average density approximations close to the ISO Standard 12647-3 ink color L*a*b* values can be extracted from the data. These are shown in Table 1 (below).

In the INCQC 2002-2004 evaluation, points will not be lost if the chroma of the color inks is greater than the ISO specification or if the L* lightness value of black is less than the ISO value of L*=40. Therefore, to print the INCQC color hexagon well, printed ink densities should be close to the old repro standards of about D=0.9 for the CMY inks and D=1.1 for the black, with the caveat that the magenta ink density should be closer to 0.95.

These densities are for the narrow-band, polarized filter type of densitometer. For other types, there are different recommendations (see Table 2, below).

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Table 1

* Using a new color difference formula recommended by CIE Technical Committee TC 1-47

So, taking the Ifra densitometer and the larger samples of data, in order to meet the ISO L*a*b* values one should print using densities of cyan=0.90, magenta=0.93, yellow=0.88 and black=0.96. The relatively low density for black results from selecting data that closely matches the ISO specification for black (L*=40, a*=1, b*=4). In practice, probably every ink manufacturer supplies their black news ink with an even darker black than that specified by ISO, i.e. with a L* value lower than 40. In the recommendations of previous national and international printing standards, before the latest ISO Standard, a common density level for many types of densitometer/filter set was a printed density of 1.1. This provided a good level of print quality. From the same data, a density level of 1.1 resulted in the following average L* values:

Data Range                    Average L* Samples

Black Ink Density            Density 1.085 - 1.115 
35.95         01

                 Density 1.05 - 1.15 36.24 53

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Table 2

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The bottom line

These densities should get the printer fairly close to the standard. If you can get a CMY 30:22:22 graybar to look gray too, at these densities, so much the better. Normally this means that some preparation work has to be done on the press to make sure that the dot gain values for the CMYK inks are nearly equal (within the ISO Standard of a 6-percent maximum difference in the midtones for any two inks).

Newspaper publishers can now register entry of one or several titles for entry to the INCQC 2002-2004.