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Colour: The Challenge of Matching Newsprint Shade


September 1, 2002
By Pulp & Paper Canada

ABSTRACTThe Newsprint Association of America has contracted the production of “NAA Newsprint Shade Tolerance Cards” to be visually matched under specified illumination by newsprint producers. Painting…

ABSTRACT

The Newsprint Association of America has contracted the production of “NAA Newsprint Shade Tolerance Cards” to be visually matched under specified illumination by newsprint producers. Painting on board produces a permanent colour. Moreover since the colourants used in the coating formulation are unavailable to the papermaker, papers that match their L*a*b* coefficients or visually match under one illumination, can be expected to mismatch under end use conditions. Papers matched to the card will match each other robustly only if their spectra deviate in the same way from the spectrum of the card. Papers with very different DIP content are hard to match. To minimize this problem, it is recommended that more than one target shade be available covering different levels of DIP content. Printers can further improve this match by using papers in the compatible ERIC values.

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A colour problem

From one perspective all newsprint is the same colour: The colours of all rolls of newsprint occupy only a tiny fraction of the space of all colours we can see. However, our eyes happen to discriminate subtle differences in shade within this tiny range of colours spanned by newsprint. In particular, we can notice the colour difference between adjacent pages in a newspaper if the colour match is not precise. Still, a printer wants to order rolls from different corners of the continent made on very different paper machines from different wood species and then wants to interleave pages from these different rolls. How can that printer specify colour expectations to all suppliers so they all can match the same target shade? One solution would be to send each a swatch of material to match as done by other industries. However, newsprint discolours too quickly. Alternatively, we can specify the absolute colour coordinates of the target shade. An instrumental match to these precise colour coordinates is equivalent to a visual match under the light of standard illumination. Specifying absolute colour coordinates has met with mixed success because of at least three related challenges:

1. Colorimeters have to be both very precise and very accurate,

2. A given target may be hard to match with the furnish and dyes available to different mills, particularly if the mill uses a different content of DIP (deinked pulp) from the target,

3. Matching under one standard illumination does not guarantee matching under different lighting.

The third problem is termed “metamerism” and occurs when materials may look the same colour when viewed in light where their colour coordinates match, but look different colours under other lighting. Newsprints in general, and recycled newsprints in particular, are likely to exhibit metamerism. The best way to overcome metamerism is to match the entire reflectance curve to the target shade, and not just to match the colour coordinates.

The NAA solution

Frustrated by the specifying absolute colour coordinates, the NAA (Newspaper Association of America) commissioned the manufacture of shade references made by painting onto card. The user places the card over the paper, and views the newsprint through a pair of slots cut into the card. The region between the slots is painted the target colour. Around the outside are comparison regions indicating “too red”, “too green” etc. Figure 1 shows the spectrum of the target colour (dark line) along with the spectra of the tolerance targets.

The user matches the newsprint to the shade card in a light booth adjusted to CIE Illuminant D50, which simulates indoor illumination, and a graphic in the corner of the card is so designed to indicate if the illumination is close enough to D50.

To the extent that the end-use lighting matches D50, all papers that match the card should match each other. At least in one common condition the colour-matching problem is solved without the use of finely tuned instrumentation.

To the extent that the end-use lighting differs from D50, some papers may exhibit metamerism. We expect more metamerism if the papers’ reflectance spectra are significantly different. Herein lies the key issue — the goal was to have rolls of newsprint from different mills arrive at the printer looking close enough in colour that interleaved pages will match. That could happen even if the papers are all metameric with the shade card, as long as the papers approximate the shade card at D50 with similar furnish and dye. We therefore look at the separate tasks of matching the card and matching other papers.

Can the reflectance spectrum

of the NAA card

ever be matched?

Colour in the shade card comes from coating that uses colourants not found in real paper. The best fit to components of paper may not be capable of following the curves of the card’s reflectance spectrum. We illustrate with Figure 2, showing the spectrum of real newsprint along with the target spectrum of the card. The reflectance spectrum of the shade card exhibits at least three challenging features:

1. In the blue part of the spectrum between 400 nm and 500 nm, the newsprint spectrum rises monotonically with a smooth convex-downward curvature. This part of the spectrum arises from the tail of an absorption peak in the ultraviolet caused by a lignin chromophore and controlled by bleaching. The spectrum of the card contains a kink at 425 nm that cannot be matched by the chromophores of real paper.

2. Less challenging, but still problematic, is the deep absorption peak at 580 nm in the target spectrum. In the newsprint example, a pair of dyes absorbing on either side of 580 nm produces a broader reflectance minimum at 580 nm. The spectral match to the card is poor and invites metamerism, but metamerism between papers would be less if papermakers approximated the 580 nm minimum with the same dyes.

3. Least challenging and most problematic is the level of reflectance above 620 nm. Residual ink from recycled furnish largely determines this region of the newsprint spectrum [1]. The reflectance spectrum of the shade card reaches a value characteristic of recycled newsprint, and the paper example in Figure 2 is typical of virgin newsprint. It is very difficult for a papermaker to match the spectrum of another newsprint with a different level of DIP without adding or removing residual ink.

A papermaker could best fit the NAA card using a furnish high in DIP and searching out a dye with a narrow absorption at 580 nm. The fit in the blue end of the spectrum will be mediocre at best.

Ease of matching and fitting

Matching the colour of a newsprint to a target with very different DIP content is problematic because the reflectance spectrum of virgin newsprint rises sharply between 600 nm and 700 nm to a reflectance about 20% higher than the typical recycled newsprint. The tristimulus X value is particularly sensitive to this rise, so a virgin paper matching to a recycled newsprint target would need to be dyed to a lower reflectance below 620 nm to compensate for overshooting the target above 620 nm. This overcompensation causes two problems: it is simply harder to achieve a match in L*a*b* colour coefficients when the DIP component in the furnish is very different; and any match achieved will be more metameric.

The greater difficulty in achieving a robust match to the expectations of a particular customer whose model is very different in DIP constitutes a market disadvantage. As long as there are both customers wanting high DIP newsprint as well as customers wanting low DIP newsprint, ease of colour matching does not skew the market. However, the choice of a colour target corresponding to high DIP furnish by the NAA for universal application throughout the USA is prejudicial to the interests of mills making virgin or low DIP newsprint. Even if the makers of low DIP newsprint meet the requirements of matching the target under the light of CIE Illuminant D50, the match to the target will be more metameric. A second shade control card modeled on low DIP newsprint would return more freedom to the market without sacrificing quality control.

Consequences

for paper metamerism

The paper spectrum in Figure 2 is obviously very different from the target spectrum, however the D50 colour coefficients match precisely ((E*=0.1) and the colours are indistinguishable in a colour booth with D50 illumination. When the lamps in the colour booth are switched to another illuminant, the metamerism is pronounced. This papermaker has succeeded in meeting the NAA’s requirements. Those requirements do not address metamerism. The printer’s goal of interleaved pages from different mills with an unbroken continuum of shade could still be achieved as long as the mills used the same level of DIP and the same dyes. So a printer could specify that suppliers match the NAA card and also that they contain a particular range of DIP and this may well improve the match between interleaved pages in the reader’s lighting environment. Actually it is not only the DIP content, but the reflectance spectrum associated with the residual ink as quantified by the ERIC measurement (Effective Residual Ink Concentration) [2]. Alternatively, the printer could batch incoming paper based on ERIC value and run compatible rolls for the same day.

The inability of real paper to follow the spectrum of the shade tolerance card in the 400 nm to 500 nm range need not cause metamerism between newsprints, as long as all those newsprints mismatch the NAA shade tolerance card in the same way. The same conclusion holds for the absorption feature at 580 nm –newsprints need not be metameric as long as they use similar dye compositions to mismatch the NAA card in the same way. Total elimination of metamerism between newsprints made at different bleaching levels and fines content, from different species, using different dyes and fillers, will remain an elusive goal even if the shade tolerance card is used and paper is sorted by ERIC value.

The role of Illuminant D50

So far we have focussed on the metamerism arising in going from a D50 matching environment to other end use viewing conditions. Another basic metameric problem arises between Illuminant C and Illuminant D50. Current ISO standards call for the measurement of paper colour using Illuminant C, which is based on the light from an overcast sky. This is done for historical continuity and because Illuminant C excites fluorescence about the same as indoor lighting. The red to blue mixture in C (or its newer version D65) differs markedly from the mixture in indoor lighting in general and D50 in particular. D50 places much more emphasis on red light than does Illuminant C. Consequently, papers manufactured to match target colour coefficients measured under Illuminant C will match visually in a colour booth set to Illuminant C, but will likely mismatch when viewed in a booth set to D50. Since the printing and graphic arts community views paper and prints under D50, they see these papers that were supposed to have matched and obviously don’t. Instrumental colour control of paper could regain credibility with printers if all colour coefficients were specified and measured with respect to D50. The ISO would contribute less to the problem and more to the solution if D50 were included in standard practice. Paprican is proposing this change to ISO.

Recommendations

There should be more than one target shade corresponding to different ranges of DIP content. When the printer is choosing among papers matching the shade tolerance card, they would experience less metamerism if they printed a given job on rolls with compatible ERIC values.

When printers and papermakers specify and measure colour with colorimeters, they should use Illuminant D50 in their colour calculations.

Acknowledgements

The author thanks Nancy Somerville for reflectance measurements and Ivan Pikulik for reviewing the manuscript.

References

1. B. Jordan and S. J. Popson, “Measuring the concentration of residual ink in recycled newsprint”, PPR 990, JPPS 20(6):J161-J167 (1994).

2. Tappi Test Method 562-pm97 “Determination of effective residual ink concentration by infrared reflectance measurement”.P&PC

Byron D. Jordan is a principle scientist for Paprican, specializing in optical properties.


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