Danny Rich

Consistent Determination of the Opacity of White Inks

Danny Rich, Sun Chemical Corporation

Opacity is also an important property of paints, plastics and papers. So all of these industries have developed some type of index related to the visual percept of opacity. In all of these indices, the measurement includes the determination of a contrast ratio. Contrast is also a perceptual attribute in graphic reproduction, related to the visibility or detectability of print over the background. Examples include the readability of information on packages, printed pages and barcodes. The printing ink business had attempted to adopt the measurement techniques developed by the paint and decorative coatings businesses. This involved laying down a standard film of coating onto a substrate that is partially white and partially black and reading the luminous reflectance (CIE Y tristimulus value) for each of the two areas. The contrast ratio is the ratio of the luminous reflectance of the film over the black area divided by the luminous reflectance of the film over the white area. The ratio is then scaled to 100 to simulate a percentage

The theory behind the use of the contrast ratio is based on the concept that a fully opaque film will not be influenced by the reflectivity of the substrate upon which it is printed. Thus, a film printed over a white substrate and a film printed over a black substrate should have identical reflectance readings and the ratio will be unity or scaled to 100. Since it is unlikely that a 2 to 3 micron thick ink layer will ever be able to produce the scattering power of a 50 to 100 micron paint film or a 1000 micron plastic chip, it will be important to quantify the luminous reflectance of the ink over the black substrate with great precision and accuracy.

In reviewing the current practices in the ink production laboratories, it was observed that several different issues might be contributing to the lack of agreement on the determination of the level of opacity of a printed white ink. The first issue that was noted is the use of various contrast cards on which the inks are printed. There is a range of card types, uncoated, matte coated and glossy sealed surfaces, each designed to interact in a nearly ideal way with the ink that is being tested. Thus, the same ink printed over an uncoated card, whose black area has only a 20% reflectance and over a sealed card whose black area has a 1% reflectance will produce very different estimates of the opacity. The second issue noted was that of measurement geometry. In times past, it was assumed that white materials produced a fully diffuse, uniform flux of reflected light and any reasonable geometry would capture and characterize that flux equally well. Today, we know that this is not the case. So in reviewing the measurement systems used to determine opacity the following geometries are identified in standard documents: 6 inch integrating sphere with diffuse influx of light onto the specimen and the capture of the directional efflux along the normal to the specimen (d:0), directional influx at 15 degrees from the normal to the specimen and an integrated diffuse efflux collection using an integrating cube (15:d) and directional influx at 45 degrees from the normal to the specimen and directional collection of the efflux along the normal to specimen. Finally, there is the geometry that involves hemispherical diffuse influx from an integrating sphere and directional efflux at 8 degrees from the normal to specimen. The first two methods are standardized in the paper industry, the third method is a standard in the graphic arts industry and the last method is used by both the paint and plastics industries.

An experiment was undertaken to systematically study these two sets of effects and determine how they might be related to each other and if possible a correction or correlation function would be derived. This would allow laboratories using two different methods to inter-relate their measurements and specifications in a meaningful way. The experiment involved 27 prints of a white ink on various paper and film substrates and readings of the contrast ratio taken using different types of backing materials, both white and black. The readings were compared to predictions using the Saunderson correction for both internal and external surface reflection and good agreement was obtained between predictions for inks which were in optical contact with the substrate and for those which had no optical contact with the substrate.

A special specimen holder was prepared using 3D additive manufacture that contained an optical trap and a special white ceramic backing plate. The reflectivity of the white ceramic was very near to 100% and the optical trap reflectance was very near to 0%.

Three different instruments, with well-known, but vastly different geometries were used to collect data on the same set of specimens. Statistical analysis of the predictions of the different instruments showed that a simple linear correction for the light lost in the interface would provide a very high correlation between measurements from instruments with different geometries.

An experiment on the assessment of opacity was carried out to better understand the differences between determinations of opacity with spectrodensitometers and the determinations using an opacity meter. The results show that that the majority of the differences are due to the specimen preparation and presentation and not to differences between the instruments. It is concluded that a more consistent or absolute determination of opacity requires that the backing white have a reflectance close to the intrinsic reflectivity of the white ink and that the black area have a reflectance of less than 6% absolute. Guidance is given on how to achieve these “absolute” readings by correcting the measured reflectance of the ink over the white area and over the black area.

Determination of the Whiteness of Paper Substrates Containing Fluorescent Whitening Agents Under CIE D50

Danny C. Rich, Veronika Lovell, and Robert Marcus, Sun Chemical Corporation

Changes to the ISO 3664 standard for the visual and instrumental assessment of print on paper substrates has been causing problems for converters who try to use these standards to assess the quality of print relative to a digital proof.  One possible cause was thought to be a result of the use of CIE D65 illuminant and CIE 10° observer data by the paper industry and the use of CIE D50 illuminant and CIE 2° observer data by the graphics industry.   This study investigated whether the CIE metrics for paper appearance and whiteness are applicable to the substrates used in modern graphic reproduction.  Both visual and instrumental data given in this report indicate that the correlation between the two measurement conditions and the CIE recommendations are moderate to poor and that there needs to be a new index developed for use in grading and specifying production stocks that will match proofing stocks.  It is shown that differences between the print and proof may not be due to the lighting.

A set of 30 white paper specimens were selected, 15 with some level of fluorescent OBA added and 15 without any fluorescent additives. A visual experiment was designed in which observers judged the level of whiteness in a rank order experiment, thus developing the basis for assessing the metric of whiteness.  The specimens were presented to twenty observers and the rank orders of each observer were compared and then averaged.  The agreement between the scale of the observers and the CIE Whiteness index, computed as per ASTM E313 was 0.87 for the papers without OBA and 0.82 for the papers with OBA.  In contrast, the ISO Brightness index gave correlations of 0.93 for the non-OBA containing substrates and 0.87 for the OBA containing substrates. The Ganz equations, adopted by the CIE and recommended in ASTM E313, contain 5 adjustable parameters.  A modern spectrocolorimeter with an ISO 13655 conforming source was used to collect D50 measurement data on the 30 specimens.  The measurements were submitted to a multiple linear regression, just as Ganz had performed, to develop new estimates for the coefficients for CIE D50 and the 1931 CIE standard observer rather than CIE D65 and the 1964 CIE supplementary observer.  The two data sets were analyzed separately and in combination.  When analyzed separately, both data sets, those with fluorescence and those without fluorescence, produced the same set of whiteness index coefficients.  The resulting equations reported correlations of 0.96 for the non-fluorescent papers and 0.94 for the papers with the fluorescent additive.  It can be concluded that the CIE equations do not produce the best agreement to visual judgments under CIE D50 as required by ISO compliant graphic reproduction systems.