Consistent determination of the contrast ratio of white inks

Translucency is a critical property of print on flexible packaging. It forms the basis for obtaining full color gamut, hiding the contents of a package and even creating the scanner readable digital codes for point of purchase tracking. The print industry has followed the paint and decorative coatings industry, the paper industry and the plastics industry in attempting to identify measurement methods that will accurately describe translucency or its opposite, opacity. Both of these properties are defined in terms of the contrast ratio, the ratio of the luminous reflectance of a coating placed over a black and a white substrate. In this paper, the theory of diffuse reflectance spectroscopy is used to define the measurements used to determine the contrast ratio. A brief experimental study illustrates the strong points and weak points of traditional contrast ratio measurements. A better model of contrast ratio determination is hypothesized, and an experiment is designed and reported that demonstrates both the validity of the hypothesis and a method for obtaining consistent estimates of the contrast ratio.