Perceptually Uniform Quantities in Graphic Technology

Abstract

The latest versions of ISO graphic technology standards recommend using approximately uniform “colour tone values” (CTV) related to reference colour stimuli by an inverse cubic function to control colour reproduction in printing. These recommendations have sparked discussions about the concept and term “tone value”, which until now in Russia has been associated only with the “dot area” of halftone elements, and the concept of “perceptual uniformity”, which has traditionally been associated with a logarithmic or inverse cubic relationship between the perception of lightness and the light stimulus. Currently, the generally accepted assessments of differences between colours in graphic technology are the CIELAB and CIEDE2000 colour differences, on the basis of which the author proposed formulas for calculating «uniform tone values». The article proposes a method for assessing the uniform-contrast values associated with printing tones, based on the deviation from a linear relationship with the accepted uniform-contrast value. The value adopted for this purpose is the “tone value by CIELAB colour difference”, which provides practically uniform colour differences between tones. A technique for visualizing single colour scales with linear tone rendering of different types of tone values is proposed. The research was carried out using reference characterization data sets for offset printing on coated paper FOGRA39 and FOGRA51. The research showed that “optical density” and “lightness” are not uniform-contrast values for process colours (such values must take into account all characteristics of visual perception), “colour tone values” CTV have significant deviations from uniform-contrast, and the calculation and evaluation of uniform-contrast tones is most accurately carried out on the basis of the CIELAB and CIEDE2000 colour differences themselves. However, they also do not allow one to obtain tones of process colours that can be clearly assessed as uniform-contrast. The research cast doubt on the fact that generally accepted mathematical models based on the inverse cubic dependence of the visual perception of light and colour on light or reference colour stimuli, on the uniform CIELAB colour space or the CIEDE2000 colour difference, describe the relationship between visual perception and stimuli with sufficient accuracy for the tones of process colours.