The Evolution of “Tone Value” Parameter in Graphic Technology Standards

Standards governing the control process for the production prints establish “tone value” as one of the primary parameters determining its visual characteristics. The parameter is one of the most uncertain and ambiguous in printing colour reproduction: over the course of several decades, the term itself, its definitions and calculation formulae have changed. At the same time, this parameter plays a key role in printed product quality control - adjusting the colour reproduction of printing system to reference printing conditions. The draft fourth edition of ISO 12647-2 provides a rather controversial definition of "tone value" as an indication of apparent intensity (brightness or lightness), and proposes the use of approximately uniform visual spacing of printed tones calculated using a formula unrelated to the generally accepted uniform CIELAB colour space and colour differences. The paper deals with retrospective analysis of "tone value" based on standards, demonstrating its development trend from apparent dot area and density to uniform visual spacing of the printed tones and colour. The analysis of the calculation formulae made it possible to identify the physical and mathematical meaning of the parameter and draw the following main conclusions. The printed tones are associated with the gradation of the surface concentration of the light-absorbing substance, the model of which is the printed single colour scales. The use of a halftone screen is only one way to form such a gradation. The printed tones of real inks change all the reference colour stimuli of visual sensations and the perception of colour itself, and not just lightness. The result of this paper was the concept of “uniform tone value” as a percentage indicator of visual perceptions - colour compared to unprinted substrate (paper) and the fully covered (solid). New formulae for calculating «uniform tone value» based on colour differences are proposed, and recommendations are given for the use and control of different types of tone values in production conditions.