Improving Print Standards by Specifying Isometric Tone Reproduction for the Overall Process

With the advent of computer-to-plate (CTP) and complementing digital proofing, color variability among printers has increased significantly. This variability is due in large part to the addition of a new variable to the plate-making process. This new variable is the ability to apply tone reproduction (TR) or "dot gain" curves in the plate imaging process. Printers employ these tone curves in a great variety of ways. Industry guidelines such as SWOP and GRACOL provide very little guidance as to the tone reproduction of the final printed product. CTP systems could be used to calibrate a printing process to match a tone reproduction specification. TR curves are normally computed individually for each process color, using density measurements. With normal differences in ink trapping, this individual color method can lead to gray balance errors. Our method of computing TR curves uses an "isometric tone scale" of equal values of overprinting cyan, magenta, and yellow. The paper describes how to compute TR curves using colorimetric measurements of this isometric tone scale. We learned that "isometric tone curves" provide a useful method for calibrating different printing systems to match a benchmark proof. The study demonstrates that tone reproduction and gray balance are effective methods of controlling a visual match from press sheet to proof. We suggest that existing printing standards could be improved substantially by the addition of a tone reproduction and gray balance specification.