How Many Delta Es Are There in a Delta D?

Process control in the pressroom is currently set up around the idea of meeting specific tolerances in terms of density, not in terms of colorimetry. ISO 12647-2 is changing this direction. This standard specifies the range of acceptable color of the four process solids in terms of delta E, rather then in the traditional Delta D (delta density). As CIELAB makes its way into the pressroom, the question of conversion of tolerances is becoming an issue. The standard does allow for densitometric process control during the press run, but it does not specify equivalent tolerances for density. This paper analyzes data from three press tests on web offset and sheetfed presses, with two different ink formulations, and with a total of thirty printing stocks to show that within a press run, running to a density tolerance will assure a colorimetric tolerance. The results agree quite well with theoretical calculations. A further analysis of hardcopy proofs from twenty-one proofing system vendors demonstrates that single channel density measurements cannot be used to assure a colorimetric match between a proof and a press sheet.