Surrogates to Status Reflection Density: A Comparison of Variation
Bruce Leigh Myers, Ph.D., Rochester Institute of Technology
Status reflection density has been used for decades as a process control metric for offset lithography as an indirect method of measuring ink film thickness. Many practitioners have presumed that +/- 0.05 D represents common cause variation for the measurement of solid ink density when printing standard process colors. The relationship between ink film thickness and colorimetric values for process color inks is specified by ISO 2846, however this standard does not address process control parameters. It does, however, outlines a useful methodology for evaluating the relationship between ink film thickness and colorimetry.
Today, printers are adopting alternative process control metrics, including L* and spectral density to maintain consistently in offset printing. For example, foil substrates may require that printers adopt spherical geometry instruments and use L* as a surrogate for density, and the increased adoption of extended gamut printing together with the limitations of status density when measuring spot colors has led to spectral density being used in place of status density
The presently proposed study analyzes the agreement between status density and the alternative metrics when measuring colors that vary in density from different sources, including press sheets, ink proofs made with a wedge plate, and density variation as produced with a Kodak Approval. Standard process color inks and select spot color inks will be utilized in the analysis, and each metric will be measured with several available instruments. In addition, the variation inherent in each method will be reported, and the aforementioned methods of reproducing density variation will be compared with a goal of documenting limitations to each. Resulting data will be analyzed using methods defined in ISO-2846 and by using Tukey Mean Difference plots when appropriate.
- For process and selected spot colors, what is the difference in colorimetric versus density variation when comparing status density and L* using directional (0/45 or 45/0) instruments?
- For process and selected spot colors, what is the difference in colorimetric versus density variation when comparing status density and L* using spherical (d/8) instruments?
- For process and selected spot colors, what is the difference in colorimetric versus density variation when comparing status density and spectral density using directional (0/45 or 45/0) instruments?
- Are off-press methods (i.e.: ink proof with wedge plate and Kodak Approval) reasonable alternatives to a press run for evaluating density variation?
An offset press run using a #2 stock will be made with the specific purpose of illustrating variation in spot and process color densities. In addition, ink proofs will be made from those same inks using a Little Joe Ink Proofer with a wedge plate, and the inks will be simulated with a Kodak Approval.
The results will be measured with an array of instruments including Densitometers (e.g. Techkon DENS), SpectroDensitometers (e.g. X-Rite eXact, Techkon Spectrodens, Konica Minolta FD-7) and Spherical instruments (e.g. X-Rite SP64).
Using the average of the status density readings from the densitometers and spectrodensitometers for each respective printing method, comparisons will be made to the surrogate densities (i.e.: L* directional, L* sphere, and spectral density). The graphing procedure outlined in ISO-2946 to plot colorimetric variation versus ink film thickness will be modified to indicate colorimetric variation versus density, and the areas of the respective rectangles will be used to indicate the relative applicability of the surrogate methods when compared to status density in terms of process control.
Further, using the status density readings as benchmark, each surrogate method will be evaluated for systematic bias d through an independent samples t-test, and proportional bias analyzed with an analysis of a Tukey Mean-Difference plot.
In addition, the proofing methods will be compared to the press run to assess the appropriates of these methods in simulating ink film thickness. Here, systematic bias will be analyzed through an independent samples t-test, and proportional bias analyzed with an analysis of a Tukey Mean-Difference plot.