Back in the dark ages of densitometry we utilized calculated values like Hue Error and Grayness to answer the above question. The reality we have been dealing with at ColorMetrix over the last several years is that spectral data will ultimately rule in our industry. While I do not expect press operators to start looking at spectral curves in making all their decisions, I can see some cases like the determination of yellow ink contamination that make a compelling case for purchasing instruments that are capable of displaying or collecting spectral curves.
While traveling last week, I was informed by a savvy technical associate from an ink manufacturer that looking at the spectral curve of the yellow ink on press is a very important part of most visits he makes to customer sites to assist in solving technical problems. The area of the curve to look at is from about 550 nanometers to 650 nanometers. If there is a dip in this part of the curve you are most likely dealing with a contaminated yellow.
I was able to create the image above, because our ProofPass.com solution now includes the ability to make single measurements utilizing an EyeOne. So the “Library Reading” (blue line) in the image above is an ideal yellow ink (uncontaminated draw down) and the “Sample Reading” (red line) is an actual press sheet. Notice the dip in the red line from about 540 nanometers to 660 nanometers. This show contamination of the Yellow ink and has of course changes the gamut vs. printing with the ideal yellow.
I also learned that this contamination is in part due to the changing formulation of inks. As printers demand higher and higher pigment loads in ink, the ink film thickness we put on the sheet becomes less and less. Well, with less ink film thickness it stands to reason that our lightest color ink is going to be more susceptible to contamination be it from other fountains of even from an excessively “dirty” sheet.
As always thanks for reading, and do not forget to look for us at the PIA/GATF Color Management Conference in Phoenix December 4-6 if you are attending.