Background information - ink film thickness & solid ink density
Offset printing presses are designed to lay down a film of ink, in the presence of water, onto a substrate - usually paper. The ink forms the image while the "water," more accurately fountain solution," prevents the non-image area on the printing plate from accepting ink. For the process to work, there needs to be a critical ink/water balance with the goal of having an ink film thickness between one micron.
If the ink film thickness is too great, the result can be "ink tailing/misting." In addition, the non-image background may take on ink resulting in "catch-up" (sometimes mistaken for "scumming"):
On the other hand, if the ink film thickness is too thin, the result can be a breakdown of the ink on the sheet causing low contrast, loss of sharpness, and mottle:
So, from a color tolerancing point of view, because the function of ink is to filter light and allow us to see color and because its thickness also effects the integrity of the printing process - ink film thickness on the sheet becomes an important metric to measure and tolerance in presswork.
Bottom: CMYK at low ink film thickness/solid ink density.
There is no practical way to directly measure the ink film thickness on a press sheet. However, there is an indirect way and that is to measure the solid ink density (SID) using an instrument called a densitometer.
Color tolerancing through densitometry
Measuring SIDs in the solid ink patches in the color bar with a densitometer does not actually provide information about the color being printed. However, because it indirectly provides information about ink film thickness (which impacts color and tone reproduction) SID values are valuable for process control and defining variation during a press run where the instrument, ink, and substrate remain the same.
Top: Commercial sheetfed, Middle: Magazine/heatset web,
Bottom: Newsprint/coldset web.
From a color point of view, the assumption is that all three chromatic colors vary in the same direction and therefore remain in relative balance. When that happens there is a shift in color saturation (higher SIDs = higher saturation) as well as tone reproduction (higher SIDs = higher dot gain/TVI). If one color, e.g. Cyan, is at the maximum low point while another color, e.g. Magenta, is at the maximum high then the result may be a visible color bias in the presswork.
While a densitometer can also be used to monitor variations in non-process, i.e. spot/Pantone colors, usually a printed sample of the target color, including a high/low density tolerance reference, is used instead since this helps both print specifier and supplier visualize the acceptable range of color change as SIDs naturally vary during the press run.
Addendum: Densitometer set up - "Status" condition
Densitometers are set by their manufacturers to an industry defined "Status" which defines the total response of the instrument including light source, optics, filtering, and receptor for given wavelength. The primary responses for the print business are "Status E" and "Status T" (ANSI PH2.18 and DIN 16536). In addition, densitometers are available with or without polarizing filters. Dry ink density readings from polarizing and unpolarized densitometers as well as those set to Status E vs Status T will not agree. Typically European instruments are set to Status E and use polarizing filters while North American instruments are set to Status T and do not use polarizing filters.
The important thing to be aware of is that if SID information is shared outside of the printshop - then the Status of the instruments that were used to determine SID values must be known. In addition, it is critical that all instruments within the printshop are set to the same Status. In North America, where many of the presses and their closed-loop color control systems are from Europe, it is not unusual to find the press set to Status E polarized while the handhelds are set to Status T unpolarized which can easily result in quite a bit of confusion in production.