The SIDs should be determined at each press after the calibration then recorded for normal press operation. It has been my experience both preG7 and postG7 introduction that when presses are installed and/or curves are calculated that the vendor asks the press operators "what are your standard ink densities?". This question obviously shouldn't be asked as this should be determined by results of hitting the desired LAB target values.
This is where I would disagree with G7.
IMHO solid ink density and Lab values serve different functions and should used accordingly.
The function of a printing press is to lay down a film of ink that is as close a facsimile of the halftone dot on the plate as possible. This is a mechanical function. SIDs are an indirect measurement of the thickness of that film of ink. So SIDs are important to the press operator for that reason.
Presses do not make color. SIDs do not measure color.
Color comes from the ink hue and its relationship with light. Spectrodensitomers measure light and report the hue as a Lab value. This is a perceptual function. The press operator has very little control over the Lab values since they are determined by the choice of ink hue, substrate, and lighting over which the press operator has little control.
I don't think that you can say one is more important than the other because they are separate, albeit related, metrics.
At the end of the day the goal is to align presswork color to the proof. Lab value information can help achieve that goal.
It is reasonable for the vendor to ask the press operators "what are your standard ink densities?" because the answer can provide an insight into problems between prepress and pressroom as well as general print manufacturing and process control deficiencies.
The problem in this industry is that standards like ISO 12647 are very poorly written and the specifications are often ambiguous, or useless as a means to implementation. There is also a general lack of clear information about how the process works.
There is also a problem with using numbers whether they are SID or Lab values. Numbers have a certain sense of absoluteness about them. For example, the number 4 is unambiguous and specific. The number 4.3 is even more unambiguous and specific. However if an instrument says that there is a delta E of 4.3 between two colors or that the SID should be 1.43 - those numbers may not be meaningful. For example, here is a chart showing the difference in reported Lab values for seven different spectrophotometers measuring the same color patches on 80lb gloss paper. Each color bar represents a specific instrument (i.e. all green bars are one instrument - all red bars another).
In this case there is a delta E as great as 7 between some of these instruments. Even with the same instrument there is a delta difference based on the color that's being measured.
Instruments do not agree - even from the same manufacturer. Different instruments do not report the same values (density or Lab) from measured samples. So you have to be very careful when quoting those values. They provide guidance but they are not absolute.
best, gordo
. It is not the ability of the ink to hit the ISO targets that is in question, or the print shop running at a density that lines everything up at their place. It is specifying a set number, regardless of print shop. As the chart above shows, it just cant be done. While the number is definitely a usefull tool on press, and for the print shop itself, a specific value for density is just not something that can be specified by the ink manufacturer.
