The concern raised was about inter-instrument agreement rather than the consistency of an instrument's readings.
You are correct Gordo. I was wandering off course. The point I was trying to make is that you are in search of unwavering perfection then you might want to consider a career outside of print.
I guess that in print we aim for perfection but make plans for what to do when we miss.... and .... well .... we miss a lot. The trick is to agree on what counts as a miss and what counts as a hit.
Now where can you find an Applied Mathematician and Color Scientist at this time of day?
Well the eminent
John Seymour has been posting on this subject about 24 hours ago on the Linkedin ‘Group of Prepress, Print & Color Management Professionals’.
I’ll save him some time and post it here verbatim:
It is not uncommon to see 1 deltaE (ab) of difference between two spectros of different make and model. Even two instruments of the same make and model are likely to differ by half that amount.
The rule of thumb is that the variability of a measurement device be no more than 0.3 times the acceptable tolerance. Thus, if the customer tolerance is 4 deltaE ab, then all the instruments in the supply chain must agree to within 1.2 deltaE ab.
First, use a round-robin check to see if the current agreement is acceptable. Take a bunch of representative samples of the work being printed, and send them around to everyone in the supply chain. Make sure the samples get remeasured at the starting point to make sure that the samples didn't change somewhere along the way.
It is important that the samples be production materials, since agreement is very much dependent on the nature of what is being measured. BCRA (Ceram/Lucideon) tiles are a great QC tool for spectros, but, unless you are only manufacturing very glossy ceramic tiles, the results may be misleading.
I have found that gloss level and OBA content are the two biggest obstacles for inter-instrument agreement.
Second, if the level of agreement is not less than 0.3 times the customer tolerance, then you can <try> to force everyone in the supply chain to use the same make and model instrument. Coca Cola has done this. There is likely to be resistance, of course.
And it may be impossible. Inline (on press) measurement cannot be the same make/model as handheld devices. These devices need to make measurements in microseconds, whereas handheld devices can take a leisurely 100 milliseconds.
If worse comes to worse, you may need to resort to different target values depending on individual devices. (I can easily say that as an armchair quarterback, since I am not the one who has to deal with dealing with multiple target values!) But, the thing is, while different devices may not agree with the absolute L*a*b* values, they pretty much agree on relative changes in color.
I should add... many packaging printers have to deal with making on-press or press side measurements of pre-laminated materials (clear films). The print buyer would like to see target values for the post-lam product (the potato chip bag). A conversion from one to the other is required, but if this is not available, the printer must resort to two sets of target values.
...but if this is true it upholds my concerns.
