I am way more comfortable now that any variations I see are actually on the media and not leakage from adjacent patches.
No longer!
I scanned a single sheet of 2920, 6mm x 6mm patches on a glossy 13x19" 3 patches were way off. 8, 10 and 11 dE compared to 3 scans I had made the day before respectively. All others were reasonably close. Ave dE was .12.
Those three patches were all on the right most column and each spaced with a good row in between. Interestingly, those patches were printed with RGB 78,0,0 0,78,0, 0,0,78 and are quite dark on the Espon 9800 with L* of 7 to 10. The patches to the left were all similar in L*. Doing the math, it appears that light was leaking from the white margins. It would take a tiny misregistration of about to produce that sort of shift. As large as it is, it's only the effect of about .5% light contamination from "white." Misregistration errors are far more damaging to darker patches than lighter ones. The same error at L*=50 would only produce about 1 dE difference.
The difference between this and the other scans, which match nicely, is that I enabled dual scan mode. This measures 2 rows with a white light, then, backs up a row and measures the same two rows with a uV LED. Also, each row is measured in alternating directions, right to left, step a row, left to right, step ... repeat.
There are other factors that should be considered. The I1Isis does not use the color changes from one patch to another to determine boundaries. They are purely a function of dividing up the total row length into precise, segments and building a window that should exclude leakage from adjacent patches. This makes the patch spacing linearity extremely critical. This is magnified when the patches range over a long distance as is the case of the 13x19, single page, 2920 patch print.
Recommendations:
When profiling larger than US letter size and you have the choice, select Profile, not Landscape, when using letter size. The longer the rows, the more error potential when interpolating where those patch boundaries are.
Use 6.5 or 7.0 mm spacing, in steps of .5 mm. This is because other fractions produce patch sizes that vary. The patch pixel width on the print is .25 mm and so .2 or .3 mm slider settings will result in different patch sizes across a row. Dumb.
Let the print dry. There could be subtle dimension changes due to variations in inking and drying. While the colors stabilize in a few minutes, the dimensions may not. I've seen some limited evidence of this.
I need to look into this more closely but I believe these recommendations will avoid most of the registration issues.
Edit for minor dim corrections.
Edit: Explanation found!
I tried to repeat scans on the target that showed the dE > 10 deviations and was not successful until I noticed that the position of the Isis XL together with the use of very a large target was the problem.
I had placed the Isis XL on a table about 28" in width, closer to exit edge. I had also select dual scan mode - I normally scan with just M2. Because the paper is fairly long at 19 inches, a portion of it would bend when extending past the table edge. This increases the drag and makes the drag asymmetrical. When in the dual scan mode the scanning motion is to read with a white lamp left to right then step forward and read right to left. Then the device reverses paper direction and steps back to the previous scanned row where the process is repeated with the uV LED. This exacerbates the effect of asymmetrical drag gradually producing an offset that resulted in enough leaked light from adjacent patches to produce the larger error. The asymmetrical drag is made worse because the 13x19" 250 gsm paper's weight and inertia is quite an increase over my normal targets that are US Letter size and lie flat on both sides of the Isis.
Subsequently, I have changed the orientation so that the paper always lies flat and doesn't extend past the tables with these long papers. Since then there has been no recurrence.