My Epson 7900 shows similar behavior. I think this phenomenon is way more common than most people might think.
In my 7900 I have Piezography Pro inks loaded and I print with QTR. I first discovered this several years ago when linearizing a QTR quad file. (A quad file is not an ICC profile; it is more like a "media setting" file that controls how much ink and which inks are laid down for each pixel value [0-255] in the grayscale image being printed). The linearization process starts with a Piezography-provided grayscale target (see attached) that is printed then measured. The target is constructed in "column/row" order with patches from 255 to 0 running down column 1 first, then continuing in column 2 etc. I started noticing an occasional very odd repeating "ripple" in the measurements using my i1Pro2. The patches were scanned row by row. See the attached graph for an especially bad example of the measurement ripple. Most of the time it was not this bad, but still noticeable.
The data shows a bump/drop in density
every 18 measurements. After a LOT of investigation I finally realized that the target had 18 rows (!) and ink density was trending higher in a non-linear fashion towards the end of the target when being printed which resulted in the measurement ripple when plotted from lowest L* to highest L*.
The actual cause of this is hard to pin down, but it could be any of several things (just my guesses here):
- print head heating up causing change in droplet characteristics resulting in non-linear change in ink density as page is printed
- movement of head, carriage and ink lines (sloshing) causes pigment density to change as page is printed
- change in ink line pressure during printing
- mechanical non-linearity in carriage movement/page feed that results in changed dither/dot placement as page is printed
In any case, it seems this behavior is more "normal" in large format printers than one would expect. It also seems to be worse with slower printing speed and high printing quality settings. When I printed the same target at a lower quality setting (720x1440 versus 2880x1440), the problem was completely eliminated. And I'm guessing that even within the same printer model, one machine may exhibit this more than another.
My solution going forward to control this to the extent possible was to start doing two things:
- Use scrambled (randomized) patch targets
- Use smaller patch sizes (but still meeting minimum requirement for whatever spectro is being used)
The smaller the overall target, the smaller the accumulating effect, and scrambled patches helps to average out the density drift.
And of course, this is one big reason why we use smoothing algorithms in making profiles (and in linearizing QTR quad files!).
Dave
Edit: Minor wording change to add clarity.