I would like to calibrate a flatbed scanner using a IT8 target. I am curious: are the targets usually printed on paper without optical brighteners? It seems like the fluorescence of typical 'bright white' paper could mess up the calibration if the ink used in the color patches wasn't totally opaque. Since I can't dig up anything about this, maybe it is not a real problem, but let me know.
Ink is not opaque, it is translucent.
You are correct. IT8 targets are printed on paper without optical brighteners. The nature and variety of paper surface optical brighteners makes their involvement problematic.
My IT8 that came with the Epson V850 is made on Kodak Endura paper. It doesn't have OBAs but it does have a fluorescent substrate and produces significant shift under uV as demonstrated with these I1Pro 2 white patch readings using M1(including D50 levels of uV) and M2 (uV cut):
L*a*b* M1: 90.5 0.9 -3.7 L*a*b* M2: 90.4 -0.3 0.4
This is a similar shift as Epson's Premium Glossy Photo which doesn't have OBAs but does have a fluorescent substrate.