The spectral measurements are done on both sides of the inkjet paper and by that include the inkjet coating's reflection of at least one side of the paper. That coating side reflection does not tell whether the coating is the same as the coating of another paper with the same coating side reflection but at least they are similar on that aspect of white reflection. Together with the weight measurement and the reflection of the uncoated paper base side it creates some evidence that it could be the same paper. For ICC printer profile selection the same white reflection is an important thing to start from. Nevertheless the way the coating interacts with the applied inks can still differ and by that make a distinction between the papers that have that same white reflection. That will also influence the usability of the profile beyond the paper white part if used for both papers. Gloss difference will not be detected in the spectral plots either so the print can still look different that way too. I did see some difference in gloss between the Canson and the Ilford quality. Usually satin to gloss finish have less impact on the profile selection though. So when the basic paper ingredients are the same, one(custom) profile can often be used for both finishes. It is different when we go to semi-matte etc.
Met vriendelijke groet, Ernst
January 2014, 600+ inkjet media white spectral plots.
Ernst, yes indeed the profile, in this case from IGFS, is interchangeable with the others tested, including the white-point as these can be considered identical. How the ink-receptor layer interacts with ink however can be different. When going for the maximum possible on the CABP (Canson Baryta Photographique), i find it can be pushed a bit further than IGFS, both in saturation (amount of ink, thus also Dmax without smearing or blocking shadows) and in resolution (fine details). Btw i use Epson 4900.
My personal preference is the Canson in this case, however with its own media settings and profile.
I do mainly portraiture, and with CABP the printed image appears more 3-dimensional, has some "alive" feeling to it, as if you are looking at the person(s) through a rectangular window.