Thanks for that quick and helpful reply.
I'm not trying to do anything high-powered — just scanning family photographs, and finding (unless it's my imagination!) that recent scans look "duller" than earlier scans when viewed on-screen. And blaming that on a gradual deterioration of my scanner. My only object is to view photos on-screen, or maybe print them, and have them look more like the originals. But I'm not seeking perfection, or consistency across different display devices, with all the complexities involved in that.
I realize that, if the scanner is producing the same output from different inputs, this can't be remedied later. I was hoping that the "drift" of my scanner wasn't of that type, but would preserve relative differences, while not keeping the absolute values as they were in the original. But I don't know if that's a realistic assumption.
I used a calibration target purchased from http://www.coloraid.de/ . I scanned it , but I haven't done anything with the profile generated from it, except to save the profile in a .icm file, while I try of discover how to make use of it.
I hope this makes it clearer what I'm trying to do, and what I'm trying to avoid doing. "Just apply a profile" describes exactly what I want to do — apply the profile I generated to the photos I scanned, to compensate for scanner drift. I realize the photos may still look slightly different on different monitors and printers. Conceptually that seems simple, although it may not be simple behind the scenes, and I'd rather not get involved in a full CMS to do it. But it begins to look like I may give Gimp a go, though I first want to check out the links you gave to http://www.color.org/opensource.xalter
One question: I've seen that some software, including Gimp and Photoshop, claims to be able to insert a profile in a jpg, but why do they not take a further step and produce a new jpg with the effect of the profile taken into account?
Once again, many thanks.