When scanning color transparencies or prints, the scanner records the colors as RGB values, but the scanner may not record the range in a linear fashion. It may emphasize some or scan with an overall cast.
Using a scanner profiling target can help you to get more accurate results. The idea is that you scan the target in a special profiling mode which disables any current scanner profile, while adjusting the scanner controls to get a full range on the target with a little room to spare - that is - you don't want the whitest patch to end up being 255 or the blackest patch to be 0,0,0 - you want to be slightly conservative so you don't clip colors or tonality. It may take several test scans to arrive at the proper scanner settings. Once you do, you have to save those software settings so your subsequent scans are made with the same settings.
Once you have the scan, bring it in to Ps and make sure that it's straight and it doesn't have any spots or hairs going across the color patches. If it does, carefully clone them out so all the patches are clean. Then import the scan of the IT-8 into your profiling module. It's usually pretty much a push button process to generate the profile.
The next time you scan, make sure you load your new custom scanner profile into the Input profile section of your scanner software and then load the same settings you used to make your profile and scan.
The profile will take into account how your scanner "sees" the film and correct the scan to be very close to the original, all without tweaking the scanner controls. You may, however want to make changes to the scanner control when scanning specific pieces of film - film that has a color cast or that is over or under exposed. That's okay.
The scanning software should give you a choice on whether to embed the input (scanner) profile in the final tiff file or convert to a chosen Photoshop working space. Use whatever choice you're most comfortable with, unless you happen to have a Howtek drum scanner using Trident, which has a bug in the convert to profile function. I do those conversion after opening the files in Ps.
Sometimes you'll find that the scanner profile makes scans that are too dark, and there is a workaround for that. Take a copy of the scan of the profile target, open it in Ps, and using Curves, place a single point in the middle of the master RGB curve and pull it down, darkening the image slightly. Save that and use that modified target to make a new profile. The profiling software will be tricked into making a scanner profile that provides a lighter image. You can do alternate versions similarly if you feel you want more or less saturation or even want to emphasize or de-emphasize a certain color on a consistent basis. Just make sure you modify the target scan in the opposite direction of where you want it to go in the final use.
Using an IT-8 or Hutchcolor profiling target on your scanner is just like making a custom profile for your printer - it "fingerprints" the scanner and provides a known numerical state of how the scanner records color.
There are scanner targets for reflective prints, for Velvia, Provia, Ektachrome and Kodachrome, among others. In theory, you would want a custom profile for each type of film, but in practice, a single profile made from an E-6 target will cover all of your E-6 films, while a profile from a Kodak reflective target will work well for most prints.
Kodachrome is always the odd man out. It generally does not scan how you expect it to, usually resulting in bluish shadows using E-6 targets. Since I've never gone to the trouble of procuring a Kodachrome target, I just override my scanner settings, and take the blue out of the black and that seems to put everything just where it wants to be.
A typical Ektachrome rebate edge will read 8R, 6G, 5B. A Velvia rebate edge will be more like 5,4,3, but Kodachrome comes in with the blue channel up around 20-25. The dyes are seen differently. You can compensate with a Kodachrome profile or you can do it manually.
Peter