Hi Piero!
You need to check if your images are of the same season, or
if they are from different seasons, because sun angle is
an important factor. If the images are taken during the
same season, but different years, you can use a regression
procedure explained in the following articles:
Vogelmann. 1988. International J. Remote Sensing. "Detection
of forest change in the Green..." 9(7):1187-1200.
Jensen, Rutchey, Koch, and Narumalani. 1995. Photogrammetric
Engineering and Remote Sensing. "Inland wetland change..."
61(2):199-209.
If this method applies to your work, and you can't get these
articles, let me know. I can mail or fax them to you.
Caio,
Paula