We proved  that the integrated absorption intensity due to intersubband optical  transition in semi- conductor nanostructures, e.g., a quantum well (QW) is a function of the  correlation length  of the interface roughness profile  only,  being  independent of the  roughness amplitude.  This  gives us the possibility of finding  a reliable  solution  for a challenging problem  in literature, viz.,  single-valued estimation of the  correlation length  (so, of the  interface roughness profile) from an experiment done with  merely  one sample.   This  experiment is different from  those  suggested so far,  done  with  two samples  assumed to be various, but  must  have  the  same  roughness profile (of equal  roughness  amplitude and  equal correlation length). This  assumption turns out to be crucial,  but  non-controllable and  often not  realized  in actual experiments.