We prove the integrated absorption intensity due to intersubband optical transition in a quantum well (QW) to be a function of the correlation length of the interface roughness profile and independent of the roughness amplitude. We then develop a novel way to perform single-valued estimation of the interface roughness profile of QW from experiments conducted merely on one sample. The new method that we propose in this paper would be replicable and more economical than the traditional counterparts, which usually require at least two samples.