The so-called characteristic curve describing photosensitivity change is elaborated and shown to be a powerful tool for understanding and characterizing photosensitive growth both at a fundamental and practical level. It has been used successfully to diagnose when optimal hypersensitization has been achieved and the physical basis for this is explained. By way of example, previous results using 355 nm hypersensitization are re-examined. Evidence of single site-selective glass relaxation through direct laser excitation offers a new approach to accessing and studying induced relaxations. (C) 2004 American Institute of Physics.