In this study continuous wave thermal lens spectrometric measurements
are made using solutions of Methylene Blue in short pathlength sample
cells (0.1 cm). Under the conditions described, rapid photobleaching o
f Methylene Blue is observed, which degrades the thermal lens signal w
ith potentially serious consequences in terms of detection limits. The
phenomenon has been studied and an empirical relationship developed t
o describe the behaviour of the thermal lens signal when photobleachin
g occurs.