The triplet quantum yield Phi(T) of thioxanthone (TX) was measured in different solvents by means of time-resolved thermal lens spectroscopy. It was observed that the experimental signal depends on the probe light wavelength. This effect which was mainly ascribed to the population lens (PL) arises from the change in solution composition after excitation, and adds its contribution to the thermal lens signal. From the transient absorption spectrum of the triplet state of TX, the change in refractive index associated with this PL was calculated as a function of the wavelength. Therefore, probing the photolens at an appropriate wavelength allowed accurate measurement of Phi(T). It was found that besides departing seriously from unity, Phi(T) decreases from 0.85 in hexane to 0.56 in methanol.