A well known photo-thermal “Mirage Effect” was used for the determination of Phase Transitions of Liquid Crystals. In Mirage effect, usually a LASER beam passing through the Liquid Crystal sample is shifted as the sample is heated. This is due to the large magnitude of the refractive index gradient developed near the vicinity of the sample. A large shift occurs at the Phase Transitions. A large change in the intensity of the probe beam with temperature indicates the phase transition. A change in the intensity of the LASER source, affects the accuracy of the measurement. Hence, the probe LASER beam from a LASER DIODE is flashed by electronic signal modulation. The flashing beam is detected by a Pin photodiode, is given to a Lock in Amplifier. A continuous change in the magnitude of the refractive index gradient in the vicinity of the heated sample changes the phase of the detected signal with reference to the flashing frequency of the probe beam. This phase change is independent of the change in intensity of the probe beam. The phase transition temperatures of two well known and widely studied nO.m compounds viz., N-(p-n-butoxy/pentyloxy benzylidene)-p-n-methyl anilines (4O.2 and 5O.2) are measured. The transition temperatures obtained through this technique agree with those reported from differential scanning calorimeter technique.