A simple interferometric technique for the study of radiation induced transient heating is demonstrated for the specific case of a glass substrate irradiated with pulsed 193‐nm excimer laser radiation. The technique relies on the fact that the optical path length of (633 nm) probe laser light within the substrate is sensitive to heat‐induced changes in the refractive index and thickness of the substrate. Interference between probe laser reflections from the front and back surfaces of the substrate allows these optical path length changes to be deduced from measured changes in sample reflectance. The technique offers fast response and is shown to be nearly quantitative.