Abstract A sensitive optical grating technique has been developed for flash photolysis experiments. The sample is excited by an optical fringe pattern obtained by interfering two light waves derived from a pulsed high power laser. Due to the photochromatic modifications within the sample, a spatially periodic refractive index and/or absorption coefficient distribution is set up, producing a phase and/or amplitude grating respectively. This grating is easily observed through the Bragg diffraction of a second, low intensity, continuous laser beam. The relaxation time of this grating is directly related to the intramolecular relaxation time of the photoexcited states. Preliminary experiments on the cis-trans photoisomerism of an azo dye shows that optical density changes of less than 10 −4 can be detected, below the limit of resolution with classical transmittance methods.