Technique for measuring the rate of a chemical reaction in which a short, intense flash of light initiates a chemical reaction and some spectroscopic probe is used to measure directly the change in concentration of one of the reactants, most often a free radical. The light pulse is provided by a flash lamp or a laser, while the detection technique is usually either absorption or fluorescence spectroscopy. Reactions occurring on timescales of less than 1 μs can be measured using this technique.