A fluorescence assay by gas expansion (FAGE) instrument based on laser-induced fluorescence designed both for laboratory validation studies and for field measurements of OH and HO2 is described. Laboratory validation centers around the development of techniques for assessing interference from O3, photolysis and calibration of the OH signal. Excitation at 308 nm will be employed, reducing the extent of O3 photolysis, together with detection of OH at high and low rotational levels. The latter technique is based on the production of rotationally excited OH in the 0(D) + H20 reaction. Calibration is centered on discharge flow and water photolysis. Optimization of the experiment relies on detailed photochemical modeling, and the proposed techniques are discussed. Attention is also paid to the possible effects of the supersonic flow field on the fluorescence assay, and the feasibility of developing a computational fluid dynamics model is discussed.