A method for photochemical reactor characterization includes an application of using dyed microspheres exposed to UV irradiation under a collimated-beam system. Particle specific fluorescence intensity measurements are conducted using samples form the collimated beam and flow-through reactor results using flow cytometry. A numerical model may be used to simulate the behavior of the reactor system to provide a particle-tracking algorithm to interrogate the flow and intensity field simulations for purposes of developing a particle specific estimate of the dose delivery. A method for measuring UV dose distribution delivery in photochemical reactors is provided that includes introducing microspheres labeled with a photochemically-active compound in a UV reactor, The labeled microspheres are harvested downstream of the irradiated zone of a UV reactor and exposed to UV irradiation under a collimated beam of UV irradiation. The method further includes quantifying a UV dose-response behavior, conducting fluorescence intensity measurement on the labeled microspheres from the UV reactor, and developing an estimate of a dose distribution delivered by a UV reactor based on the numerical deconvolution of the sum of the UV dose response behavior and fluorescent intensity of exposed microspheres.