Continuous-flow microstructured technologies are now recognized as promising alternatives to batch processing for organic photochemistry, especially when light emitting diodes (LEDs) are employed as light sources. To evaluate and optimize productivity and energetic efficiency, the knowledge of the incident photon flux density is crucial. In this context, the objectives of the present work are dual: first, to transfer the classical actinometry method with Reinecke’s salt to a continuous-flow LED-driven spiral-shaped reactor and, second, to propose a revised one-dimensional equivalent model for the accurate determination of the incident photon flux density in this microreactor. Experimental measurements were carried out under controlled conditions. The effects of the spectral domain and radiant power emitted, the tubing length, the presence of gas-liquid Taylor flow, and the material of the support plate were especially investigated. For this purpose, an expression was established for the revised one-dimensional Cartesian model, taking into account the diffuse emission of the LED array and the reflection induced by the material of the plate in which the tubing was inserted (i.e. the reflection by the backside of the microreactor wall). In this way, the incident photon flux density could be estimated with an acceptable level of accuracy, which was not the case if the usual one-dimensional (1D) model was applied (collimated emission and no reflection).