In this study, we utilized the widely recognized potassium ferrioxalate actinometer (Reaction 1), of which the quantum yield has been explored across an extensive wavelength range. We employed several pseudo-monochromatic UV-A LED light sources, and their spectral distributions were determined using a spectrophotometer (Supplementary Figure ). The decadic molar attenuation coefficients for the potassium ferrioxalate complex and its subsequent products were measured for relevant wavelengths within the emission spectra (Supplementary Figure ). An important conclusion from these measurements is the minimal attenuation coefficient of the resultant products compared to the initial material. This underscores the potassium ferrioxalate complex as the principal absorbing species, simplifying the absorbance assessment.