In this paper, the spectral radiant power absorbed by a solution of patulin was calculated as a function of the concentration of the light absorbing substance when it was irradiated with a UV multi-wavelength lamp. Two equations are presented. The first one contains a simplification and, consequently, is only applicable for solutions with low light absorber concentrations. The second equation does not include the simplification, so its calculation is more difficult, but it is applicable in all cases. A comparison between the data obtained with both equations is presented, showing that both models coincide for patulin concentrations below 50mg/L and diverge substantially for higher concentrations.Both equations are obtained considering the linear spherical emission model and are calculated using the Simpson integration method. The incident spectral radiant power at the surface and the bottom of the reactor was also calculated.The data obtained with the non-simplified equation reach an asymptote for high-patulin concentration values that coincides with the incident spectral radiant power at the surface of the reactor, meaning that for high concentration values, all the radiation that reaches the surface of the reactor is absorbed by the whole volume of the solution.The ratio between the spectral radiant power absorbed and the incident spectral radiant power at the surface was evaluated as a function of the depth of the reactor and the absorbance coefficient, concluding that for a specific absorption coefficient, the absorbed radiation increases asymptotically with the depth of the reactor. The higher the absorption coefficient values, the faster the ratio values increase. The asymptotic value increases if the absorbance coefficient also does because the greater the absorption coefficient, the fewer the beams that exit the solution. For the known values of the absorbance coefficient for fruit juices, it was concluded that almost all the radiation entering the solution is absorbed for depths of much less than 1mm.