The present invention relates to a method for determining an effective dose of monochromatic or polychromatic light from one or more light sources to inactivate microorganisms present in a biological fluid, preferably a non-transparent fluid. Moreover, there is provided a method for the inactivation of microorganism in a biological fluid in a flow-through-reactor. Moreover, the invention advantageously provides a flow-through-reactor with one or more thermostated light sources. The invention further provides a method of controlling the light sum dose of monochromatic or polychromatic light emitted from one or more light sources to effectively inactivate microorganisms present in a biological fluid in a batch reactor.