The system focuses technically on the development of the thin film detection and sensoring element for the photochemically induced colour change, potentially useful in the phototherapy. Such elements are understood as a very effective tool for the calibration of the visually simple assesment of the time variable associated with the discussed colour change. It is in fact a memory element which transforms the continuous time axis with any possible precision to the cumulative form. It can be than evaluated and associated with a specific dose of light of a charactersitic wavelength. The system is cheap, technically simple, and econimicaly feasible.