The system coprises a thin element for the visual monitoring of the exposition dose. Its application is expected in various practical fields, such as phototherapy, museums, skin protection, food safety, phramaceuticals safety, construction materials, etc. The time variable is calibrated via the colour change. It is based on the utilisation of different stabilites of many tested chemicals. Their light induced decomposition is manifested as decolorization. The decolorization is described kinetically, and then associated in a cumultaive form with a certain characteristic dose. The dose reflects the amount of absorbed light, and it has many practical consequences in the mentioned fields.