The application relates to a detecting device (1) for indicating the intensity of a predetermined type of radiation present in electromagnetic radiation incident on the detecting device, wherein the detecting device comprises: a filter element (2) for filtering the incident electromagnetic radiation, wherein the filter element is configured to filter off electromagnetic radiation with a wavelength of above 590 nm from the incident electromagnetic radiation; a converging element (3) configured to increase the density of photons of the predetermined type of radiation present in the incident electromagnetic radiation; and - a sensor element of material (4) arranged to receive the incident electromagnetic radiation that has passed through the filter element and the converging element for indicating the intensity of the predetermined type of radiation present in the incident electromagnetic radiation by change of the color of the sensor element of material, wherein the material is represented by the following formula (I).