A concealed sprinkler including a body having a proximal portion and a distal portion. The material of the reflecting mirrors, and the materials of the plate and the holding members have approximately equal coefficients of linear expansion. Calibration factors are computed to compensate for variations in emitted electron yield among the types of the materials, by identifying, for each material type, one or more horizontal regions on the surface that are made-up of the material type, and computing a calibration factor for the material type based on at least one of the detector outputs at the identified horizontal regions.