The invention provides a system and related equipment for the precise measurement of the output characteristic of a light source, e.g., a dental light curing unit (LCU) or light for photodynamic therapy, using a light collector, a light detector, and a computer programmed to deliver the value of the output characteristic of the light source to the user. The systems allow for the determination of a proper exposure time or the selection of a light source as needed for a specific application.