The irradiator has an emitter with a base on at least one side. It also has a socket with a receptacle in which at least part of the base extends. The socket is connected to a sensor element adapted to receive electric, magnetic or optical pulses. The part of the socket nearest the receptacle is provided with a marking element. The marking element is formed to provide electric, magnetic or optical pulses which can be received by the sensor element. A stimulating arrangement may be provided which is operative connection with the marking element to stimulating the pulses. The stimulator may be e.g. a voltage source or a light source.
