 An infrared screen blocked out the visible light so that it couldn't be seen from the outside.The beam from the transmitter was directed at the reflector (a parabolic mirror) in the control hut at Myrtletown, which focussed the rays to a pin point on the cathode of the PE cell.Breaking the beam closed a contactor on to an 80 Volt DC supply and connected it in parallel with a service contactor.