The SPT.LIGHT of UNiKA phase tester is the second generation of SPT-1 phase tester. It consists of two units, a transmitter and receiver. The SPT.LIGHT transmitter creates a pulse that is sent through an audio system. The receiver then picks up the signal at various intervals in the system, and indicates whether or not the system is in or out of phase. Every SPT.LIGHT phase tester has been thoroughly tested and offers the best value at an affordable price.