The photocell in an oil burner is used as a safety device, it monitors the combustion chamber for a flame signal. It provides a monitored feedback signal back to the Control PCB. During the start-up process the photocell checks there isn't a flame or stray light in the chamber, only then does the PCB allow the system to start the next phase and establish the flame. Once the flame has been lit the photocell starts to monitor that the flame remains lit, if the flame signal is dropped the PCB will initiate a burner lockout.