1. A method of testing an optical flame detector, comprising: (i) irradiating the flame detector with optical energy from a remote test source which is different from optical energy emitted by a real flame; (ii) processing by a first processing module the received optical energy to determine if measured characteristics of the optical energy correspond to predetermined characteristics of a predetermined test signal from an optical test source; (iii) operating the flame detector in a test mode if the processing indicates the received optical energy is a test signal from the optical test source; (iv) if the processing indicates that the received optical energy is not a test signal from the optical test source, operating the flame detector in an operating mode, wherein a second processing module of the flame detector is responsive to optical radiation generated by flames to initiate an alarm mode; and (v) repeating steps (ii), (iii) and (iv) if the processing indicates that the received optical energy is not a test signal.