Abstract: There is described a detector which is suitable for use as a fire detector. The detector has two channels which allow for discrimination between energy received from a fire and a “false fire”. False fires are sometimes detected as a result of radiation from a so called “cold”, black body radiation source which flickers at a frequency of between 1 and 20 Hz. In the past, these gave rise to false alarms being triggered. The invention overcomes the problem by having a notch filter which when used in combination with another filter, ensures that detected radiation at, or around, 4.3 &mgr;m is transmitted to a sensor. A processor then compares the received value with a value computed by interpolating between signals received from two other channels. If a threshold value is exceeded an alarm is triggered. The invention thus overcomes disadvantages with prior art systems as signals from cold black body sources are rejected as false.