Recognition and processing of wave forms

A flame detector has a sensor for producing a digital waveform (8A) representing the flickering flame. Its turning points are detected and the magnitudes 60A,60B,60C... between successive such points are measured. The value of each such magnitude is plotted on a graph (8B) whose horizontal axis represents the current value and whose vertical axis represents the immediately preceding value. Points such as E and F are produced lying in regions away from the diagonal B and from the axes. If the detected waveform originates from a regularly varying interfering source, the corresponding points will lie on or near the diagonal B. If the interfering source is a waveform undergoing step changes, the corresponding points will lie adjacent to the axes. In this way, discrimination between flames and interfering sources of radiation can be made.