has been carried out by the sensors each of these outcomes now becomes the start of a decision tree in which additional data from the sensors is made use of by the processor 5 it will be understood that the analysis suggested by the scheme of fig2 and tables 1 and 2 is being carried out continuously also in a complete instrument further data analysis will be performed that is not relevant to this invention and this could lead to further hardening of the u2018 possible u2019 and u2018 probable u2019 categories analysis of the output of the array detector 2 reveals that only a single source is present in the target area which will typically be a hot object such as a halogen lamp or an electric fire in a simple case the output of the array 2 may be sufficient to determine that the object has no flame like characteristics however modulation of the source often occurs in practice for example due to objects moving in front of it and this can cause flame like characteristics which might result in the output from the array detector wrongly identifying the source as a flame in the presence of such a.