System for detecting a hot spot in an infra-red detected landscape

A system for detecting a hot spot in a infra-red detected landscape comprising:

• a landscape scan system for converging a main optical beam emanating from various regions of said landscape onto an infra-red detector circuit;
• an optical filter means interposed in the path of said main beam for producing from said main beam consecutively in time at least two secondary beams each representing a part of the main beam spectrum;
• and, a signal comparison circuit having an input connected to said detector circuit for comparing the detector circuit output signals corresponding to the received secondary beams and providing an output signal representing the results of the comparison, said signal comparison circuit comprising analog-to-digital converter means for converting signals from said detector circuit into digital signals, a digital store for storing said digital signals representing at least one of said secondary beams, a subtraction element for forming the difference between the data in the store and the data supplied by the detector circuit representing the other secondary beam, inverter means switchable from a first position to a second position, means for changing the positions of said inverter means such that in said first position, output signals from said converter means are applied to first inputs of said substraction element and output signals from the digital store applied to second inputs of the subtraction element, while in said second position of the inverter means, output signals from the digital store are applied to said first inputs of the subtraction element and output signals from the analog-to-digital converter means are applied to the second inputs of the subtraction element.