An infra-red proximity fuze system for a homing missile is provided that has Mercury Cadmium Telluride detector cells cooled to at least -40.degree. C., and a frequency response range of 5-7 microns, so as to be sensitive to target skin radiation due to kinetic heating and insensitive to jet-exhaust plume radiation. Three optics/detector modules are equidistantly spaced around the missile axis and each has first and second detector elements the three first elements being connected in a common channel to constitute a guard beam and the three second elements being likewise connected to constitute a firing beam, the guard beam field being displaced angularly from the firing beam field in the forward missile axis direction by about 6.degree..