Infrared detection apparatus

Claim 1

Infrared detection apparatus for providing an output substantially free from background comprising a signal channel of sequential input signals, each corresponding to a line of scan of a scene and a background reference, each signal including a sample corresponding to radiation received from said background reference and a plurality of succeeding samples corresponding to radiation received from said scene, circuit means for deriving a plurality of output signals, each corresponding to a respective one of said input signals in which the sample corresponding to said background reference is subtracted from each of the samples corresponding to said scene including an input terminal, an output terminal and a reference terminal, a capacitance connected between said input terminal and said output terminal, a resistance and a normally-open switch connected between said output terminal and said reference terminal, means for applying the output of said signal channel between said input terminal and said reference terminal, means for closing said normally-open switch during the time of occurrence of the sample of each input signal corresponding to said background reference, the reciprocal of the on-time of said switch being large in relation to the noise bandwidth of said sample corresponding to said background reference, the time constant of said capacitance and said resistance being greater than the on-time of said switch.