Background Suppression With Variable Modulation Transfer Function (MTF) Imaging Systems

A technique is described for suppressing the lower spatial frequencies in the object using an optical system with a time -varying MTF. The technique is applicable to imaging radiometers operating in the infrared region. The required variations in the MTF are produced by causing the image to move in a programmed sequenceof small amplitude circular gyrations on the face of the detector. The gyrations are performed at a frequencyhigh compared to the system frame rate. A series of calculations are described which indicate that, at lowspatial frequencies, average suppression ratios of at least 500 to 1 can be achieved with this technique. Introduction This paper discusses a technique of background suppression on an optical level by means of suitably matchedor tailored pairs of modulation transfer functions which, when subtracted from one another, suppress the lowerspatial frequencies in the object scene. This, in turn, enhances the detectability of point sources ortargets in the presence of structured backgrounds. The technique is intended to be applied to image- formingradiometers operating at or near the diffraction limit in the infrared region of the spectrum.Figure 1 shows a somewhat idealized pair of modulation transfer functions of this type. The MTF labeledT (w) initially drops steeply, then levels off, and finally falls to zero at the cutoff frequency, w =1.0.