Brazilian Navy

Detection of airborne optical emission sources in Infrared Search and Track (IRST) systems is usually carried out using the blackbody temperature or emissivity difference between the emission source and the background. Recent countermeasure techniques include emissivity tailoring and temperature distribution tailoring across the emission source area to avoid the formation of "hot spots " which effectively embeds the emission source in its background. A technique relying on coherence rather than irradiance is presented, allowing detection with poor signal-to-clutter ratios. The technique has some similarity with Fourier Transform Spectroscopy (FTS), and its key components are a Michelson interferometer, which measures the coherence profile of the scene in the field of view, and an interference filter, which uses the background illumination to create amplitude minima in the interferogram envelope. Unlike FTS, the interferometer moving mirror scans only a tiny portion of the interferogram, this being the region surrounding the first minimum of its envelope. It is shown that this envelope is a sinc function, and that at this minimum the phase of the interferogram undergoes a π phase step, which is used to define the position accurately. When a partially coherent emission source comes into the field of view, the modulus of the net complex degree of coherence of the scene increases, and the phase step position changes; this latter optical feature is used to declare detection. We present a simple theoretical model and compare it with experimental results for highly emissive sources having various coherence lengths in the presence of incoherent background illumination. Agreement between the experimental results and the theory is discussed