Passive Detection of Motion Transverse to the Optical Viewing Axis

The passive detection of velocities of objects moving transverse to the optical viewing axis is discussed from the standpoint of the spatial Fourier transform of the image. A simple algorithm is developed which yields the velocity, as a function of time, of an object moving against a complicated background. This algorithm relies on monitoring magnitude and phase of two prominent spatial Fourier components. Measurement of the Fourier components is most simply accomplished with the direct electronic Fourier transform (DEFT) sensor. This device employs coupling between the image intensity, surface acoustic waves, and electron current in a CdS film to decompose the image into Fourier components. Experimental results show strong confirmation between theory and experiment. The paper concludes with a brief comparison of the DEFT implementation versus other systems in terms of speed, sensitivity, and cost.