Scale-Invariant Recognition of Three-Dimensional Objects by Use of a Quasi-Correlator

Introduction Several different methods of extending twodimensional #2-D# optical pattern recognition toward operating in three-dimensional #3-D# space have been proposed recently. In 1997 the optical correlator was extended to process a 3-D scene containing scattered targets. The basic idea behind this technique is to extend the optical Fourier transform #the correlator 's building block# from two to three dimensions. The 3-D correlation was first demonstrated by a 3-D joint-transform correlator #JTC#. 2,3 As an alternative, we later proposed a hybrid correlator with general complex filters, which can be regarded as a 3-D extension of the 2-D Vander Lugt correlator. 8 Recently, we proposed a 3-D optical quasi-correlator #OQC#. In this method, the spatial spectrum of several images from the 3-D tested scene is compressed into a single computer-generated hologram #CGH#. This CGH is then accordingly filtered and coherently illuminated. As a result, a correlation space b