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Real-time enhancement of submicron defects using photorefractives

L. Hesselink

We have developed a new approach using photorefractives for real-time inspection of periodic masks or cracks and defects in non-periodic objects. The approach is based on Fourier transform, holographic recording of the object, filtering, and phase-conjugate readout using a photorefractive crystal. These processes are performed simultaneously to allow real-time operation. The object to be inspected (top part of the figure) is placed in the input plane, and the defect-enhanced image in the bottom part appears at the output plane, in a time limited only by the time constant of the photorefractive material. This time constant is material and light intensity dependent and ranges in our experiments from 50 to 250 msecs. This method differs from previous approaches in that all operations are carried out in the Fourier domain, no object dependent mask is needed, and real-time operation is achieved without the need for careful alignment of filters and masks.