Abstract

A method is presented to enhance defects in periodic objects in real time. A low-intensity reference wave interferes with the Fourier transform of an object beam to form a hologram in a photorefractive crystal. The nonlinear properties of the crystal perform a filtering operation, suppressing the high-intensity Fourier components of the periodic structure, and phase-conjugate read-out results in a defect-enhanced image. The mask to be inspected is placed in the input plane, and the defect-enhanced image appears at the output plane, in a time limited only by the time constant of the crystal. Using ESSO with a total incident light intensity of ~20 mW/cm², the response time is ~50 ms. Defects of 10 × 100 μm² have been easily detected with high SNR, and performance limitations are discussed.

© 1985 Optical Society of America