POLYMERS that exhibit the photorefractive effect-a light-induced modulation of refractive index-are emerging as attractive materials for optical devices and processing systems(1,2). Here we demonstrate one such application using our recently developed high-efficiency photorefractive polymer(2). The polymer provides a nonlinear medium in which real-time all-optical image correlation, and hence pattern recognition, can be accomplished, This forms the basis of an optical security system, whereby documents are encoded with practically invisible phase masks (such masks are difficult to forge), which may then be rapidly screened to verify the authenticity of the documents, The wavelengths at which our optical system operates are compatible with commercial low-power semiconductor laser diodes, and the system can be integrated into a compact device at low cost.