Distributed kinoforms in optical security applications

A phase-only referenceless computer-generated Fourier hologram, which gives a nonsymmetric image in the reconstruction process, is known as a kinoform. A new optically variable device designed for optical security purposes is described that is formed from a matrix of kinoforms. The device is characterized by both rainbow holographic effects and hidden features. The first hidden feature is a slit-shaped image, which contains a line of text or other information. Under normal illumination conditions, this first element shows rainbow holographic and kinematic effects. The second hidden feature is a dark image visible at an angular range different than that of the first feature. This second image is faint under white light illumination, but appears clearly in laser light. In the proposed device, both rainbow holographic images and hidden Fourier-plane images are reconstructed from the whole hologram area, which contains a matrix of single superposed recordings. In this way, the use of special encoded subfields that alert counterfeiters is eliminated. Careful analysis of the image and spectral planes of the device made with expensive and specialized equipment may allow imitation of the device using optical holographic techniques. The device is calculated using a modified iterative Fourier transform algorithm and manufactured in a binary or a multiple phase-step technique. We recall several types of optically variable devices and consider their security properties for document protection. (C) 1996 Society of Photo-Optical Instrumentation Engineers.