<title>Amplitude-encoded phase-only filters for pattern recognition: influence of the bleaching procedure</title>

Abstract

The main techniques for optical pattern recognition are based on the correlation methods. In the past few years several correlation filters related to classical matched spatial filters have been proposed1'2 and different measures of performances have been introduced3. Recent works have shown that the problem of low light efficiency of optical correlators can be avided with the help of several types of pure phase-only filters . They have also good features for recognition process such as high discrimination capability and high sharp correlation peaks. Optical realization of the phase-only5flters (POF) based on holographic recording method is very difficult ' . Using digital methods one cn easily codify and modify the filter depending on the applications. The binary encoding of the phase-only filter (or its binary version) is the best candidate for recent applications in pattern recognition. Nevertheless most of graphic devices controlled by computer such as plotter, laser printer or the majority of spatial light modulators can only modulate the amplitude. For such a reason several methods have been recenly proposed to obtain amplitude encoded phase-only filters (AEPOF) , which can be written on an amplitude modulating device. However, amplitude modulation does not provide high light efficiency of the optical correlator. The use of phase materials for spatial filters recording allows better utilization of laser light in recognition systems and keeps laser power requirements lower. One of the more popular conventional phase materials is the bleached holographic emulsion. Bleaching removes the attenuation giving more light in the output correlation plane, but also in the case of classical matched filtering distortion and increased scattering noise occur in the reconstructed avefront which degrades the recognition capability of the filter . Binary amplitude encoded computer generated holograms do not suffer from distortion after bleaching11, because in this case the distribution of the phase hologram is identical to the distribution of the amplitude hologram without bias. The reconstruction is not more disturbed than the reconstruction of the amplitude hologram. In this paper we analyze the usefulness for pattern recognition of the bleached mplitude encoded phase-only filters based on Burckhardt's method . The silver halide (sensitized) gelatine (SHSG)12 is proposed to record computer generated phase-'only filter. The SHSG provides higher signal to noise ratio (SNR) that conventionally bleached eiitulsions because of absence of scattering centers after processing.