RESUMO
A set of photoinitiating systems (PIS) for free radical photopolymerization was studied using time-resolved spectroscopic experiments, real-time FTIR and holographic recording. It is shown that the efficiency of the photoinitiating system can be drastically increased when a redox additive is added to the conventional dye/coinitiator system by virtue of photocyclic behaviour. The homogeneous photopolymerization process was found to reach a fast vitrification, limiting the conversion at about 55%. By contrast, holographic recording underlines the differences in photoinitiating system reactivity, allowing diffraction efficiencies close to unity for the most reactive PIS.
RESUMO
Two methods of reducing the dynamic range of the transmittance of computer-generated interconnection holograms are presented and compared. The holograms are used in an optical implementation of an associative memory to connect the input and the output planes but are representative of more generalN(4) interconnection holograms. Because the holograms play a double correlation-reconstruction role, the standard spectrum-smoothing techniques (e.g., random phase) cannot be applied. We show, in computer simulations and optical experiments, that by using deterministic phase functions that can be realized in the optical system (defocusing the hologram or controlling the phases of the diffraction spots of a Dammann grating used in the system input) the hologram dynamic range can be lowered, reducing the errors during the hologram binarization and increasing the hologram's diffraction efficiency.
RESUMO
We present an optical implementation of an improved version of the Kohonen map neural network applied to the recognition of handwritten digits taken from a postal code database. Improvements result from the introduction of supervision during the learning stage, a technique that also simplifies the map layer labeling. The experimental implementation is based on a frequency-multiplexed raster computer-generated hologram used to realize the required N(4) interconnection. The setup is shown to be equivalent to a 64-channel correlator. Computer simulations are used to study various detection and classification procedures. The results of the optical experiments, obtained with binary phase computer-generated holograms, are presented and shown to be in excellent agreement with the simulations.
RESUMO
In the first part of this paper we present frequency multiplexed raster (FMR) optical implementation of neural networks. A hidden difficulty for hardware (optical and electronic) implementation is that the dimension of the synaptic matrix is twice that of the input and output matrices or vectors. For 2-D images, which is we believe one of the greatest potentialities of neural networks, the synaptic matrix is 4-D and cannot be directly implemented in optics. We propose FMR as a method to fold this matrix into a 2-D format. In the second part of this paper we describe the system built in our laboratory showing the feasibility of FMR optical neural networks. The system is built from an optical input module, a fixed synaptic matrix coded on a transparency, a CCD camera, and a microcomputer which performs the thresholding and feedback operations. In a later stage the fixed matrix will be replaced by a programmable matrix.
RESUMO
Our study was conducted in two stages. In a first stage, elaboration of a reliable biomechanical model for the lumbar spine, permitting to calculate for each lumbar level, for a definite posture and a given patient, the normal and tangential forces as well as the overall stresses applied to the disc, and represented by the intradiscal pressure of the nucleus pulposus and the overall stresses in the annulus fibers. This biomechanical model was constructed with the help of radiographs and formulas of forces and moments balance in comparison with a reference axis system. This biomechanical model was then used to compare stresses on the disc in various sitting positions, in order to determine an ergonomic adaptation of the sitting position at work.
