Peter Allan Jansson

Least-Squares Polynomial Filtering of Images by Convolution

A method of reducing image noise is proposed. Each element of a filtered image is obtained by evaluating a least-squares polynomial that has been fitted to a square or circular region surrounding a corresponding element of the noisy image. This is done by convolving the image with an appropriate filter function that is finite in extent and can be computed once and for all, given the polynomial degree. We derive expressions for such filters of both square and circular symmetry. The method is optimum where image noise is additive and gaussian, and the object has a polynomial form. Noise and object spectra need not be known. The proposed method is compared with Wiener filtering. Implementation is discussed.

Hot-Pressed TGS for Pyroelectric Detector Applications

Although pyroelectric detectors were proposed long ago, only recently has pyroelectricity been employed in a practical infrared detector. TGS (triglycine sulfate) has generally proven to be the pyroelectric material of greatest value for use in high-detectivity detectors. Detectivities D* on the order of 3 × 10 are typically available commercially. Other materials have proven useful where very short laser pulses must be resolved or where mechanical strength, resistance to thermal shock, high Curie temperature, or low manufacturing costs are prerequisites. One of the problems in the construction of TGS detectors, particularly those having large area, has been the need to grow large single crystals and to cleave, lap, and etch thin plates from these crystals without breaking them. In this letter we describe the preparation of hot-pressed TGS and give evaluations of a few detectors made from the material. These preliminary results indicate that the material appears to be comparable with single crystal TGS when employed in pyroelectric detector applications. It could even prove to be more useful because of superior mechanical properties and ease of fabrication. We believe this to be the first time an organic ferroelectric material has been successfully hot pressed. We prepared the TGS reacting glycine with sulfuric acid. The strong temperature dependence of TGS water solubility enabled purification of the compounds by several simple recrystallizations. The final precipitate was collected and dried to a powder. A die was machined (Fig. 1) from tool steel and then hardened. We lapped the surface of the spacer piece to a final clearance of 0.025 mm. A small quantity of the powder was placed between the die faces, and the opposing pieces were rotated to facilitate an even distribution of the powder. We have not yet attempted to control the size of the powder particles. A water slurry of the powder may also be employed at this stage. A force of 20,000 lb. was applied to the die and held for a few minutes at about 173°C, near the decomposition temperature of TGS. By this means we were able to produce large plates of fused polycrystalline material (Fig. 2). The resulting material, pure TGS, was optically translucent, mechanically rigid, and about 0.1 mm thick. The process is similar to the preparation of KBr pellets used in sample preparation for infrared spectroscopy. The temperature range for successful pressing, however, is fairly narrow. We noted some discoloration of the TGS plates from corrosion of the tool steel die. The use of a soft stainless steel die eliminated this problem, but the die then suffered permanent deformation of the faces during pressing. Use of a ceramic die or one of a harder stainless alloy is recommended for future experiments. Electrodes were placed on the surface of a TGS plate by vacuum evaporation of aluminum. The plate was mounted on a piece of 0.012-mm thick aluminized polyester film, and electrical contact to the top surface was made with the aid of conducting silver cement. We polarized the material by connecting the electrodes momentarily across an electrostatic potential of 50 V. The assembly Fig. 1. Die for hot pressing TGS.