Showing papers in "Journal of the Optical Society of America in 1986"

Minimum-variance synthetic discriminant functions

Abstract: The conventional synthetic discriminant functions (SDF’s) determine a filter matched to a linear combination of the available training images such that the resulting cross-correlation output is constant for all training images. We remove the constraint that the filter must be matched to a linear combination of training images and consider a general solution. This general solution is, however, still a linear combination of modified training images. We investigate the effects of noise in input training images and prove that the conventional SDF’s provide minimum output variance when the input noise is white. We provide the design equations for minimum-variance synthetic discriminant functions (MVSDF’s) when the input noise is colored. General expressions are also provided to characterize the loss of optimality when conventional SDF’s are used instead of optimal MVSDF’s.

Mechanism of the ultraviolet laser ablation of polymethyl methacrylate at 193 and 248 nm : Laser-induced fluorescence analysis, chemical analysis, and doping studies

Abstract: The formation of C2 in the ablation of polymethyl methacrylate (PMMA) by 193- and 248-nm laser pulses was studied by laser-induced fluorescence. The formation of C2 is observable at a fluence at 248 nm that is well below the fluence threshold where significant etching takes place. The opposite is true at 193 nm. The velocity distribution of the diatomic form was also measured and found to approach but not exactly fit a Maxwell–Boltzmann equation. Average translational energies as high as 6 eV were recorded even at the fluence threshold for this product. The other products of the laser ablation of PMMA at 193 or 248 nm are methyl methacrylate (MMA) and a solid that is a low-molecular-weight fraction of PMMA. Although the products are the same at both wavelengths, the mix is quite different. At 193 nm, 18% of the ablated polymer is MMA, whereas at 248 nm less than 1% of the polymer appeared as MMA. A semilog plot of the mass of material removed versus the fluence shows three distinct regions. The central portion at each wavelength (80–300 mJ/cm2 at 193 nm; 600–2000 mJ/cm2 at 248 nm), which corresponds to rapid etching, is identified as ablative photodecomposition. At lower fluences, the etching efficiency falls off rapidly, indicating that there is a threshold fluence. At fluences above the range for ablative photodecomposition, the etching levels off, probably because of the secondary absorption of the incoming photons by the products. The mass of material ablated per joule of energy absorbed in the ablated volume was remarkably similar at both wavelengths. It is suggested that ablative photodecomposition involves both a one-photon process that produces MMA and low-molecular-weight polymeric fragments and a many-photon process that gives rise to products such as C2 with high translational energy. In order to establish the role of the many-photon process, samples of PMMA doped with acridine were exposed to laser pulses. The presence of as little as 2% of acridine lowers the threshold for significant etching at 248 nm from 400 to 90 mJ/cm2. The absorption characteristics of the doped polymer at this wavelength show that the acridine molecules absorb >10 photons each, whereas the absorption of the polymer changes little. The drop in the threshold energy for etching should therefore be attributed to the many-photon decomposition of the acridine and the consequent ablation of the polymer as a whole.

Fundamental Limitations in Inverse Source and Scattering Problems in NDE

Abstract: Quantitative NDE is, by its very nature, a discipline within which inverse source and scattering problems abound Determining the shape of a scattering obstacle from the obstacle’s scattering amplitude or the index of refraction distribution of an inhomogeneous object from scattered field measurements performed in one or more scattering experiments are examples of inverse scattering problems encountered in quantitative NDE Determining the value of a wavefield (eg, the pressure of a sound wave) over some surface from measurements of the wave at points removed from the surface is a special case of an inverse source problem Pulse echo and transmission tomography, holographic imaging and emission tomography are further examples where the inverse source or scattering problems arise