Hadamard transform spectroscopic imaging: Raman microscopy and photothermal densitometry.

Abstract

Two distinct research projects were undertaken. Both involved the application of Hadamard transform spatial multiplexing techniques to spectroscopic imaging. The first project was the development and application of high resolution Hadamard transform photothermal deflection densitometry. The second project was the development of Hadamard transform Raman microscopy. Spatial multiplexing is a general means to reduce local power densities often encountered in conventional tightly focused laser imaging. By initially defocusing the laser and efficiently distributing the radiation, even thermally sensitive materials can be investigated. This is called the spatial distribution multiplex advantage. The photothermal densitometer employed beam condensing optics as a means to obtain high spatial resolution while still retaining the spatial distribution multiplex advantage. The densitometer optics were characterized according to the modulation transfer function, a rigorous evaluation of imaging capability. In addition, the photothermal instrument was applied to densitometry of blotted proteins. The system's performance was compared directly with the performance of a high dynamic range video densitometer. The photothermal densitometer performance compared favorably with that of the video system. Hadamard transform Raman imaging was demonstrated to be feasible with the implementation of a source-encoded system capable of one-dimensional imaging. The instrument included a coarse Hadamard mask and employed beam condensing optics for higher spatial resolution. The first Hadamard transform Raman microscope was successfully developed. The instrument used a two-dimensional encoding mask to spatially multiplex the Raman signal which was collected and magnified with microscope optics. It was possible to obtain near-diffraction limited spatial resolution spectroscopic images. The spatial distribution multiplex advantage was demonstrated with the non-destructive imaging of thermally sensitive polycrystalline samples. Multichannel detection was incorporated into a Hadamard transform Raman microscope. The system combined one-dimensional Hadamard encoding with a two-dimensional charge-coupled device detector. Raman images with 127 x 128 total pixels having submicron spatial resolution were acquired in only a few minutes. A second multichannel Hadamard microscope was also developed. The microscope was coupled to a single stage spectrograph for higher throughput. Raman images with 255 x 256 pixels were acquired. Systematic artifacts present in the images as a result of defects in the Hadamard mask were investigated. In addition, a first order correction was applied to these artifacts. The instrument was applied to the study of relatively weak Raman scattering defects in highly ordered pyrolitic graphite electrodes. The microscope was shown to be an effective investigative tool.