Optical Computed Tomography by Detection of Photons Preserving initial Polarization Through a Scattering Medium

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An optical CT apparatus using photons preserving initial polarization was designed on the basis of results of fundamental experiments. A diode laser, a phase-modulator, a translation stages and a detector were mounted on a small optical bread board. A personal computer was used for three purposes; control of translation stages and the gain of the lock-in amplifier, acquisition of the projection data and reconstruction of the optical CT image. The apparatus had the ability to suppress the diffused light in the transmitted light up upto 110dB.The following applications of the optical CT apparatus were investigated.(1) Two dimensional distribution of fruits ingredientsOur optical CT apparatus was applied to a pimento and a summer orange and their optical CT images were reconstructed from projection data. The internal section and the core were able to be recognized, although the spatial resolution of images was not sufficient. Three diode lasers, which had different emission wavelengths in … More dividually corresponding to the absorption lines of water, sugar (sucrose) and chlorophyll A were employed to take the projection data of the fruit. Two dimensional distribution of absorption coefficient of the reconstructed images depended on the wavelength of laser. It means the possibility that two dimensional distribution of every ingredient in food is obtained by our apparatus.(2) Detection of foreign matter in foodX ray is applicable to the detection of metals in food, but is not effective for pieces of plastics. Our apparatus was applied to detection of foreign matter in food. The projection data of the milk in a bottle in which a piece of plastic was set as a foreign matter. The existence of a piece of plastic was clearly recognized in the reconstructed image. The image by our method is easily influenced by the change of refractive index, which is common problem in the methods detecting photons travelling near the straight line. The sensitivity of our method to the boundary of refractive indices is also expected to be used for the detection of foreign matter in food. Less