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Imaging into thick media is currently a major area of optics, or more specifically, of image science. In one form, the object is a mildly scattering structure, in which techniques such as optical coherence tomography and confocal imaging apply. In the other form, that of highly scattering media, these techniques break down, and the technique of photon migration comes into play.We have for some time worked in these areas and continue to do so. In this chapter, we summarize what we regard as some of the most significant results achieved over several years, then describe in more detail some results obtained during the past year. The method of photon migration, using the first arriving light, is a common way to image, in transmission, absorbing structures embedded in a thick, highly scattering medium. In this technique (Fig. 1.1), the object is backlighted, and the light emerging from the medium is imaged. If the illuminating source is a short pulse, then the pulse emerging from the medium is significantly stretched because the light is multiply scattered inside the medium, and the photons follow various paths of different length. The light that emerges first, since it was scattered the least or scattered mostly in the forward direction, will form the best image of whatever is embedded in the medium. This is the principle of the first arriving light, as first given by Duguay and Mattick. A time gate is required to separate the first arriving light from the remainder.
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