An instrument is described which measures small differences in optical reflectivity or transmissivity of two specimens. The apparatus has been used to investigate surface effects such as corrosion or deformation of thin surface layers as well as metallurgical transformations or compositional modulation. A complete ``differential spectrogram'', i.e., scan through the visible and near uv region, is performed in less than 1 min.
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Holbrook et al. (1973) studied this question.
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