The spectral reflectance of various samples of pressed powders of MgO and BaSO 4 were determined in the near infra-red (up to 2. 5 μm). The paper describes two semi-absolute methods used, the results yielded being converted into absolute values by extending the measurements in the visible region where the behaviour of the samples was well known. Above 1.1 μm the results show that different samples which remain substantially comparable in the visible, begin to depart significantly from each other depending on the origin of the powder. The study of MgO also illustrated the strong influence of moisture or even of the duration of rest in a normal laboratory atmosphere, whereas BaSO 4 is largely free from such defects. MgO samples were therefore discarded. The paper reports the studies carried out on samples of BaSO 4 from the different sources.
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Morren et al. (1972) studied this question.
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