A linear differentiating refractometer

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Abstract
A new version of the linear critical-angle refractometer for liquids is described: a 1728 element linear CCD array is used to scan the intensity profile of the output image, within which the position of a cut-off edge is a linear function of refractive index. The precision in locating this cut-off edge is greatly increased, especially for milky liquids, by differentiating the intensity profile numerically. Further information about complex particulate liquids is found to be given by the shape of the differentiated profile, and alternative internal and external illumination modes are provided which may give more information. The present instrument covers the refractive index range 1.3000 to 1.4700, with an accuracy of +or-0.0001 for transparent liquids. The light source is an LED at 635 nm. The intensity profile, the differentiated intensity profile, and the refractive index value are all computed, displayed and printed by a desk-top computer.

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