A method is described for measuring the specular optical reflectance R($) of the interface between a fluid and a glass prism, the incident and reflected light being in the glass, and the angle of incidence 4 being varied about the critical angle. Transparent, absorbing and some turbid fluids give R y ) close to theoretical predictions, and experimental reflectance data are analysed to give the real (n') and imaginary (n") refractive indices of the fluid. Other turbid fluids gave R(4) data that differed strongly from theoretical expectation. This is attributed to heterogeneity of the fluid on the optical wavelength scale, and problems of refractometly for such fluids are discussed.

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High-precision critical-angle refractometers, with automatic refractive index computation and readout, are now available from several manufacturers. In such instruments light is incident from within a transparent reference medium, typically an optical prism, onto a sample material in good optical contact with the prism. The critical angle of reflection is measured at the interface between the prism of known refractive index and the sample material of unknown refractive index. Critical-angle refractometers are calibrated for transparent samples but are commonly used for measurements of refractive index in optically absorbing or optically heterogeneous materials. The instrument will then read an apparent refractive index which differs from the true refractive index of the sample. The optics of the critical-angle refractometer are investigated to quantify errors which arise from the neglect of absorption and heterogeneity in the transparent sample interpretation of the critical angle. Refractive index reading errors will be important when they become larger than the refractive index precision of modern instruments, about 10 −5 . In critical-angle refractometry it is shown that this occurs for samples of surprisingly weak absorption. The critical-angle and Brewster-angle methods are compared for optically absorbing and heterogeneous samples. Errors arising from sample absorption in the Brewster-angle method are shown to be much less than in the critical-angle method.