Abstract
We present a practical algorithm based on the Fourier power spectrum for expanding the measuring range of a critical-angle refractometer (CAR). Refractive index (RI) information is extracted from the gradual change in the relative reflective ratio. The Abbe-type refractometer has a limited measurement range based on Snell's reflective law. When the RI of a liquid sample is beyond the upper limit of the Abbe-type refractometer, it can be obtained by using the Fourier power spectrum of reflectivity. The maximal value of the Fourier power spectrum of the reflective ratio is associated with the RI of the liquid. Supported by theoretical analysis and experimental results using sugar solutions with RI that varied from 1.359 to 1.3766 tested through the reflective CAR, the algorithm is found to be computationally effective and robust to expand the measuring range of the Abbe-type refractometer.
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