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Form and Wavefront Metrology

Working Group 4.21

Refractometry

The high-accuracy refractometer is used to determine the refractive index of solid and liquid samples with very low uncertainty at selected wavelengths in the visible spectral range. The achievable measurement uncertainty depends decisively on the optical and mechanical properties of the measurement object. For solid samples, an uncertainty of the refractive index of 1 · 10-6 can be achieved. For liquid samples, the achievable uncertainty additionally depends essentially on the temperature coefficient of the refractive index. The achievable uncertainty amounts to 1 · 10-6 + α · 0.01 K, α being the temperature coefficient of the refractive index of the sample.
Refractive index measurements with very low uncertainty are based on the minimum of deviation method. In addition to the angle measuring instrument (goniometer) with corresponding measuring facilities, special instruments are used to determine the refractive index of the surrounding air.

Fig. 1: Principle of the refractive index measurement

For the measurement on solids, the test specimen must be optically transparent (glass, plastics, etc.) and have the form of a prism with two plane measuring faces at an angle. Depending on the measurement uncertainty desired, the prism must meet increased requirements as to the flatness, size of the surfaces and the homogeneity of the glass.

The refractive index of optically transparent solid materials in form of a prism is determined by measuring the apex-angle and the deviation-angle. The apex-angle is measured with an additional autocollimator. For the measurement of the deviation-angle, a monochromatic beam of a collimator - deviated by the prism under test - is detected. The refractive index of the test specimen at a discrete wavelength is calculated from the angles and the measured environment parameters.

Dispersion curves can be determined in the same way by varying the wavelength of the light used. For the measurement, spectral lamps with the discrete wavelengths 404.7 nm (Hg nh), 435.8 nm (Hg ng), 479.99 nm (Cd nF'), 508,6 nm (Cd), 546,07 nm (Hg ne), 589 nm (Na nD) or 643.85 nm (Cd nC') are used.