The Abbe refractometer is an optical instrument for determining the refractive index n of liquids.
As a total reflectometer, its measuring principle is based on the fact that the critical angle of total reflection at an interface depends on the refractive indices of the optical media involved. Total reflection occurs here at the interface between the glass used, with a known and high refractive index n', and the optically thinner liquid with the lower refractive index n to be measured.
The optical design of this refractometer, developed by Ernst Abbe, enables precise determination of the critical angle with only a thin layer of liquid applied between the two glass prisms. The refraction of light depends on the wavelength of the incident light and, depending on the liquid, is highly temperature-dependent. In the past, a sodium vapour lamp was usually used as a monochromatic light source. Today, white light from an incandescent or halogen lamp and an optical filter with a wavelength of 589 nm are usually used. This corresponds to the sodium D line. The prisms are thermostatically controlled to keep the temperature constant during the measurement. The usual measurement temperature is 20°C. However, for various reasons, such as the melting, boiling and decomposition temperatures of the material being measured, it may be necessary to measure at a different temperature.
The wavelength and temperature used are specified as indices.
