Krüss refractometer refers to gemmological refractometers manufactured by A. Krüss Optronic GmbH of Hamburg. The firm produces both traditional analogue refractometers, with a high-index glass hemisphere and an internal sodium-equivalent yellow filter, and digital refractometers, with electronic readouts and built-in temperature compensation.

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The standard Krüss analogue gem refractometer reads refractive indices in the range of approximately 1.30 to 1.81, sufficient to identify the great majority of gemstone species used in the trade. The instrument is supplied with a calibrated yellow LED or filtered tungsten illuminator equivalent to the sodium D-line at 589.3 nanometres, the standard wavelength for published gem refractive index data. Distant-vision and conventional reading techniques are supported, and birefringence is measured by rotating the stone on the hemisphere.

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Krüss instruments are well-regarded for the optical quality of the hemisphere and the durability of the housings. Like all refractometers, they require a refractive index liquid (typically methylene iodide saturated with sulphur, RI 1.81) for proper coupling, and the hemisphere must be protected from scratching since it is the wearing surface that ultimately limits instrument life.