A refractometer is a laboratory or field device for the measurement of an index of refraction (refractometry). The index of refraction is calculated from Snell's law and can be calculated from the composition of the material using the Gladstone–Dale relation. There are four main types of refractometers: traditional handheld refractometers, digital handheld refractometers, laboratory or Abbe refractometers, and inline process refractometers. There is also the Rayleigh Refractometer used (typically) for measuring the refractive indices of gases.
In veterinary medicine, a refractometer is used to measure the total plasma protein in a blood sample and urine specific gravity.
In drug diagnostics, a refractometer is used to measure the specific gravity in human urine.
In gemology, a refractometer is used to help identify gem materials by measuring their refractive index.Gemstones are transparent minerals and can therefore be examined using optical methods. As the refractive index is a material constant dependent on the chemical composition of a substance, it provides information on the type and quality of a gemstone. Classification with a special gemstone refractometer is an easy-to-use method with which the authenticity and quality of a stone can be evaluated. The gemstone refractometer is therefore a piece of basic equipment in a gemological laboratory. Due to the dependence of the refractive index (dispersion) on the wavelength of the light used, the measurement is normally taken at the wavelength of the sodium-D-line (NaD) of 589 nm. This is either filtered out from daylight or generated with a monochromatic light-emitting diode (LED). Certain stones such as rubies, sapphires, tourmalines and topaz are optically anisotropic. They demonstrate birefringence based on the polarisation plane of the light. The two different refractive indexes are classified using a polarisation filter. Gemstone refractometers are available both as classic optical instruments and as electronic measurement devices with a digital display.
In marine aquarium (pool maintenance) keeping, a refractometer is used to measure the salinity and specific gravity of the water.
In home brewing, a refractometer is used to measure the specific gravity before fermentation to determine the amount of fermentable sugars which will potentially be converted to alcohol.