A refractometer is a laboratory or field device for the measurement of an index of refraction (refractometry). The index of refraction is calculated from the observed refraction angle using Snell's law. For mixtures, the index of refraction then allows the concentration to be determined using mixing rules such as the Gladstone–Dale relation and Lorentz–Lorenz equation. Refractometry Standard refractometers measure the extent of light refraction (as part of a refractive index) of transparent substances in either a liquid this is then used in order to identify a liquid sample, analyze the sample's purity, and determine the amount or concentration of dissolved substances within the sample. As light passes through the liquid from the air it will slow down and create a ‘bending’ illusion, the severity of the ‘bend’ will depend on the amount of substance dissolved in the liquid. For example, the amount of sugar in a glass of water.[1] Types There are four main types of refractometers: traditional handheld refractometers, digital handheld refractometers, laboratory or Abbe refractometers (named for the instrument's inventor and based on Ernst Abbe's original design of the 'critical angle') and inline process refractometers.[2] There is also the Rayleigh Refractometer used (typically) for measuring the refractive indices of gases. In laboratory medicine, a refractometer is used to measure the total plasma protein in a blood sample and urine specific gravity in a urine sample. In drug diagnostics, a refractometer is used to measure the specific gravity of human urine. In gemology, the gemstone refractometer is one of the fundamental pieces of equipment used in a gemological laboratory. Gemstones are transparent minerals and can therefore be examined using optical methods. Refractive index is a material constant, dependent on the chemical composition of a substance. The refractometer is used to help identify gem materials by measuring their refractive index, one of the principal properties used in determining the type of a gemstone. Due to the dependence of the refractive index on the wavelength of the light used (i.e. dispersion), the measurement is normally taken at the wavelength of the sodium line 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.[3] In marine aquarium keeping, a refractometer is used to measure the salinity and specific gravity of the water. In the automobile industry, a refractometer is used to measure the coolant concentration. In the machine industry, a refractometer is used to measure the amount of coolant concentrate that has been added to the water-based coolant for the machining process. In homebrewing, a brewing refractometer is used to measure the specific gravity before fermentation to determine the amount of fermentable sugars which will potentially be converted to alcohol. Brix refractometers are often used by hobbyists for making preserves including jams, marmalades and honey. In beekeeping, a brix refractometer is used to measure the amount of water in honey.