A refractometer, also known as a refractive index meter, is an instrument that uses light to measure the concentration of liquids. It is used to determine the refractive index, birefringence, and optical properties. The refractive index is one of the important physical constants of a substance. Many pure substances have a specific refractive index. If a substance contains impurities, its refractive index will change, resulting in deviations. The more impurities present, the greater the deviation.

The refractometer is mainly composed of a high-refractive-index prism (lead glass or cubic zirconia), a prism reflector, a lens, a scale (internal or external scale), and an eyepiece.

Refractometers include handheld refractometers, sugar refractometers, honey refractometers, gem refractometers, digital refractometers, automatic refractometers, and online refractometers.