The digital refractometers are optical instruments that employ the measurement of refractive index to determine the % Brix of sugar (VT871), % Fructose (VT872), % Glucose (VT873) and % Invert Sugar (VT881) in aqueous solutions. The method is both simple and quick. Samples are measured after a simple user calibration with deionized or distilled water. Within seconds the instruments measure the refractive index of the sample and convert it to % Brix or % by weight concentration units.

A refractometer measures the index of refraction for a material, which could be a gas, a liquid or a transparent solid such as glass. They achieve this by passing light through the sample and measuring the refraction — the amount that the light bends.