Standard refractometers measure the extent of light refraction (as part of a refractive index) of transparent substances in either a liquid or solid state; this is then used in order to identify a liquid sample, analyse 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

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.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.