- An instrument or device which measures the refractive index of a given material or medium. An example is an instrument which measures the critical angle of a medium.
- noun an instrument which measures the refraction of the eye.
- a device used to measure the sugar content of grape juice or must. The device relies on the characteristic of a liquid to bend, or refract, light at different angles according to the levels of sugar dissolved in the liquid. The light strikes a scale that is calibrated to show the sugar content and the possible alcohol level of the finished wine using the Baumé, Brix or Oechsle scale.