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.

Most commonly, aqueous solutions are measured, providing an indication concentration. Examples measuring aqueous solution concentration include the specific gravity of urine, coolants for engines or machine tools, and the salinity of water in aquariums or aquaponic systems.

Refractive index is dependent on the wavelength of light. A known reference wavelength must, therefore, be used. This is typically provided by filtering daylight or using a narrow-band LED. The temperature of the sample will also affect its refractive index and must therefore be within specified limits to achieve the stated accuracy for a refractometer. The most accurate refractometers used closed-loop control of the sample temperature.

Types of refractometer include:

• Handheld analogue refractometers are held up to a light source, such as the sun, so that light is directed through the sample, a prism and lenses onto a measurement scale. The angle at which light is totally internally reflected determines the position of a shadow line on the scale, which can then be viewed through an eyepiece.
• Handheld digital refractometers work in essentially the same way as the traditional analogue refractometers, with a shadow line indicating the angle at which total internal reflection occurs. However, rather than simply holding the refractometer up to the light, an LED light source is normally included. This can improve accuracy by controlling the wavelength more accurately. The eyepiece and glass scale is also replaced by an array of photodiodes, enabling the shadow line to be digitally detected and the relevant measurement result displayed numerically on a screen.
• Abbe refractometers are bench-top instruments designed for more accurate measurement. They therefore typically include some temperature control of the sample. Abbe refractometers also include an optical arrangement designed to eliminate Abbe error, which can be caused by different viewing angles. Abbe refractometers may be either analogue optical instruments or more typically today, digital instruments.
• Inline process refractometers continuously measure the refractive index of a fluid as it flows through the sensor.
• Brix refractometers are calibrated to measure the sugar content of a solution using a scale where 1 degree brix is equal to 1% sucrose by mass. This scale is widely used in the food industry and simple handheld Brix refractometers are used for home
• Gemstone refractometers are used to indicate the chemical composition of gems. Since some gems have a refractive index which is dependent on the polarization of the light (birefringence), gemstone refractometers may also involve polarization filters.