Refractometry is the analytical method of measuring substances' refractive index (one of their fundamental physical properties) in order to, for instance, assess their composition or purity.
A refractometer is the instrument used to measure refractive index ("RI").
The RI of a substance is strongly influenced by temperature and the wavelength of light used to measure it; therefore, care must be taken to control or compensate for temperature differences and wavelength.
RI measurements are usually reported at a reference temperature of 20 degrees Celsius, which is equal to 68 degrees Fahrenheit and is considered to be room temperature.
Therefore, the refractive index of water at 20 degrees Celsius, taken at the Sodium D Line, would be reported as 1.3330 nD20.