A refractometer is a laboratory or field device for the measurement of an index of refraction (refractometry). The refractive index of a certain sample varies for nearly all materials for different wavelengths. This dispersion relation is characteristic for every material. Temperature has a very important influence on the refractive index measurement. Therefore, the temperature of the prism & the temperature of the sample have to be controlled with high precision.
Automatic refractometer
Automatic refractometers automatically measure the refractive index of a sample. The automatic measurement of the refractive index of the sample is based on the determination of the critical angle of total reflection.An LED light source is imaged under a wide range of angles onto a prism surface which is in contact with a sample. Depending on the difference in the refractive index between prism material and sample the light is partly transmitted or totally reflected. The critical angle of total reflection is determined by measuring the reflected light intensity as a function of the incident angle
This method of detecting the angle of total reflection is independent on the sample properties. It is even possible to measure the refractive index of optically dense strongly absorbing samples or samples containing air bubbles or solid particles . Furthermore, only a few microliters are required and the sample can be recovered. This determination of the refraction angle is independent of vibrations and other environmental disturbances.
Watch: How to use a refractometer https://www.youtube.com/watch?v=Qd9uJ9JitJw