The refractometer operates on the principle of critical angle measurement. Inside the refractometer, there is a light source, a prism, and an image detector. Light rays with varying angles are emitted to the prism and process interface. Rays with a steep angle are partly reflected to the image detector and partly refracted to the process. Rays with a low angle are totally reflected to the detector. The critical angle is the angle at which the total reflection begins. 

The CCD camera inside the refractometer detects a bright field and a dark field corresponding to partly and totally reflected light, respectively. The position of the borderline between these fields indicates the critical angle, which is proportional to the refractive index and thus the solution's concentration. As the measurement principle is based on physics and the law of refraction, it is drift-free. 

A built-in temperature sensor measures the temperature T on the interface of the process liquid. The sensor converts the refractive index nD and temperature T into concentration units. Vaisala Polaris process refractometer can indicate different scales, for example, Brix, liquid density or concentration by weight. The diagnostics program ensures that the measurement is reliable.

Learn more about the Refractive Index Measurement Principle (PDF) and the law of refraction.