Abstract: A refractometer, comprising a frame structure, in which are arranged a light source, an optical window and a light sensitive detector, whereby the optical window is arranged to be positioned into contact with a solution to be measured and a beam of rays from the light source is arranged to be directed to an interface between the optical window and the solution, whereby part of the ray beam reflects back from the solution and part of it is absorbed partially into the solution and an image is created, in which the location of a borderline between a light and a dark area depends on the critical angle of total reflection, this angle being a function of the concentration of the solution, and whereby an information of the location of the borderline is arranged to be transformed into an electrical form by the light sensitive detector.