An exemplary method for measuring a refractive index of a substance being measured through an optical window, includes arranging the optical window in contact with the substance being measured, directing light to the interface of the optical window and substance being measured, where part of the light is absorbed by the substance being measured and part of it is reflected from the substance being measured to form an image, in which the location of the boundary of light and dark areas expresses a critical angle of the total reflection dependent on the refractive index of the substance being measured, and examining the formed image. Light is directed on a first structure and to desired angles on an interface between the optical window and substance being measured. Light reflected from the interface of the optical window and substance being measured is directed on a second structure.