Optical measuring device (refractive index meter) with which the concentration of liquids or solids can be determined by means of light refraction. A variant of such a device was developed by the German physicist Ernst Abbe (1840-1905), who was the director of the observatory in Jena and owner of the Zeiss company. In viticulture, it can be used to determine the sugar content in the form of the must weight and thus the potential alcohol content already in the grapes in the vineyard or in the grape must. In a drop of grape must enclosed between two prisms, light is diffracted depending on the sugar content and the value is usually displayed on a scale in several common units of measurement (for example Brix, KMW, Oechsle).
The more concentrated the sugar solution, the more strongly the light is refracted. This is used to determine the degree of ripeness, which (among other things) serves as a decision aid for the ideal time of so-called physiological ripeness during the grape harvest. In order to obtain a representative value, at least 100 berries must be taken from different zones of the grapes and necessarily also from several grapes from different locations in the vineyard. These berries are crushed together in a container and then some juice is extracted for the measurement. By taking several measurements on different days in a row, the must weight development is determined. Regarding other measuring methods or measuring instruments, see under analytical testing.