A refractometer is a tool used in wine making to measure the sugar content, i.e. the brix in grapes.
A refractometer looks similar to a handheld microscope. To use, place a drop of juice from a fruit or vegetable onto the glass plate and look through the device to read the meter that measures the amount of brix of the juice.
Besides wine, refractometers identify brix to measure the approximate amount of sugars in fruits, vegetables, juices, wine, soft drinks and in the starch and sugar manufacturing industry.