VALOA BRIX is a non-invasive refractometer that measures the refractive index of a liquid across a glass wall.
The sugar content of wine is generally measured in one of two units, Brix (= %) or grams/liter (g/l). 10 g/l = 1 Brix. Adding sugar to liquids makes their refractive index go up.
In wine, it is the sugar content that affects the index most. Alcohol affects the index as well, but since the alcohol content is always indicated on the label, its effect can be taken into account.
The measurement is based on the amplitude of the Fresnel reflection. Cloudy liquids cannot be measured, because the back-scatter from the cloudiness interferes with the measurement. The color of the bottle does not have an effect on the result, because the absorption of light in the glass, if any, is compensated for by the multiple-beam optics.
The wall thickness of the bottle also has no effect on the result, due to the design of the optics. A patent is pending.