A refractometer is a device which measures the content (brix or sugar, honey, urine, salt, battery fluid…) of a solution by the refraction of light. Using an optical refractometer is simply a matter of placing several drops of the sample liquid on the angled prism and sealing the clear plate on top of it. Look through the eyepiece while pointing the refractometer at a source of direct light (do not look directly at the light with the naked eye).
The use of a digital refractometer eliminates the uncertainty associated with optical refractometers; a digital refractometer uses an LED light to shine through the sample. When a liquid sample is applied to the measuring surface only some of the light is transmitted through the solution and is lost; while the remaining light is reflected onto a linear array of photodiodes creating a shadow line. The refractive index is directly related to the position of the shadow line on the photodiodes. Once the position of the shadow line has been automatically determined by the instrument, the internal software will correlate the position to refractive index, or to another unit of measure related to refractive index, and display a digital readout.
One benefit to a refractometer is that most models have an automatic temperature conversion, so samples can be taken directly from a production line. The second benefit is that only a very small sample is needed. A couple drops of liquid will give an accurate reading.