There are certain hindrances with using handheld refractometers, the main one being; obtaining an accurate measurement. When using an optical refractometer, one must place their sample on the prism and look through an eyepiece to determine the measurement of an index of refraction.
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.
To learn more about Sper Digital Refractometers please watch this video