A traditional handheld refractometer is an analog instrument for measuring a liquid's refractive index. It works on the critical angle principle by which lenses and prisms project a shadow line onto a small glass reticle inside the instrument, which is then viewed by the user through a magnifying eyepiece. aplication area: Agriculture, Alcohol, Beverage, Food, Fruits & Vegetable, Honey, Parmaceutical, Salinity, Sugar, Urine Specific Gravity.