Traditional handheld refractometer

A traditional handheld refractometer is a type of refractometer, a handheld analog instrument for measuring refractive index that works on the "critical angle" principle. It utilizes lenses and prisms to project a shadow line onto a small glass reticle inside the instrument, which is then viewed by the user through a magnifying eyepiece.

In use, a sample is sandwiched between a measuring prism and a small cover plate. Light traveling through the sample is either passed through to the reticle or totally internally reflected. The net effect is that a shadow line is formed between the illuminated area and the dark area. It is at the point that this shadow line crosses the scale that a reading is taken. Because refractive index is very temperature dependent, it is important to use a refractometer with automatic temperature compensation. Compensation is accomplished through the use of a small bi-metal strip that moves a lens or prism in response to temperature changes.