Traditional handheld refractometer

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.

In use, a sample is placed 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.

Compensation is accomplished through the use of a small bi-metallic strip that moves a lens or prism in response to temperature changes.