A refractometer is a gem-testing instrument that reads the refractive index of one polished facet. The facet lies on a drop of contact liquid on a glass of higher index.
In more detail
The reading is where a shadow edge falls on the scale. A standard instrument has a glass of about 1.88 and 1.81 liquid, so it cannot read stones above 1.81. A doubly refractive stone gives two edges; their gap is the birefringence. Real readings have a tolerance of about 0.005 on a well-polished facet.
How we measure it
Our Refractometer models a standard instrument: glass 1.88, liquid 1.81 and yellow light at 589 nm. The Fresnel equations, for both polarizations, light each point of the scale. It is a teaching model, not a test of a real stone.
Related terms
- Shadow edge: The shadow edge is the line between the dark and the bright part of a refractometer scale. It falls at the refractive index of the stone.
- Over the limit (OTL): Over the limit (OTL) is the refractometer reading of a stone whose refractive index is higher than the contact liquid's. The scale stays dark down to the liquid line, so there is no number to read.
- Contact liquid: Contact liquid is the drop of high-index liquid between a stone and the glass of a refractometer. It fills the air gap so that light can pass from the glass into the stone.
- Refractive index: Refractive index is the speed of light in a vacuum divided by its speed in the stone. A higher index bends light more where it crosses a surface at an angle.
- Birefringence: Birefringence is the difference between the two refractive indices of a doubly refractive stone. It shows how strongly the stone splits light into two rays.