Refractive index: how diamonds and gems bend light
Refractive index is a measure of how much a material slows and bends light as it enters. It is the speed of light in a vacuum divided by its speed in the material, so a higher number means a stronger bend. Diamond's refractive index is about 2.42, against about 1.5 for glass, which is a large part of why a well-cut diamond returns so much light.
3D Jewelry Designer1 min read
Why a high index matters
A high index also traps light. Inside a diamond, light that meets a facet at more than about 24 degrees from the perpendicular reflects back inside instead of escaping; in glass, that angle is about 42 degrees. A good cut uses this: light that enters the top bounces off the lower facets and comes back out of the top as brilliance.
The index differs slightly for each colour of light. That spread is dispersion, which splits white light into flashes of colour called fire.
Refractive index in a 3D render
3D software usually calls it IOR, short for index of refraction, and uses it to bend each ray at every facet. A diamond given the index of glass renders like glass.
In the studio, diamonds get real refraction, dispersion and fire, and coloured stones keep their saturation instead of turning grey or black.
The free 3D jewellery renderer renders your own file in the studio's engine. More in how to render diamonds realistically and jewellery rendering in the browser.
Questions
How is refractive index measured?
With a refractometer. Each gem species has its own narrow range, so the reading helps identify a stone. Diamond is beyond the range of a standard refractometer, so other tests identify it.
Related
- Dispersion (fire) in diamonds and gemstones
Dispersion is how a gem splits white light into spectral colours, and fire is the flashes of colour you see. How it is measured and how it renders in 3D.
- Diamond brilliance: what it is and how cut affects it
Diamond brilliance is the white light a polished stone returns to the eye. How cut creates it, how it differs from fire, and how a 3D render shows it.
- How to render diamonds realistically
A diamond render looks real when the stone refracts at diamond's index, light bounces inside it, dispersion adds fire and the lighting has contrast.