Fresnel reflection is the share of light that a surface reflects instead of letting through. The Fresnel equations give that share from the two refractive indices, the angle and the polarization.
In more detail
At the crown, part of the light reflects straight off and never enters. As light leaves the stone, part of it reflects back inside. A higher index reflects more: about 17% at normal incidence for diamond.
How we measure it
The simulator uses the full Fresnel equations, per polarization in doubly refractive stones. The Brightness & Tilt Analysis and the Cut Grading Tool use Schlick's approximation at entry and exit. The light census and the light map use none. The Refractometer uses the full equations for both polarizations to light its scale.
Related terms
- Schlick's approximation: Schlick's approximation is a short formula for Fresnel reflectance: R(θ) = F0 + (1 − F0)(1 − cos θ)^5. F0 is the reflectance at normal incidence, ((n − 1)/(n + 1))².
- Total internal reflection: Total internal reflection happens when light inside a stone meets a facet beyond the critical angle. The facet then acts as a perfect mirror, and all the light reflects back into the stone.
- Polarized light: Polarized light vibrates in one direction across its path, instead of in every direction. A polarizing filter passes only the light that vibrates along its own direction.
- Ray tracing: Ray tracing follows straight lines of light through a model of a stone. At each facet it applies refraction and reflection, and it records where the light comes from or where it leaves.
- 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.