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.
In more detail
It is fast and close to the full Fresnel equations for unpolarized light. In our engine, F0 is 0.172 for diamond and 0.204 for moissanite.
How we measure it
The Brightness & Tilt Analysis and the Cut Grading Tool's brilliance apply it at the entry and exit facets. The simulator uses the full Fresnel equations instead.
Related terms
- Fresnel reflection: 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.
- Brightness & Tilt Analysis: The Brightness & Tilt Analysis is a free Digital Gemology tool. It measures how much dome light a cut returns face-up, and how this changes with tilt.
- 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.