In 3D computer graphics, Schlick’s approximation, named after Christophe Schlick, is a formula for approximating the contribution of the Fresnel factor in the specular reflection of light from a non-conducting interface (surface) between two media.^[1]

According to Schlick’s model, the specular reflection coefficient R can be approximated by:

$${\displaystyle R(\theta )=R_{0}+(1-R_{0})(1-\cos \theta )^{5}}$$

$${\displaystyle R_{0}=\left({\frac {n_{1}-n_{2}}{n_{1}+n_{2}}}\right)^{2}}$$

${\displaystyle \theta }$

${\displaystyle \cos \theta =(N\cdot V)}$

${\displaystyle n_{1},\,n_{2}}$

${\displaystyle R_{0}}$

${\displaystyle \theta =0}$

${\displaystyle n_{1}}$

In microfacet models it is assumed that there is always a perfect reflection, but the normal changes according to a certain distribution, resulting in a non-perfect overall reflection. When using Schlick’s approximation, the normal in the above computation is replaced by the halfway vector. Either the viewing or light direction can be used as the second vector.^[2]