Unreal Engine 4.18
The UE4 Fresnel node is actually a “Facing Ratio” node (aka Perpendicular / Parallel) with some extra control.
It basically allows controlling material effects according to the incident angle the surface is viewed at, which is a hugely important feature for designing advanced material effects.
The steepness of the value / angle curve.
Base Reflect Fraction:
The value at perpendicular angle.
An option to connect World Space surface normals input to affect the output of the Fresnel node.
* Tangent Space normals must be converted to World Space by using a Transform Vector node.
A value of 1.0 for the Exponent parameter, and a value of 0.0 for the Base Reflect Fraction will produce a linear “Facing Ratio” (“Perpendicular / Parallel”) falloff blend.