:<math> R_{\perp} = \frac{Z_s\cos\theta - \eta_0} {Z_s\cos\theta + \eta_0} </math>
where θ is the incident angle between the propagation vector of the incident field and the normal to the surface and <math>\eta_0 = 120\pi \; \Omega</math> is the intrinsic impedance of the free space.
<table>