[BluePill]

Hi,

I'm currently stuck on how to derive the orientation factor for RET. Basically, the common equations for RET are:

κ² = (cos θ_T - 3cosθ_Dcosθ_A)² = (sin θ_Dsin θ_AcosΦ - 2cosθ_Dcosθ_A)²

I've also attached a diagram associated with it:

source: http://www.olympusfluoview.com/applications/fretintro.html

Can somebody point me to the right direction on how to derive this? It isn't simple geometry in my opinion.