Could someone provide an equation for calculating frustrated internal reflection? Like for a partially reflective laser mirror or a beam splitter. I believe that it depends on the refractive indexes of a first medium and a third medium if a second medium separating the two (like a mirror) is thin enough that evanescent wave coupling allows light to be transmitted through to the third if it's in a certain angle range. However, I couldn't find an equation to calculate this. I found one for total internal reflection but not for this. I could use some help on this, or otherwise something that explains this. Thank you.
Frustrated total reflection obviously means that the usual total reflection at a surface to a medium with lower refractive index $n$ becomes less than total because the thickness of the lower $n$ medium becomes comparable to the evanescent wave damping length penetrating the lower $n$ medium.
This can be calculated by using the Fresnel equations with multiple surfaces. See Fresnel equations