Reflection and transmission in FDTD

Is there anybody who knows about calculating the reflection and transmission (coefficients) with a plane wave excitation in FDTD? Especially, i don't know about reflection because the incoming wave is mixed with it. I think I should extract the reflection wave from the total field. But it's not clear. Is there any standard method? Any comments or material(paper, notes, etc..) will be great.
Thank you for your time.

Check chapter seven of Allen tavflove book on computation electrodynamic, you wilfind some idea of how to caculate reflection and transmission co-efficeint there.Or you can check the shaum series book on electromagnetics. There is an analytical method of calculating what you want in the book.Good luck