I recently learned about etendue - which is important because it is conserved.
The only derivation of etendue conservation which I was able to understand is via hamiltonian optics (https://en.wikipedia.org/wiki/Hamiltonian_optics).
a) Is it possible to define etendue at wave optics level?
b) If not, does this mean that etendue isn't a proper physical thing?
Pointers to literature are welcome.