  We introduce generalized Husimi functions at the interfaces of dielectric
systems. Four different functions can be defined, corresponding to the incident
and departing wave on both sides of the interface. These functions allow to
identify mechanisms of wave confinement and escape directions in optical
microresonators, and give insight into the structure of resonance wave
functions. Off resonance, where systematic interference can be neglected, the
Husimi functions are related by Snell's law and Fresnel's coefficients.
