where ω ω ~ ~ and ω ω ~ ~ 0 are the complex frequencies of the perturbed and unperturbed cavity modes, and H 0 and E 0 are the electromagnetic fields of the unperturbed mode (permeability change is not considered for simplicity). Expression (1) relies on stored energy considerations. The latter are intuitive since common sense dictates that the maximum change in resonant frequency occurs when the perturbation is placed at the intensity maximum of the cavity mode. However energy consideration in electromagnetism is only valid for Hermitian systems for which energy is conserved. For cavities, energy is conserved only in the limit of very small leakage (infinite Q's), so that Expression (1) is only valid in this limit. For instance, it is apparent that Expression (1) predicts a change of the Q factor ( I m ( ω ω ~ ~ − − ω ω ~ ~ 0 ) ) only if Δ Δ ϵ ϵ is complex, i.e. only if the perturber is absorbent. Clearly this is not the case and it is well known that a dielectric perturbation may either increase or decrease the Q factor.