  Microcavities and nanoresonators are characterized by their quality factors Q
and mode volumes V. While Q is unambiguously defined, there are still questions
on V and in particular on its complex-valued character, whose imaginary part is
linked to the non-Hermitian nature of open systems. Helped by cavity
perturbation theory and near field experimental data, we clarify the physics
captured by the imaginary part of V and show how a mapping of the spatial
distribution of both the real and imaginary parts can be directly inferred from
perturbation measurements. This result shows that the mathematically abstract
complex mode volume in fact is directly observable.
