  If an atom is able to exhibit macroscopic dark periods, or electron shelving,
then a driven system of two atoms has three types of fluorescence periods
(dark, single and double intensity). We propose to use the average durations of
these fluorescence types as a simple and easily accessible indicator of
cooperative effects. As an example we study two dipole-interacting V systems by
simulation techniques. We show that the durations of the two types of light
periods exhibit marked separation-dependent oscillations and that they vary in
phase with the real part of the dipole-dipole coupling constant.
