Lifetimes of metastable levels of chromium in ruby are determined using an unusual technique. The fluorescence from the chromium is measured using an ac technique in which the excitation is modulated and the fluorescence is detected synchronously. The novelty lies in the measurement of the dependence of the signal on the modulation frequency and extracting the lifetime from the frequency dependence. The experiments and data analysis demand a detailed understanding of the methods of lock‐in amplification and synchronous detection and hence they can serve as a useful primer on the subject. The observed values for the lifetimes show excellent agreement with previous measurements.