Abstract: A direct band-gap semiconductor (54) is exposed to intensity-modulated photon radiation (56) having a characteristic energy at least as great as the energy gap of the semiconductor. This produces a time-dependent concentration of excess charge carriers through the material, producing a luminescence signal (58) modulated at the same frequency as the incident radiation but shifted in phase by an amount related to the lifetime of minority carriers. In a preferred embodiment, the phase shift of the luminescence signal is determined by transforming it to a modulated electrical signal and mixing the electrical signal with a reference signal modulated at the same frequency and having a phase which is known relative to the incident radiation. Minority carrier lifetime is calculated by integrating a direct current component of the mixed signal (F.sub.dc) over a 2.pi. range in phase of the reference signal.