Many photocathodes require excellent vacuum conditions to function and will become "poisoned" when exposed to contaminates.  Additionally, using the photocathodes in high current applications will slowly damage the compounds as they are exposed to ion back-bombardment.  These effects are quantified by the lifetime of the photocathode.  Cathode death is modeled as a decaying exponential as a function of either time or emitted charge.  Lifetime is then the time constant of the exponential.