the present invention is directed to systems and methods which utilize a cavity ringdown spectroscopy technique for measuring vapor transmission rate through a barrier film by first allowing the vapor to accumulate over a period of time after a steady state condition has been achieved . in this manner , in one embodiment , water permeation through a plastic film , even at very low permeability rates , can be accumulated over time . the accumulated water vapor is then measured and a calculation made as to permeation per unit of time . one main advantage of the accumulated method is that minimum vapor detection is limited only by the ability to produce a good seal around the edges of the plastic film in the sample cell .