The photoluminescence properties of conjugated polymers are currently of great interest, since the radiative decay of singlet excitons is the process by which light emission occurs in polymer light-emitting diodes. We describe here accurate measurements of photoluminescence efficiencies in solid films of conjugated polymers, using an integrating sphere to collect the emitted light. Values in excess of 40% are measured for cyano-substituted derivatives of poly(p-phenylenevinylene) (PPV). In PPV itself, measured photoluminescence efficiencies of 27%, combined with time-resolved measurements of the luminescence decay, suggest that the species produced by photoexcitation is predominantly the singlet exciton responsible for luminescence. The photoluminescence spectrum and the rate of radiative decay can be strongly affected by the presence of dielectric or metal interfaces. We model here the effects of optical interference in a number of multi-layer structures. We compare these results with measured photoluminescence efficiencies in LED-like structures, and discuss the implications for the efficiency and emission characteristics of polymer light-emitting diodes.