An analysis of the diffraction efficiency of photorefractive holograms in ferroelectric crystals is shown to provide a novel technique for determining the nature and magnitude of the equivalent electron drift field. For a Fe doped lithium niobate crystal (0.05% per mole) we find that the total field consists of an intensity independent internal field of 8.5 KV/cm and a photo-generated field given by the conversion factor 1.4Ã 10⁶ V-cm/W.