An image intensifier tube comprising a sealed envelope having therein means for controllably liberating oxygen within the envelope, and an iput screen including an oxygen conditioned layer of fluorescent material and an overlying layer of photoemissive material which may be oxidized to improve the photo-to-electron conversion efficiency of the input screen. A method of improving the conversion efficiency of an image intensifier tube input screen including the steps of exposing a layer of fluorescent material to oxygen prior to the deposition of an overlying layer of photoemissive material, and subsequently exposing the layer of photoemissive material to oxygen, if desired, after the tube envelope has been sealed.