Define Glass photo-emissive tubes?

Glass photoemissive tubes are those which consist of a curved metal cathode, coated with a photosensitive material. While light strikes its surface, cathode emits electrons which are attracted to positively charged anode, causing a current to flow that is directly proportional to the light intensity. The phototube is either evacuated or filled with an motionless gas at low pressure. In the later, the electrons emitted from cathode strike gas molecules that release more electrons and thus amplify the current response to the fight radiation. Although tubes are available for wavelengths well beyond both ends of the visible spectrum, two tubes are needed to cover a range of 200-800 nm.