1) Optical device 2) Vacuum tube 
A phototube or photoelectric cell is a type of gas-filled or vacuum tube that is sensitive to light. Such a tube is more correctly called a `photoemissive cell` to distinguish it from photovoltaic or photoconductive cells. Phototubes were previously more widely used but are now replaced in many applications by solid state photodetectors. The pho.....
a photosensitive tube in which the photoemissive effect is utilized