Video camera tube

The video camera tube was a type of vacuum tube used to capture television images between the 1930s and 1980s.
A video camera tube uses a lens to focus the image on a photoelectric material (a material which emits electrons when light hits it) housed at the front of a glass tube.
An electron gun at the back of the tube, like those used in a cathode ray tube, fires electrons at the material. When the photoelectric material is hit with brighter light, it produces more electrons, which cause more of the electrons fired by the electron gun to bounce back.
By measuring the number of electrons which bounced back, the video camera tube is able to tell how bright each part of the image is, and create a signal suitable for display on a cathode ray tube.