What is a cathode-ray tube (CRT)? A cathode-ray tube (CRT) is a vacuum tube which is used to create images in the form of light emitted from the fluorescent screen. In its simplest form, it contains a source of electrons called an electron gun aimed at a fluorescent screen, with a means to accelerate and deflect the electron beam which is used to create images in the form of light emitted from the fluorescent screen.