A tube with a beam
A cathode ray tube is a sealed glass vacuum tube with an electron gun at one end and a coated screen at the other. The gun fires a beam of electrons, magnets or electric plates steer the beam, and the screen glows where the beam strikes it. The device was developed in the late nineteenth and early twentieth centuries and became the heart of oscilloscopes, radar displays and, later, television sets. When engineers building the first electronic computers needed a way to see results or to hold data, the tube was already familiar. Its construction is related to the vacuum tubes that powered many early computers.
Early output
The earliest electronic computers of the 1940s mostly produced results on printers, punched cards or panels of lights. Seeing the state of the machine often meant reading a bank of indicator lamps. A screen offered a faster way, and engineers began attaching oscilloscope-style tubes to computers to show data and to monitor what the machine was doing.
One of the best-known early uses of a screen with a computer was in air defense in the 1950s. Systems in the United States such as SAGE used CRT displays to show radar tracks as moving symbols, and operators could point at a target with a light gun or light pen. This brought a kind of interactive display to a large audience of users, years before personal computers.
The tube as memory
Cathode ray tubes were used for more than display. In the late 1940s, researchers at the University of Manchester, including Frederic Williams and Tom Kilburn, developed a way to store bits as patterns of charge on the face of a tube. A beam wrote a dot or a dash at each spot on a grid, and the charge left behind could be sensed and refreshed repeatedly. It was one of the first forms of random-access electronic memory.
A small experimental machine at Manchester, nicknamed the Baby, ran a stored program in 1948 using such a tube for memory. Developments from that work led to a larger Manchester computer and to commercial machines that used the same type of memory, which is generally called the Williams or Williams-Kilburn tube. IBM and others also experimented with tube-based memory in early machines, including the IBM 701, which used banks of CRT storage tubes. Memory of this type was faster than delay lines but temperamental, and it was gradually replaced by magnetic-core memory. The wider story is told in the history of main memory.
Vector displays
Many early graphics screens did not paint a grid of dots. Instead they drew lines directly by steering the beam from point to point, a style known as a vector or calligraphic display. Because the beam only needed to trace the shapes, the images could be sharp and bright. But the picture faded quickly, so the computer had to redraw it constantly, and complex drawings could flicker.
Research in the 1960s used vector displays for interactive graphics, including systems where users drew on the screen with a light pen. Games and demonstrations of the era, such as a space combat game developed on a DEC PDP-1 at MIT in the early 1960s, relied on this kind of screen. These experiments laid groundwork for computer graphics and for the later arcade era.
Some displays used storage tubes, which retained a drawn image without constant refresh. They were useful for detailed drawings and text on remote terminals, though erasing meant clearing the whole screen.
Raster screens and terminals
Television technology provided a different approach: scan the beam in rows across the screen and vary its brightness to form an image. This raster method needed memory to hold the picture, and as memory costs fell in the 1970s, it became practical. Raster screens could show text, shapes and filled areas, and they could use mass-produced television-like tubes.
Video terminals, which paired a screen and keyboard with a connection to a larger computer, replaced many printing teletypewriters in the 1970s. Users could see what they typed and edit it on screen. When microcomputers arrived, many plugged into ordinary televisions or inexpensive monochrome monitors, and then into color ones. This is a central chapter in the story of home computing and early video games.
Color and quality
Color CRTs used three electron guns and a mask that directed each beam to red, green or blue phosphor dots. Sharpness and color accuracy varied widely. Early home computer displays often struggled with text on ordinary televisions, and better monitors were an upgrade many buyers paid for. Over the years, resolution, refresh rates and tube shapes improved, and the tubes became less bulky, though they remained deep, heavy and power-hungry compared with later flat displays.
Replacement
The CRT dominated computer screens for most of the twentieth century. During the 1990s and 2000s, flat panel technology matured, becoming thinner, lighter and cheaper, and the market for tube monitors shrank. Manufacturers stopped producing most CRTs in the following years. Some people continue to appreciate them for retro games, old computers or particular uses, while archivists value working tube displays as part of vintage restoration.
The glass tube linked several eras: it was a diagnostic tool in laboratories, a memory device in experimental machines, a console in air defense, a terminal on office desks and a television-like monitor in the home.