A cathode ray tube, or CRT, is a vacuum tube that fires a beam of electrons at a coated glass screen to produce light. For most of the 20th century, it was the core technology behind televisions, computer monitors, radar screens, oscilloscopes, and medical displays. While flat screens have replaced CRTs in most settings, the technology still shows up in surprising places today.
How a CRT Creates an Image
Inside a CRT, a heated element at the narrow end of the tube releases a stream of electrons. This stream is focused into a tight beam and accelerated toward the wide, flat end of the tube, which is coated with phosphor, a material that glows when electrons strike it. By steering the beam with magnetic or electric fields, the tube can hit any point on the screen and make it light up.
In a television or monitor, the beam sweeps across the screen left to right, one horizontal line at a time, while slowly moving downward. When it finishes a full pass of the screen, it snaps back to the top-left corner and starts again. This process, called raster scanning, happens fast enough that the phosphor glow from earlier lines hasn’t faded by the time new ones are drawn. Your brain’s persistence of vision fills in the gaps, making the rapidly painted lines look like a single, steady image. String enough of these frames together and you get smooth motion, similar to how a film projector works.
Television and Computer Monitors
The most widespread use of CRTs was in home televisions and desktop computer monitors. From the late 1930s through the early 2000s, virtually every TV set contained a cathode ray tube. Color CRTs used three electron beams (one each for red, green, and blue phosphors) firing through a perforated metal sheet called a shadow mask to produce the full color spectrum. This was the standard display technology for decades, and it shaped how video content was produced. Game developers, filmmakers, and graphic designers all designed their work around the visual characteristics of CRT screens.
LCD televisions didn’t overtake CRT sales globally until the fourth quarter of 2007, when LCDs captured 47% of the TV market compared to CRT’s 46%. That crossover marked the beginning of a rapid decline. Within a few years, major manufacturers stopped producing CRT televisions entirely.
Oscilloscopes and Signal Analysis
Outside the living room, one of the most important applications of CRT technology has been the oscilloscope. An oscilloscope uses a CRT to draw electrical signals as visible waveforms on a screen, essentially turning invisible voltage changes into a graph you can see in real time. The beam’s vertical position corresponds to voltage, while it sweeps horizontally to represent time. Engineers, electricians, and scientists relied on CRT oscilloscopes to diagnose circuits, test equipment, and study everything from radio waves to heartbeats.
CRT oscilloscopes typically use electrostatic deflection (steering the beam with charged plates rather than magnets) because it responds fast enough to display high-frequency signals accurately. The deflection of the beam is directly proportional to the applied voltage, so the waveform on screen is a faithful representation of the electrical signal being measured.
Radar Displays
Radar was one of the earliest high-stakes applications for CRTs. During World War II, military radar systems used cathode ray tubes to show the position of aircraft, ships, and other objects detected by radio pulses. The U.S. Navy deployed CRT radar displays as early as 1945. In these systems, the electron beam swept outward from the center of the screen in a rotating pattern. When a radar pulse bounced back from an object, a bright dot appeared on the phosphor screen at the corresponding distance and angle. Air traffic control, maritime navigation, and weather tracking all depended on CRT-based radar well into the digital era.
Medical Imaging
CRT monitors became integral to radiology and diagnostic imaging. Ultrasound machines, CT scanners, MRI consoles, and patient monitoring systems all used CRT displays to show their output. The technology’s ability to render fine gradations of brightness made it well suited for medical images, where subtle differences in shading can indicate tumors, fractures, or other abnormalities. Heart monitors in hospitals displayed continuous electrocardiogram waveforms on CRT screens for decades before digital flat panels took over.
The Role CRTs Played in Science
Before CRTs became consumer products, they were scientific instruments. The technology traces back to experiments with vacuum tubes in the 1800s. In 1879, William Crookes used an improved vacuum pump to build better cathode ray tubes and demonstrated that cathode rays carry a negative charge by deflecting them with a magnetic field. This work laid the groundwork for J.J. Thomson’s discovery of the electron in 1897, one of the most important findings in modern physics. The CRT went from a laboratory curiosity to a tool that reshaped communication, entertainment, and science.
Why CRTs Still Get Used Today
CRTs have largely disappeared from everyday life, but they persist in a few niche areas. One is retro gaming. On a CRT, electrons strike the phosphor screen directly with no digital processing in between, resulting in near-zero input lag (often under one frame). Modern flat panels, even in dedicated game modes, typically introduce 20 to 50 milliseconds of delay. Skilled players can detect differences as small as 10 milliseconds, which matters in fast-paced games and competitive speedrunning. Beyond latency, many older games were designed around CRT characteristics like scanline rendering, phosphor glow, and native support for low resolutions like 240p. Playing these games on a modern monitor without an external signal converter often introduces scaling artifacts and color inaccuracies.
The other major holdout is industrial and legacy equipment. CRT monitors are still sold as replacements for CNC machining systems, older medical devices, and specialized industrial controls. These machines were built decades ago with CRT displays integrated into their hardware and software, and replacing the entire system can cost far more than sourcing a compatible CRT monitor. Ruggedized CRT displays with analog inputs remain available from specialty manufacturers for use in harsh environments where the equipment they serve is still running reliably.
Environmental Concerns With Disposal
One lasting issue with CRTs is what happens when they’re thrown away. The glass in a typical CRT contains about 8% lead by weight, used to shield viewers from X-rays generated inside the tube. That lead makes CRTs hazardous waste. Millions of old televisions and monitors have ended up in landfills or warehouses awaiting proper recycling. Breaking down CRT glass safely requires separating the leaded funnel glass from the front panel, a process that’s more expensive than recycling most other electronics. This disposal challenge is one reason e-waste programs specifically flag CRT monitors for special handling.