A cathode ray tube, or CRT, works by firing a beam of electrons at a phosphor-coated screen inside a sealed glass vacuum tube. When the electrons hit the phosphor, it glows, creating the bright dots that form an image. Every old-style TV and computer monitor before flat screens used this basic principle, and the engineering behind it is surprisingly elegant.
The Electron Gun
At the narrow end of the tube sits the electron gun, the component responsible for generating and focusing the beam. It starts with a metal plate called the cathode, which is heated to extreme temperatures, around 1,000°C (1,800°F) or higher. At that heat, electrons gain enough energy to break free from the metal’s surface entirely. This process is called thermionic emission: the metal gets so hot that electrons literally jump off it.
Once those electrons are free, they need to be shaped into a tight, controllable beam. A metal ring near the cathode, held at a slight negative charge, acts as a choke. Because electrons carry a negative charge, this ring’s electric field pushes stray electrons inward, squeezing them into a narrow column. The choke also controls how many electrons pass through at any given moment, which directly controls how bright that point on the screen will be. More electrons means a brighter spot; fewer electrons means a dimmer one. After the choke, an accelerator plate with a strong positive charge pulls the electrons forward at high speed, and additional lensing elements focus the beam into a fine point before it travels down the length of the tube.
Why the Tube Needs a Vacuum
The entire assembly sits inside a sealed glass envelope with nearly all the air pumped out. This vacuum is essential. If gas molecules were present inside the tube, electrons would collide with them, scattering the beam and losing energy before reaching the screen. The vacuum requirements inside a CRT are comparable to those in ultra-high vacuum laboratory chambers, and maintaining that low pressure over thousands of hours of operation is one of the key engineering challenges in manufacturing. The thick glass walls you see on old TVs and monitors aren’t just for durability. They’re structural, holding up against the enormous pressure difference between the vacuum inside and the atmosphere outside.
Steering the Beam
A focused beam of electrons traveling in a straight line can only hit one spot on the screen. To draw a full image, the beam needs to be swept rapidly across the entire screen surface. This is handled by a set of electromagnetic coils called the deflection yoke, wrapped around the narrow neck of the tube just past the electron gun.
These coils work in pairs. One pair deflects the beam horizontally (left to right), and another deflects it vertically (top to bottom). When current flows through a coil, it generates a magnetic field. A moving electron passing through that magnetic field experiences a force that pushes it sideways, perpendicular to both its direction of travel and the field itself. By adjusting the current through each pair of coils, the CRT can aim the beam at any point on the screen with precision.
In a television CRT, the beam sweeps across the screen from left to right, one line at a time, starting at the top. When it finishes one line, it snaps back to the left and drops down slightly to begin the next. A standard-definition TV repeated this process fast enough to paint the entire screen 25 or 30 times per second, creating the illusion of a continuous, moving image. Computer monitors often ran at higher refresh rates, 60 Hz or more, to reduce visible flicker.
How Phosphors Create Light
The wide, flat end of the tube is the screen, and its inner surface is coated with phosphor, a material that emits visible light when struck by electrons. When the focused beam hits a phosphor particle, the energy from the incoming electrons excites the atoms in the phosphor, causing them to release that energy as photons (light). The color and brightness of the glow depend on the type of phosphor and the intensity of the beam.
Black-and-white CRTs used a single type of phosphor across the screen. Color CRTs arranged three different phosphors, one red, one green, one blue, in tiny clusters or stripes across the screen. A perforated metal sheet called a shadow mask or aperture grille sat just behind the phosphor layer, ensuring that each of the three electron beams (one per color) hit only its corresponding phosphor. By varying the intensity of each beam, the CRT could mix red, green, and blue light at each point to produce a full range of colors.
Phosphors also have a property called persistence, which describes how long they continue to glow after the beam moves on. Short-persistence phosphors fade almost instantly, which is ideal for fast-moving video. Longer-persistence phosphors hold their glow a bit longer, reducing flicker on screens with lower refresh rates. The choice of phosphor was one of the design tradeoffs that determined how a CRT looked and felt to use.
Lead Glass and X-Ray Shielding
Accelerating electrons to high speeds and slamming them into a surface can produce X-rays as a byproduct. To protect users, CRT glass contains lead, typically 1 to 1.5 kilograms per screen, concentrated in the funnel and neck sections of the tube. The lead absorbs X-ray radiation before it can escape the glass, making the display safe for everyday use. This is also why CRTs are so heavy. A 27-inch TV could easily weigh 40 kilograms or more, with the leaded glass accounting for a significant portion of that weight.
Disposal and Environmental Concerns
That same lead content is what makes CRTs an environmental headache. The funnel glass generally contains high enough concentrations of lead to be classified as hazardous waste when disposed of. You can’t legally throw a CRT in a standard landfill in most places. The EPA amended its recycling regulations in 2006 to streamline how used CRTs and CRT glass are managed, and 25 U.S. states plus the District of Columbia now have electronics recycling laws on the books. If you still have an old CRT to get rid of, your local municipality or an electronics recycling program is the right path. Many retailers and waste facilities accept them, though some charge a fee to cover the cost of safely handling the lead glass.