A cathode ray tube (CRT) works by firing a focused beam of electrons at a phosphor-coated screen inside a sealed glass vacuum tube. When the electrons strike the phosphor, it emits visible light, creating the glowing dots that form an image. Every old television set and computer monitor before flat screens used this basic process, and each component in the tube plays a specific role in getting electrons from one end to the other with enough precision to draw a picture.
The Vacuum Inside the Tube
The entire process depends on a near-perfect vacuum. The interior of a CRT is evacuated to less than one millionth of atmospheric pressure. At that level, there are so few air molecules inside the tube that electrons can travel from the back of the tube to the front without colliding with anything along the way. If air were present, the electrons would scatter off gas molecules and never reach the screen in a tight, controllable beam.
Creating this vacuum is one of the more demanding parts of manufacturing. The tube is baked in an oven at 375 to 475°C to drive out trapped gases from the glass and metal components before being permanently sealed.
How the Electron Gun Produces a Beam
At the narrow neck of the tube sits the electron gun, the assembly responsible for generating and shaping the electron beam. It starts with the cathode: a small metal element (often containing tungsten) heated to high temperatures. As it heats up, some electrons gain enough thermal energy to escape the metal surface entirely. This process, called thermionic emission, is the source of every electron that eventually hits the screen.
Once electrons are freed from the cathode, a series of components shape them into a usable beam. A control grid, a metal ring held at a slightly negative voltage, sits just in front of the cathode. By adjusting this negative voltage, the grid controls how many electrons pass through. A stronger negative charge repels more electrons back toward the cathode, dimming the beam. A weaker charge lets more through, brightening it. This is how the tube controls the brightness of each point on the screen.
Beyond the control grid, an accelerator plate held at a high positive voltage pulls the electrons forward with considerable force. The beam then enters a lensing region: two adjacent metal tubes held at different voltages. The difference in voltage between these tubes creates an electrostatic “lens” that focuses the stream of electrons into a tight point, much like a glass lens focuses light. Without this focusing step, the beam would spread out and produce a blurry spot on the screen instead of a sharp dot.
Steering the Beam Across the Screen
A focused beam aimed at a single point isn’t useful for creating images. The beam needs to sweep across the entire screen surface, and it does this through deflection. There are two main approaches: electrostatic and magnetic.
Electrostatic deflection uses pairs of metal plates inside the tube. One pair controls horizontal movement, and another controls vertical movement. Applying voltage to these plates creates an electric field that pushes the beam up, down, left, or right. This method is extremely fast, capable of redirecting the beam at rates of a million inches per second, which made it the standard choice for oscilloscopes and instruments that needed rapid, precise beam positioning.
Magnetic deflection uses coils of wire (called a yoke) wrapped around the outside of the tube’s neck. Running current through these coils generates magnetic fields that bend the electron beam’s path. Television sets almost universally used magnetic deflection because it works well at the higher voltages needed for large, bright screens and produces better resolution at bigger sizes. The tradeoff is that magnetic coils respond more slowly than electrostatic plates, but since TVs draw images in a predictable scanning pattern rather than jumping randomly across the screen, that limitation doesn’t matter.
Some specialized displays actually combined both methods, using magnetic coils to position the beam in a general area of the screen and electrostatic plates to quickly draw characters within that area.
How Phosphors Turn Electrons Into Light
The front face of the tube is coated with a layer of phosphor, a material that converts the kinetic energy of incoming electrons into visible light. Phosphors are made of a transparent host material doped with a small amount of an activator element, typically a metal. When a high-speed electron slams into the phosphor, it excites the activator atoms, pushing their electrons to a higher energy state. Those excited electrons then relax back to their ground state and release the excess energy as a photon of visible light.
Different phosphor compositions emit different colors of light and glow for different durations after being struck. The duration, called persistence, matters because the beam can only be in one place at a time. As it sweeps across the screen, previously hit areas need to keep glowing long enough for the beam to finish one complete pass and return. If the phosphor fades too quickly, the top of the image goes dark before the bottom is drawn, causing visible flicker. If it persists too long, moving objects leave ghostly trails. CRT designers chose phosphors with persistence tuned to the display’s refresh rate to balance these concerns.
Building a Complete Image
In a standard television CRT, the beam starts at the top-left corner of the screen and sweeps horizontally to the right, tracing a single line. It then snaps back to the left (a movement called horizontal retrace, during which the beam is briefly turned off) and draws the next line slightly lower. This continues until the beam has covered the entire screen from top to bottom, completing one “field.” In analog TV systems, this happened in an interlaced pattern: the beam first drew all the odd-numbered lines, then returned to fill in the even-numbered lines, completing a full frame.
The brightness at each point along each line is controlled by varying the intensity of the beam in real time. As the beam sweeps, the video signal modulates the control grid voltage thousands of times per line, making each tiny spot on the phosphor glow brighter or dimmer as needed. The result is a complete image built from hundreds of horizontal lines, each composed of continuously varying brightness levels, all drawn so quickly that your eye perceives a stable, full picture.
How Color CRTs Add Red, Green, and Blue
A color CRT uses three electron guns instead of one, each responsible for one primary color: red, green, or blue. The screen is coated not with a single phosphor layer but with a pattern of tiny phosphor dots or stripes in three types, each emitting one of those colors when struck.
The challenge is making sure each gun’s beam hits only the correct color of phosphor. This is solved by placing a metal sheet called a shadow mask (or in some designs, an aperture grille) just behind the phosphor screen. The shadow mask contains hundreds of thousands of precisely positioned holes. Because the three guns are mounted at slightly different angles, each beam passes through the same hole at a different angle and lands on a different colored phosphor dot on the other side. The red gun’s beam hits only red phosphor, the green hits green, and the blue hits blue.
By varying the intensity of each gun independently, the display can mix different proportions of red, green, and blue at every point on the screen. Your eye blends these tiny adjacent dots into a single perceived color. A bright red area has its red gun firing strongly while green and blue are suppressed. White areas have all three guns at full intensity. This is the same RGB color mixing principle used in modern displays, just implemented with electron beams and phosphor chemistry.
Why CRTs Contain Lead
Electrons striking the phosphor screen at high velocity produce a small amount of X-ray radiation as a byproduct. To prevent this radiation from reaching the viewer, CRT glass contains lead oxide, particularly in the funnel and neck sections of the tube. A typical CRT screen contains 1 to 1.5 kilograms of lead, with the lead oxide making up 11% to 28% of the mass of the leaded glass sections. The faceplate (the part you look at) uses a different glass formulation with other X-ray absorbing compounds, since lead would give the image a yellowish tint. Together, these glass layers absorb virtually all X-ray emissions before they leave the tube.