A glass bottle with an electron gun
For several decades, nearly every television and computer monitor was built around the same part: a large, sealed glass tube with most of the air removed. At the narrow back end sat an electron gun. At the wide front end, the inside of the glass was coated with materials called phosphors that glow when electrons hit them. Cathode ray tubes, often shortened to CRTs, drew pictures by steering a beam of electrons across that coating.
The vacuum matters. Electrons travel freely in a near-empty space, whereas air would scatter them before they reached the screen. It is also the reason the glass had to be thick and strong, and why the tubes were so heavy.
How the beam makes light
The electron gun heats a cathode, which releases electrons. An electric field accelerates them toward the front of the tube, and electrodes shape them into a narrow beam. When the beam strikes a phosphor dot, that spot emits light for a short time. The harder the beam hits, the brighter the spot, so the signal controls brightness by controlling the beam’s strength.
A single beam cannot light the whole screen at once. Instead, the tube relies on persistence of vision: the beam moves so fast that the eye perceives a full image even though only a small point is lit at any instant.
Steering the beam: deflection
To move the beam, a CRT uses magnetic coils around the neck of the tube (many designs) or electrically charged plates (common in small instruments such as oscilloscopes). Changing the current in the coils bends the beam’s path.
The beam follows a pattern called a raster:
- It sweeps left to right along one thin line.
- It switches off briefly and snaps back to the left, a short interval often called the retrace or flyback.
- It drops down slightly and sweeps the next line.
- After the bottom line, it returns to the top and starts again.
The number of lines and how often the full pattern repeats determine the picture’s detail and its refresh rate in the modern sense. Higher refresh rates on computer CRTs reduced visible flicker.
Interlacing
Broadcast television historically used interlacing to save bandwidth. The beam drew all the odd-numbered lines first, then all the even-numbered lines, so each full picture was built from two passes. This halved the data needed per pass while keeping the flicker rate acceptable. Computer monitors more often used progressive scanning, drawing every line in order, which was easier on the eyes for text.
Making color
A color CRT has three electron guns, one each for red, green and blue. The screen’s inside surface is covered in tiny groups of red, green and blue phosphors. To make sure each beam only hits its own color, a metal mask sits just behind the glass.
Two common designs were:
- Shadow mask: a thin sheet with tiny holes, each aligned with a trio of phosphor dots.
- Aperture grille: a set of fine vertical wires, used with phosphor stripes.
Because the three beams approach the mask from slightly different angles, each can reach only its own phosphors. The resulting dots or stripes are small enough that the eye blends them, much as the subpixels of a modern screen do.
Alignment problems could show up as color fringes, and nearby magnets could distort colors. Many sets included a degaussing circuit to clear stray magnetism from the mask.
What CRTs did well
People often remember several advantages. Because the image was drawn by light that faded quickly, motion looked clear and there was little of the smearing that early flat panels showed, a topic covered under motion blur. The tubes could display different resolutions reasonably well without the soft look caused by scaling, which is a common issue on flat panels with a fixed grid, as native resolution explains. Viewing angles were wide, and black levels in a dark room were often good, although bright rooms could wash out the image through reflections off the glass.
What CRTs did poorly
The drawbacks are the reason flat panels took over:
- Depth and weight: the tube had to be as deep as the screen was wide, roughly speaking, so large screens were bulky and heavy.
- Power and heat: driving electron beams and high voltages used a fair amount of power.
- Geometry and focus: image shapes could bow, and focus could vary toward the corners.
- Flicker: low refresh rates were visible and tiring for some people.
- Burn-in: a static image shown for a long time could wear the phosphors unevenly.
- Size limits: very large tubes were hard to manufacture and transport.
Another consideration is disposal. CRT glass contains materials that call for careful handling, so old tubes are typically treated as special waste in many places. Local rules vary, and display recycling programs often explain what they accept.
Why the design faded
As liquid crystal panels became cheaper, brighter and larger, the compactness of flat screens outweighed the CRT’s picture-quality strengths for most buyers. Flat panels also fit wall mounts, thin laptops and phones in a way a tube never could. Today CRTs survive mainly in niche uses, such as some retro gaming setups and certain specialist equipment, where their behavior with older signals is valued. Their practical legacy is large: the vocabulary of scan lines, refresh rate and resolution all began with tubes that painted a picture one line at a time.