How a CRT Television Works
For about fifty years, almost every television and computer monitor was a CRT, a cathode-ray tube. That heavy glass bulb did not have pixels the way a modern screen does. Instead, the entire picture was painted, over and over, by a single moving dot of light travelling faster than your eye can follow.
One dot, painting the whole screen
The dot starts in the top-left corner and races to the right, drawing one thin horizontal line. Then it snaps back, drops down a notch, and draws the next line. Line by line, top to bottom, it fills the screen. Then it jumps back to the top and does it all again, dozens of times every second. This left-to-right, top-to-bottom sweep is called a raster scan.
Below is a simulation. Drag Beam speed all the way down and you will see the lone dot crawling across each line, building the test pattern one stripe at a time. Drag it up and the dot moves so fast the whole image looks solid and still. That is exactly the trick your real TV was playing.
Notice the glowing trail the dot leaves behind. That is persistence, and it is not just an effect. It is how the whole thing works.
The electron gun
At the narrow back of the tube sits the electron gun. A small filament heats a metal cathode until it boils off electrons. A very high voltage, tens of thousands of volts, yanks those electrons forward and accelerates them into a tight beam aimed at the screen.
Steering the beam
A beam pointed straight ahead would only ever light up the centre of the screen. So the tube is wrapped with a deflection yoke: coils of wire whose magnetic fields bend the beam. One pair sweeps it left-to-right, another nudges it top-to-bottom. Together they steer the dot through the entire raster pattern. At the end of each line the beam “flies back” to the left almost instantly. This is the horizontal retrace, and the beam is switched off during it so you never see the return trip.
Why the screen glows
The inside of the glass is coated with phosphor: a material that gives off light for a brief moment when the electron beam strikes it. As the beam moves on, that spot keeps glowing and slowly fades. Because every spot was hit just a fraction of a second ago and is still glowing, your eye blends the fading trail into one complete, steady image. Turn the Phosphor persistence slider down in the simulation above and the trail vanishes almost immediately; turn it up and the glow lingers.
Drawing pixels: switching the beam on and off
So far the beam has been either sweeping or glowing, but what decides whether a given spot lights up? The answer is a second voltage. Near the cathode sits a small control electrode, the control grid. The voltage on it sets how many electrons get through: turn it up and the beam is strong and the spot glows brightly; drop it below a point called cutoff and the beam shuts off completely, leaving that spot black.
So to draw a row where every other pixel is lit, you switch that drive voltage up and down in step with the beam’s position: high over pixel 1, low over pixel 2, high over pixel 3, and so on. The animation below shows the drive voltage along the bottom and the row it produces on top. Watch the beam sweep across: while the voltage sits above the cutoff line the pixel under it lights up, and whenever the voltage dips below cutoff the pixel stays dark.
Green columns: voltage above cutoff, so the beam is on.
A real picture is just this with the voltage varying smoothly instead of snapping fully on and off, so each spot can be any brightness from black to full glow. Do that across every line and you have an image.
Getting colour
A black-and-white tube uses one beam and one phosphor. A colour tube uses three electron guns, one each for red, green, and blue, and the screen is coated with tiny red, green, and blue phosphor dots (or vertical stripes). A perforated metal sheet just behind the glass, the shadow mask (or aperture grille), is lined up so each gun’s beam can only ever hit phosphors of its own colour. Mix the three brightnesses and you get every colour on screen.
Why it flickers (and why it mostly does not)
The whole screen is repainted 50 or 60 times a second. That is fast enough that persistence of vision, your eye holding onto an image briefly, smooths it into steady motion. To save bandwidth, broadcast TV cheated a little with interlacing: each pass painted only every other line (the odd lines, then the even lines), two half-images called fields adding up to one full frame. It is also why old CRTs in the background of videos often show those rolling dark bands: the camera’s shutter and the TV’s scan are not in step.
So the next time you picture an old TV, remember: there was never a whole picture on the glass at once. Just one frantic dot of light, and an afterglow.