Why did CRT televisions have curved screens?


I was born in ’99, so I grew up in the last age of CRT TVs. I’m wondering why they were built to have curved screens. It’s not like flat screens weren’t a thing, because films were projected onto flat screens in movie theaters until IMAX became a thing. But I wonder what about cathode rays made curved screens a necessity until plasma, LCD, and LED televisions came about.

In: Technology

Becuase they were lit by a cathode tube. (CRT = Cathode ray tube) which needs a convex end-point. remember the “degauss” button on CRT’s, and [this](https://en.wikipedia.org/wiki/Degaussing#/media/File:Degauss-in-progress_at_Dell-Trinitron-monitor.jpg) happened. sometimes pixels got stuck due to magnetic aberrations. the convex screen was necessary at the time for CRT to work.

Because they were lit by a beam of electrons, and the aiming mechanism is a powerful magnet. Having the screen equally distant from the beam point was the best way to get it focused on the screen.

I realize this isn’t “ELI5” ish, but….I couldn’t think of a way to explain it better.

There was a combination of factors. First, spheres are stronger against implosion. A totally flat screen would be too weak to hold vacuum. Second, maybe most important, the electron beam is more like a water hose than a laser beam. An electric field focuses it to a point, and that point needs to coincide with the CRT inside surface. The focus electronics is simpler for a spherical surface because the beam length stays constant. Thirdly, for a similar reason, the scan/sweep electronics had to account for non-spherical beam lengths in order to maintain a constant transverse speed. Interestingly, they could also have moved the beam source farther back from the screen so the beam length was more constant. In other words, larger spheres have flatter surfaces. However, the market wanted thinner TVs they could set closer to the wall.

Imagine standing at a fixed point in a road, with a hose in your hand that can project water. A little way ahead of you is a large wall, and your job is to ensure that every spot of that wall is hit by the water. However you are not allowed to move from where you stand, all you can do is swivel left or right, up or down.

So acting like this you can indeed get the hose to point at every part of the wall, but the corners of the wall (which are further away from you) are going to get a weaker splosh, and it will take slightly longer for the hose water to actually hit them because the distance is longer.

A CRT operates in a similar fashion. The “hose of water” is actually a beam of energised particles and instead of swivelling the cathode, electromagnets are used to bend the direction of the beam. But the important point is that all those particles come from a fixed point (the cathode, which is the “C” in CRT).

If the inside of the screen that they’re being beamed at is flat, they have the same problem as you had with the hose: the corners of the screen are further away so the particles take longer to reach it and have less energy when they arrive. This would manifest itself as colour-washout in the corners of the screen.

Of course the perfect solution to this would be for the screen to be hemispherical, because then all parts of it are literally the same distance from the cathode emitter. But that’s not very practical for humans to watch, so manufacturers compromised by making the screen somewhat curved, which reduces the corner colour fade to acceptable levels.