Cathode Ray Tube (CRT)

Older monitors use a cathode ray tube to display images

Abbreviated as CRT, cathode ray tube is a large vacuum tube used to display an image on a screen. Generally, it refers to a type of computer monitor utilizing a CRT.

Although CRT displays (often called "tube" monitors) are really bulky and take up lots of desk space, they generally have a much smaller screen size than new display technologies.

The very first CRT device was called Braun tube and was built in 1897. The first CRT television made available to the public was in 1950. With many years since then, newer devices have seen improvements in not only the total size and screen dimension, but also in energy usage, manufacturing costs, weight, and image/color.

Mitsubishi Diamond Plus 200 Diamondtron Natural Flat 22" CRT Monitor

CRTs have ultimately been replaced by newer technologies that offer these drastic improvements, like LCD, OLED, and Super AMOLED.

SecureCRT, a Telnet client, used to be called CRT but it has nothing to do with CRT monitors.

How CRT Monitors Work

There are three electron guns within a modern CRT monitor that are used for red, green, and blue color. To produce an image, they shoot electrons at phosphor toward the front end of the monitor. It starts at the top left edge of the screen and then moves from left to right, one line at a time, to fill the screen.

When the phosphor is hit with these electronics, it enables them to glow at particular frequencies, at particular pixels, for a particular amount of time. This creates the required image using a mixture of the red, blue, and green colors.

When one line is produced on the screen, the electron guns continue with the next, and keep doing this until the entire screen is filled with the appropriate image. The idea is for the process to be quick enough that you just see one image, whether it be a photo or a single frame in a video

More Information on CRT Displays

A CRT screen's refresh rate determines how frequently the monitor will refresh the screen to produce an image. Because the phosphor glowing effect is not sustained unless the screen is refreshed, a lower refresh rate is why some CRT monitors experience flickering or out-of-place, moving lines.

What's being experienced in those situations is the monitor refreshing slow enough that you can see which portions of the screen have yet to show the image.

CRT monitors are at risk for electromagnetic interference since a magnet is what allows for the electrons to move within the monitor. This type of interference doesn't exist with newer screens like LCDs.

See How to Degauss a Computer Monitor if you're experiencing magnetic interference to the point that the screen is discolored.

Within the large and heavy CRT are not only electron emitters but also focusing and deflection coils. The entire apparatus is what makes CRT monitors so large, which is why newer screens that use different technologies like OLED, can be so thin.

Flat panel displays like LCDs can be made to be really large (over 60") while CRT displays are generally around 40" at maximum.

Recycling CRT Monitors

If you need to dispose of a CRT monitor, please know that it might contain toxic substances like cadmium or might have been built using leaded glass. This means that CRTs are considered hazardous waste.

To avoid a fine from the EPA or other regulatory agency, it's best to throw out an old CRT monitor by taking it to a recycling facility. If you're in the US, learn more about your state's hazardous waste programs.

Other CRT Uses

CRT has also been used for non-display devices, like to store data. The Williams tube, as it was called, was a CRT that could store binary data.

The .CRT file extension is clearly unrelated to the display technology, and is instead used for the Security Certificate file format. Websites use them to validate their identity.

Similar is the C runtime (CRT) library associated with the C programming language.