Contrary to what I said in my last vacuum tube explanation, today we're going to discuss the cathode-ray tube, a specialized vacuum tube that slams high speed electrons into a screen to produce images. If you have a bulky CRT TV, this is at its core.
Other tubes I've mentioned such as the triode and rectifier are used as electrical components, and the cavity and Hull magnetrons are used to produce high frequency electromagnetic waves. The CRT, on the hand, uses vacuum tube technology to produce a picture for you to see.
Diagram of early CRT made by J.J. Thompson, discover of the electron.
Image credit
Electron radiation and phosphors
A few weeks ago I made several posts involving commercially available tritium light tubes. These tubes contain a small amount of radioactive tritium (Hydrogen-3), an isotope of hydrogen that decays with a half-life of 12 years. When tritium decays, it releases electron antineutrinos and electrons, with the electron having an average energy of just under 6 kiloelectronvolts. These electrons strike the inner surface of the glass tube, where a phosphor lining is excited, releasing visible green light out of the tube. The result is a sort-of permanent light that will last decades with no electronics or battery, and no escaping ionizing radiation other than the neutrinos and a very small amount of XRays.
My tritium tube
But why bring this up? Is this just another way to shoehorn radiation into yet another post? Kind of, but the real reason is that the way a cathode-ray tube produces images is essentially exactly the same as the way a tritium tube produces light. The difference? In a CRT, we are artificially producing and guiding the electron radiation that will be providing us with light/images.
Accelerating Electrons, and how your old TV works
At their heart, all vacuum tubes are a sort of electron accelerator. The CRT, however, takes this a step further and uses quite a high voltage. The CRT in a color television set will use voltages up to 20,000 volts to accelerate electrons across the tube.
The original CRT is the Crookes Tube, created by William Crookes, which was used to discover the first XRays. It simply accelerated electrons into a wall, producing a glow. When high voltage is applied across the tube, residual air in the tube ionizes, producing free electrons that can accelerate in the resulting electric field.
Crookes Tube
Image credit
Although many CRT types exist, I'll be using the one inside a CRT TV as an example case for this post. Almost everything that applies to this specific CRT will apply to others.
As usual, a cathode filament is heated up with low voltage current and ejects electron via thermionic emission. These electrons are accelerated by a strong electric field produced by a the high voltage flyback transformer (more information on flyback transformers can be found here in my post on extracting them). Alongside the main accelerating voltage, the electron beam is also deflected both electrically and magnetically: Plates lining the tube at various voltages can deflect the beam horizontally, while large current-carrying coils above the tube use a magnetic field to bend the beam. As far as I know, most televisions use both methods, and all of the CRTs I've taken apart had the deflection coils in place.
Fun fact: You can tap into one of the deflection coils to make any CRT TV into a makeshift oscilloscope.
This is the electron gun portion of the CRT inside a TV. This part produces the beam that is then sent into the main chamber.
Image credit
CRT vacuum tubes typically fan out from the initial electron source to produce a large, rounded screen. Here's a cathode-ray vacuum tube from a TV I gutted last year:
Note the deflection coils on the stem of the tube. The hole in the top of the tube is the anode, and normally the high voltage supply connects here.
The inner surface of the screen is coated in a phosphor material that produces light when struck by high-energy electrons. The electron beam is accelerated by the high voltage source to high energies, then deflected by the coils/plates to various points of the screen. Adjusting where the beam strikes the screen produces a specific image. Changing the beam's position over time produces a moving image and lets you watch a video on your CRT.
For color televisions, a special cathode-ray tube is used which incorporates three different types of phosphors, evenly distributed in discrete specks on the screen, which produce the three colors: Red, Green, and Blue. In color TVs, three electron beams are produced in the CRT electron gun which are deposited onto the screen to produce a glowing color image.
You can then think of a television CRT as a big, controlled tritium tube without the tritium. Electrons are accelerated to ionizing energies through a vacuum and directed at a big screen of phosphor. Rather than producing microwaves or stopping backwards current, this vacuum tube can show you the news or Star Wars.
Interestingly, bigger CRTs produce non-negligible amounts of XRays because of the high voltage and reasonable current. As usual, when high energy electrons strike a material, XRays are produced. Because of this, CRT TV tubes are made using a material called lead glass, which incorporates lead-oxide into screen. This increases the density of the screen and improves its ability to stop XRays, effectively converting the screen into a decent radiation shield. Thanks to lead glass, your TV won't produce very much ionizing radiation out the front.
This does have the side effect of making the TV very heavy, which adds a large implosion risk if it is dropped and the vacuum is breached - so be careful carrying around big CRT TVs.
Cathode-Ray Tube Uses
CRTs have many uses other than serving as TV screens. Many oscilloscopes and some camcorders use or used CRTs as displays, with the deflection coils controlling the electron beam resulting in the signal displays on the oscilloscope. By using the three-dot phosphor system, color CRT screens can be produced, or by using a single phosphor black-and-white screens can be made.
Cathode-ray tubes are also surprisingly easy for a hobbyist to make - you just need a vacuum pump to make your own, or you can use an existing tube with your own high voltage supply. I previously wrote about turning a microwave oven light bulb into an XRay tube - what I was effectively doing here was making the bulb into a crude CRT, with XRays being the byproduct. It even had the green glow!
Perhaps surprisingly, cathode-ray tubes can actually be used as computer memory. The Williams Tube did just that, using a CRT to produce a matrix of dots that could be read by a computer. Invented in the 1940s (!), this tube was used for some of the first computers as a form of random-access memory.
The Williams tube memory device
Image credit
Of course, as anyone who has seen RAM modules knows, this tube is no longer used in computers and has been obsolete for decades. But it's still pretty cool.
CRTs are rapidly becoming obsolete as well, in favor of LCDs and other displays. Imagine having a CRT in your phone - it would be ridiculously large for even the smallest screen. However, many people still use CRT TVs, making the cathode-ray tube the second most common consumer vacuum tube after the ubiquitous microwave oven Cavity Magnetron.
Hopefully you learned something new from this. If you have any questions or corrections, don't hesitate to let me know.
Thanks for reading!
Images not credited are my own. You are welcome to use them with credit.