Cathode ray tube

A cathode ray tube (CRT) is a glass tube that makes pictures using tiny particles called electrons. These electrons come from one or more electron guns inside the tube. They hit a screen that glows, called a phosphorescent screen. CRTs were used in old televisions and computer monitors to show pictures and videos. They could also show information in devices called oscilloscopes.

In a CRT television or computer monitor, the screen is scanned over and over in a pattern. For color screens, three beams of electrons control red, green, and blue to make a full picture. The electrons are guided to the right spots using special fields.

CRT tubes are made of heavy glass and need to be almost empty inside. Because of this, breaking a CRT can make the glass burst outward quickly. The screens were also made with special glass to block hidden rays. Since the late 2000s, CRTs have mostly been replaced by thinner, lighter screens that are easier to make and use.

History

Discoveries

Cathode rays were first discovered by Julius Plücker and Johann Wilhelm Hittorf. Hittorf saw that these rays could make shadows on a glowing wall, showing they moved in straight lines. Later, Arthur Schuster showed these rays could change direction with electric fields, and William Crookes showed they could change with magnetic fields. In 1897, J. J. Thomson measured these rays and found they were made of tiny, negatively charged particles called electrons.

The first version of the cathode ray tube (CRT) was made by Ferdinand Braun in 1897. It was an early display device that used a screen covered with a special material called phosphor. This invention helped create televisions in the 20th century.

In 1908, Alan Archibald Campbell-Swinton suggested using a cathode ray tube to send and receive images, calling it "distant electric vision." The first CRT with a special type of cathode was made in 1922 by John Bertrand Johnson and Harry Weiner Weinhart.

Development

The technology of the cathode ray tube came from a paper by Karl Ferdinand Braun in 1897. Soon after, JJ Thomson discovered that cathode rays were made of electrons.

In 1926, Kenjiro Takayanagi showed a CRT TV receiver that could receive images with 40 lines of resolution. By 1927, he improved this to 100 lines. Also in 1927, Philo Farnsworth made a TV prototype.

Vladimir K. Zworykin named the CRT in 1929. In the 1930s, Allen B. DuMont made CRTs that lasted longer, helping TVs become more common. The first electronic TV sets with CRTs were made in Germany in 1934.

In 1947, the first interactive electronic game using a CRT screen was created. In the 1950s, CRTs changed from circular to rectangular shapes. In 1954, RCA made the first color CRTs.

The size of CRTs grew over time, from 20 inches in 1938 to 43 inches by 1989. In 1968, Sony released the Trinitron brand, which made screens brighter. In the 1990s, about 160 million CRTs were made each year.

Decline

In the late 1990s to early 2000s, CRTs were replaced by LCDs, starting with small computer monitors. LCDs became cheaper and more popular. By the mid-2000s, CRT TVs were no longer made. Some industries still use CRTs because it is hard to change their equipment. CRTs are also popular for retro gaming because some games need CRT displays to work properly.

Constructions

A cathode ray tube (CRT) has three main parts: a screen, a cone, and a neck. These parts are joined to make the bulb or envelope.

The neck is made from a glass tube. The cone and screen are made by pouring and pressing glass into a mold. The glass needs to block certain rays and let light pass through properly. This glass is very high quality, with almost no defects.

The size of a CRT can be measured by the whole screen area or just the viewable area that shows the image. Most of the weight comes from the thick glass screen, which makes up most of the total weight.

The electron gun inside the CRT sends out electrons that hit the phosphors on the screen to make the display. It has a heater that warms up a cathode, which then makes electrons. These electrons are aimed and sped up towards the screen.

There are two types of deflection used in CRTs: magnetic and electrostatic. Magnetic deflection is common in TVs and monitors, using coils to move the electron beam. Electrostatic deflection, used mostly in oscilloscopes, uses voltage on plates to move the beam.

If images stay still for too long, they can "burn-in" to the screen. Screensavers were used to stop this on computers.

The CRT is emptied of air, so most of the air is taken out. This lets the electrons travel properly without getting in the way. This process heats the CRT to remove any remaining gases.

Phosphors on the screen light up when hit by electrons, making the display. Different types of phosphors are used for different colors and purposes.

One problem with CRTs is blooming, where bright areas of the image get bigger and push away darker areas. This happens because brighter areas make more electrons, making the beam wider. Another issue is doming, where parts of the shadow mask heat up unevenly, causing colors to change in bright spots.

The size of a CRT is limited by the weight of the glass needed to hold the empty space inside. Larger screens need thicker glass, making them very heavy. Bigger screens also need higher voltages to make the image brighter, which creates more heat and radiation, so special glass is needed to block it.

Deflection angles also affect CRT design. Higher angles make the tube thinner but need more power and can cause heat buildup in the coils, limiting how large and detailed the screen can be.

Comparison with other technologies

Main article: Comparison of CRT, LCD, plasma, and OLED displays

Cathode ray tubes (CRTs) have some good points compared to newer screens like LCD and OLED. CRTs show colors well and don’t blur moving images as much. They also show images right away when a signal is sent, making them great for older video games.

But newer screens like LCD and OLED are thinner, use less power, and can sometimes show images faster. OLED screens can show very dark blacks and bright colors. Even though CRTs were popular before, newer technologies are now used because they are easier to use and save energy.

Types

CRTs came in two main types: picture tubes for TVs and display tubes for computer monitors. Display tubes often had a sharper picture and could change how the picture looked. Picture tubes always showed the whole image, which kept the picture looking nice.

Monochrome CRTs

Monochrome CRTs showed pictures in black and white. They had one electron gun and a special coating inside to help make clear images. This coating reflected light and helped with heat. When turned off, the screen would shrink to a small white dot in the center.

Color CRTs

Color CRTs showed pictures in color using three colors of light: red, green, and blue. They had three electron guns, one for each color, and a special mask to make sure each color went to the right spot on the screen. This kept the colors looking pure and correct.

Shadow mask

The shadow mask was a metal plate with tiny holes that helped make sure the right colors showed up in the right places. It absorbed electrons that might hit the wrong color, keeping the picture looking good.

Screen manufacture

Making the screen for a color CRT was a careful process. It involved applying special coatings and patterns to create the right colors and keep them separate. This helped make sure the picture looked clear and colorful.

Convergence and purity in color CRTs

It was hard to make all three electron beams hit the right spots perfectly, so special magnets were used to adjust how the beams moved. This helped keep the colors looking right and the picture clear.

Magnetic shielding and degaussing

To keep the colors looking right, some CRTs had special shields to protect them from magnetic fields. If the screen got magnetized, it could change the colors, so a degaussing coil was used to remove this magnetization when the TV was turned on.

Resolution

The resolution of a CRT depended on how close the color dots were together. Some CRTs used special patterns to reduce moiré, which is a wavy pattern that can appear when the resolution is too high for the screen.

Projection CRTs

Projection CRTs were used in projectors and rear-projection TVs. They were smaller and brighter than regular CRTs, with special parts to make them shine brightly enough for projecting images. They needed cooling because they generated a lot of heat.

Beam-index tube

Beam-index tubes were an early attempt to make color CRTs without a shadow mask. They used a special pattern on the screen and a sensor to track the electron beam. This made them simpler but also harder to produce, so they weren’t used much.

Flat CRTs

Flat CRTs had screens that were flatter than regular CRTs. They were harder to make because of issues with how the electron beam moved. Some, like the LG Flatron, were completely flat inside and out.

Radar CRTs

Radar CRTs had circular screens and were used to show radar images. They often had two colors: one bright color that showed where the radar beam was pointing and a dimmer color that stayed lit afterward to show targets.

Oscilloscope CRTs

Oscilloscope CRTs used electric fields to move the electron beam, instead of magnetic fields like TVs. This let them show very fast changes clearly. Some had special parts to make the image brighter when showing very quick events.

Microchannel plate

To make very fast events clearer, some oscilloscope CRTs used a microchannel plate. This plate made more electrons hit the screen, making the image brighter and easier to see.

Graticules

Oscilloscopes often had a graticule, which was like a ruler on the screen to help measure things. It could be part of the screen or a separate piece that you could see through.

Image storage tubes

Some oscilloscopes used storage tubes to keep an image on the screen even after the event was over. These tubes had a special grid that could hold the image until it was erased.

Vector monitors

Vector monitors were used in early computer systems and arcade games. They drew images by connecting points, unlike regular CRTs that scanned lines across the screen.

Data storage tubes

The Williams tube was a special CRT used to store computer data in the 1940s. It wasn’t a screen but a way to keep information inside a computer.

Cat's eye

Some old radios used a “Magic Eye” tube to help people tune the radio. It showed a shadow that got smaller when the radio was tuned right.

Charactrons

Charactrons were used in early computers to show text. They had a mask with letters that the electron beam could pick from to draw words on the screen.

Nimo

Nimo tubes were small CRTs that showed numbers, like on a digital clock. They had many electron guns to make the numbers appear.

Flood-beam CRT

Flood-beam CRTs were used in big video screens. They sprayed electrons across the whole screen to light up pixels, making each part of the screen a single color.

Print-head CRT

Some CRTs were used to make printed pictures by putting a charge on paper that then attracted ink.

Zeus – thin CRT display

In the late 1990s, some companies tried to make very thin CRTs, but they were never sold to the public.

Slimmer CRT

Some companies made CRTs that were thinner than usual, with names like Superslim and Ultraslim. These were a bit deeper than flat CRTs but still thinner than regular ones.

Health concerns

Ionizing radiation

CRTs can give off a tiny bit of X-ray radiation. This happens when the electron beam hits parts inside the screen. The amount of radiation from the front of the monitor is very small and is not thought to be harmful. Rules in the United States limit how much radiation TVs can give off, and most CRTs made after 2007 give off less than this limit.

Toxicity

Older CRTs might have been made with toxic materials, like cadmium, in the screen. Modern CRTs may have lead in the glass, which can be dangerous if not thrown away properly.

Flicker

Main article: Flicker (screen)

At low refresh rates, some people might see the screen flicker, especially at the edges. Most computer monitors have higher refresh rates to prevent this.

High-frequency audible noise

Some people, especially children, might hear a high-pitched sound near a CRT TV. This is because of how the TV’s parts move and vibrate.

Implosion

If the glass of a CRT is damaged, the vacuum inside can cause the tube to break apart very quickly and send pieces flying. Modern CRTs have special protections to help prevent this, but they still need to be handled carefully.

Implosion protection

Early CRTs had a glass plate glued to the screen. Over time, the glue could break down. Later CRTs used a metal rim to keep the glass strong. This rim helps prevent the CRT from breaking apart if the screen is damaged.

Electric shock

CRTs need a very high voltage to work. This voltage is much higher than normal household electricity. Even after turning off the power, some parts can keep a small charge for a short time. Getting an electric shock from the CRT itself is very unlikely.

Security concerns

Sometimes, the signals from a cathode ray tube can be picked up from far away. This can let someone see what is on the screen. This is called Van Eck phreaking. Special shielding, called TEMPEST, can help stop this. This can happen with many types of screens and electronic devices.

Recycling

Because CRT monitors have harmful materials, the United States Environmental Protection Agency made rules in 2001. These rules say CRTs must go to special e-waste recycling places. Starting in 2002, the EPA began punishing companies that threw away CRTs in landfills or by incineration.

CRTs are hard to recycle because they have lead and special materials for showing images. Some companies charge a small fee to pick up CRTs. They sell parts like copper and wires to help cover costs. The EPA calls old CRT monitors "hazardous household waste," but if they are being tested and not thrown away, they are treated like valuable items.

Different states have their own rules for CRT recycling. For example, in California, recycling is managed by CALRecycle, the California Department of Resources Recycling and Recovery. Recycling places in California must check that the CRTs come from that state.

In Europe, throwing away CRT TVs and monitors follows rules set by the Waste Electrical and Electronic Equipment Directive.

There are many ways to recycle CRT glass, including using heat, machines, or chemicals. Some companies used special furnaces to separate lead from the glass. The materials inside CRTs sometimes have rare metals. A CRT has about 7 grams of these materials.

The glass part of a CRT can be separated using lasers, special saws, or heated wires. Sometimes, this glass is melted down to make new items or used in building materials.

Unfortunately, a lot of CRT glass ends up in landfills, where it can harm the environment. In California, more CRT glass was thrown away than recycled in 2016.