Cathode ray tube

A cathode ray tube (CRT) is a special type of glass tube that can create images by using streams of tiny particles called electrons. These electrons come from one or more electron guns inside the tube and hit a screen coated with a material that glows, called phosphorescent screen. CRTs were used in old televisions and computer monitors to show pictures and videos. They could also display information like electrical signals in devices called oscilloscopes.

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

CRT tubes are made of heavy glass and need to be almost empty inside to work properly. Because of this, breaking a CRT can cause the glass to burst outward very quickly. The screens were also made with special glass to block hidden rays called X-rays. Since the late 2000s, CRTs have mostly been replaced by thinner, lighter screens like liquid-crystal displays, which are easier to make and use.

History

Discoveries

Cathode rays were first discovered by Julius Plücker and Johann Wilhelm Hittorf. Hittorf noticed that these unknown rays could cast shadows on the glowing wall of a tube, showing they traveled in straight lines. Later, Arthur Schuster showed these rays could be bent by electric fields, and William Crookes showed they could be bent by 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 invented by Ferdinand Braun in 1897. It was an early display device that used a screen coated with a special material called phosphor. This invention became the basis for televisions in the 20th century.

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

Development

The technology of the cathode ray tube came from a paper written by Karl Ferdinand Braun in 1897. Shortly 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 created 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 improved brightness. 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 difficult to change their equipment. CRTs are also popular for retro gaming because some games need CRT displays to work properly.

Constructions

The body of a cathode ray tube (CRT) is usually made up of three parts: a screen, a cone, and a neck. These parts are joined together to form the bulb or envelope.

The neck is made from a glass tube, while the cone and screen are made by pouring and pressing glass into a mold. The glass used needs special properties to block certain rays and allow light to 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 emits electrons that hit the phosphors on the screen to create the display. It contains a heater that warms up a cathode, which then produces electrons. These electrons are focused and accelerated 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 steer the electron beam. Electrostatic deflection, used mostly in oscilloscopes, applies voltage to plates to move the beam.

Over time, images can "burn-in" to the screen if left static for too long. Screensavers were used to prevent this on computers.

The CRT is evacuated, meaning most of the air is removed, to allow the electrons to travel properly without interference. This process involves heating the CRT to remove any remaining gases.

Phosphors on the screen glow when hit by electrons, creating the display. Different types of phosphors are used for different colors and purposes. Some phosphors last longer, which can be useful for seeing fast changes, while others fade quickly to avoid blurry images.

One limitation of CRTs is blooming, where bright areas of the image expand and push aside darker areas. This happens because brighter areas produce more electrons, making the beam wider. Another issue is doming, where parts of the shadow mask heat up unevenly, causing colors to shift in bright spots.

The size of a CRT is limited by the weight of the glass needed to hold the vacuum inside. Larger screens require thicker glass, making them very heavy. Higher voltages are needed for bigger screens to make the image brighter, but this also creates more heat and radiation, requiring special glass to block it.

Deflection angles also affect CRT design. Higher angles make the tube thinner but require 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 benefits compared to newer display technologies like LCD and OLED. CRTs can show colors very well and don’t blur motion as much as some other screens. They also don’t have any delay between when a signal is sent and when the image appears, which makes them great for older video games.

However, newer screens like LCD and OLED are thinner, use less power, and can sometimes show images faster. OLED screens, in particular, can show very dark blacks and bright colors. While CRTs were popular in the past, newer technologies have taken over because they are more convenient and energy-efficient.

Types

CRTs came in two main types: picture tubes for TVs and display tubes for computer monitors. Display tubes often had higher resolution and could sometimes adjust how the picture looked. Picture tubes always showed the full image without showing the edges, which helped keep 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 create clear images. This coating reflected light and helped manage 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 different 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 helped keep 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

Because it was hard to make all three electron beams hit the right spots perfectly, 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 that comes out of 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 when looking 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

In some situations, the signals from the parts of a cathode ray tube (CRT) can be picked up from a distance and used to see what is displayed on the screen. This is called Van Eck phreaking. Special shielding, known as TEMPEST, can help prevent this. It's important to note that this kind of signal picking up can happen with many types of screens and electronic devices.

Recycling

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

CRTs are hard to recycle because they have a lot of lead and special materials needed for showing images. Some companies in the United States charge a small fee to pick up CRTs. They then sell parts like copper and wires to help cover their 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, the recycling of CRTs 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 to follow the rules.

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 to remove the lead. Some companies used special furnaces to separate the lead from the glass. The materials inside CRTs, called phosphors, sometimes have rare metals. A CRT has about 7 grams of these materials.

The glass part of a CRT can be separated from the screen using lasers, special saws, or heated wires. Sometimes, this glass is melted down to make new CRTs or used in building materials like tiles or concrete.

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.