Cathode-ray tube - Coursedia

Cathode-Ray Tubes (CRTs) in Electrical Engineering#

Okay, let’s talk about Cathode-Ray Tubes, or CRTs. Think of them as the display technology that ruled the world for decades before flat screens came along. For anyone into electrical engineering, understanding how these tubes worked is pretty fundamental because they involve a lot of core electronic principles – generating electrons, controlling them with electric and magnetic fields, high voltage, vacuum technology, and signal processing.

Basically, a CRT is a big glass bottle with no air inside (a vacuum) that uses beams of electrons to draw pictures on the front surface. This front surface is coated with special materials that light up when electrons hit them. You’ve seen these in old TVs, computer monitors, oscilloscopes, and even some older radar displays. The one in a TV is often just called a “picture tube.”

While they’re not used much for new displays anymore because flat-panel technologies like LCDs, plasmas, and OLEDs are lighter, thinner, and cheaper, CRTs were incredibly important and still pop up in specific old equipment, or even for people who like old video games because of their unique display properties.

The main idea is: make an electron beam, move it around precisely, and turn it on and off to make bright or dark spots on the screen.

How They Work: Inside the Glass Bottle#

A CRT is built from a few main parts, all sealed inside a thick glass envelope to keep the vacuum:

The Electron Gun: This is at the back (the narrow part, called the neck) and shoots out the electron beam.
Deflection System: This moves the electron beam around the screen.
The Screen: The front, flat or curved surface coated with stuff that glows.

Let’s break down these parts and how they function.

The Body (The Glass Envelope)#

The CRT’s body is a sealed glass container, usually made in three sections: the screen (or faceplate), the cone (or funnel), and the neck. Together, these form the “bulb” or “envelope.”

The glass is special stuff, often called “CRT glass.” It needs particular properties.

Vacuum: First off, it holds a near-perfect vacuum inside. Why? Because the electrons need to travel from the back to the screen without bumping into air molecules. Any collisions would scatter the electrons, messing up the beam and the picture. The pressure inside is super low, much less than one-millionth of the air pressure outside.
Strength: Since there’s a vacuum inside, the outside air pressure is constantly pushing on the tube. For a decent-sized screen, this adds up to several tons of force! The glass has to be very thick and strong to withstand this. This is why CRTs are heavy and why breaking one can be dangerous – the glass can implode violently inwards and then spray outwards.
X-ray Shielding: High-speed electrons hitting things inside the tube (like the screen or internal metal parts) create X-rays. The glass, especially in the funnel and screen, often contains lead or special barium-strontium mixtures to block most of this radiation from getting out. Lead glass is common in the neck and funnel because it’s good at blocking X-rays and is relatively inexpensive. The screen often uses lead-free barium-strontium glass, which also blocks X-rays but doesn’t turn brownish over time like leaded glass can.
Electrical Insulation: The glass in the neck and funnel needs to be a great electrical insulator because there are high voltages running through components inside, especially near the electron gun and the anode connection.
Optical Properties: The screen glass needs specific optical qualities, like how transparent it is (transmittance) and uniformity, especially for color CRTs, to ensure colors look right across the whole screen. Screen curvature changed over time, from very curved to flatter, to reduce reflections. Flat screens often had glass that got thicker towards the edges, slightly reducing transmittance there.

Different parts might use slightly different glass mixes, and these mixes need to expand and shrink at similar rates when heated (like during manufacturing or operation) so the tube doesn’t crack.

Vacuum Tube / Envelope: A sealed glass or metal container from which most of the air has been removed, allowing electrons to flow freely within the tube without colliding with air molecules.

The Anode (The High Voltage Part)#

Inside the funnel part of the CRT, and often coating the back of the screen too, is a conductive layer. This is the anode. It’s connected to a very high positive voltage, usually ranging from a few thousand volts (kV) up to 30 kV or more for large color CRTs.

The anode does two main jobs:

Accelerate Electrons: It pulls the electrons coming from the electron gun forward at high speed towards the screen. The higher the anode voltage, the faster and more energetic the electrons, which makes the phosphor on the screen glow brighter.
Collect Electrons: After the high-speed electrons hit the phosphors on the screen, they cause the phosphors to emit light and also knock off some secondary electrons from the phosphor material itself. The anode’s positive voltage helps collect these secondary electrons, preventing charge buildup on the screen that could mess up the beam’s path.

In many CRTs, the inner conductive coating on the funnel (often Aquadag, a graphite paint, or evaporated aluminum in monochrome CRTs) acts as the anode. This coating is connected to a high-voltage power supply through a special connector (the anode cap) embedded in the funnel glass. The outer coating (usually Aquadag) is typically connected to ground. These two coatings act like a capacitor (often 5-10 nF), which helps smooth out and stabilize the high voltage provided by the power supply, especially useful in older designs. This capacitor can hold a dangerous charge even after the power is off, so handling old CRTs requires caution.

The high voltage is typically generated by a component called a flyback transformer.

Flyback Transformer (or IHVT - Integrated High Voltage Transformer): A specialized type of electrical transformer used in CRT displays to generate the high voltage required to accelerate the electron beam and provide other necessary voltages for the CRT’s operation. It works by rapidly switching a current on and off in a primary coil, inducing a high voltage pulse in a secondary coil when the magnetic field collapses.

The voltage generated depends heavily on the design of the high-voltage power supply. Older, less regulated supplies could have the anode voltage drop slightly when displaying a very bright image (more electron beam current), which could cause the image to slightly “bloom” or expand (see Limitations).

The Electron Gun#

The electron gun is the heart of the CRT, located in the neck. It’s a complex assembly of electrodes that create, control, focus, and accelerate the electron beam (or beams, in a color CRT).

It works like this:

Heater and Cathode: At the back of the gun is a small heating element (like a tiny light bulb filament, often tungsten). This heats up a small metal cap called the cathode. The cathode is coated with a special material, typically barium oxide or barium strontium calcium carbonate, which emits electrons easily when heated (a process called thermionic emission). This creates a cloud of electrons near the cathode surface. Color CRTs have three separate cathodes, one for each color (red, green, blue).

Cathode: The negative electrode in a vacuum tube. In a CRT’s electron gun, it’s heated to emit electrons (thermionic emission). Heater: A filament inside the cathode that heats the cathode to a high temperature, enabling it to emit electrons.

Cathodes can wear out over time due to “cathode poisoning” (a positive ion layer forms on the surface, blocking electron emission) or simply degrading from the heat and electron flow. This reduces image brightness or causes color loss in color CRTs. Adding special metals like zirconium or manganese to the cathode material helps extend its life. Scandium oxide can extend life significantly.
Control Grid (G1): Just in front of the cathode is a metal cylinder or plate with a hole in it, called the control grid (G1). A voltage negative with respect to the cathode is applied to this grid. This negative voltage repels the electrons, controlling how many electrons from the cloud can pass through the hole and form the beam. By changing this negative voltage, you control the intensity (brightness) of the electron beam hitting the screen. More negative voltage = fewer electrons = darker spot. Less negative voltage (closer to the cathode voltage) = more electrons = brighter spot. The video signal is typically applied to the control grid (in monochrome) or the cathodes (in color CRTs with shared grids) to vary the beam intensity and create the picture.

Control Grid (G1): An electrode placed near the cathode in an electron gun that controls the flow of electrons, regulating the brightness of the spot on the screen. A more negative voltage on this grid reduces the electron beam current.
Screen Grid (G2): Following the control grid is the screen grid (G2). This grid has a positive voltage (hundreds of volts) relative to the cathode. It helps accelerate the electrons coming through G1 and shapes them into a rough beam.
Focus Grid(s) (G3, G4, etc.): Further down the gun, a system of electrodes forms an “electron lens” to focus the electron beam to a sharp point on the screen. This is often done using electrostatic lenses, where different voltages are applied to metal cylinders or plates to create electric fields that bend and focus the electron path, much like glass lenses focus light. Early tubes needed external magnetic coils for focusing, but electrostatic focusing is more common in modern CRTs, especially color ones. The focus voltage is derived from the high anode voltage via a voltage divider.

Electron Lens: An arrangement of electrodes (electrostatic lens) or magnetic coils (magnetic lens) used in an electron gun to focus the electron beam to a sharp spot on the screen. Bipotential Lens: A type of electrostatic electron lens formed by two electrodes at different high potentials, often used in CRT electron guns for focusing.
Acceleration: The final stage often involves the electron beam passing through an electrode connected to the main anode voltage (tens of kV). This gives the electrons their final, high speed before they hit the screen. In many CRTs, the conductive coating on the funnel (the main anode) also participates in this final acceleration and focusing stage, sometimes referred to as the “final anode.”

All these gun components are precisely aligned and supported by glass rods or beads inside the neck. They are manufactured as a unit and then sealed into the glass neck.

Electron Gun: An assembly of electrodes in a vacuum tube that produces, focuses, and accelerates one or more electron beams.

Deflection (Moving the Beam)#

After being accelerated and focused, the electron beam needs to be moved around the screen to draw the image. There are two main ways to do this: magnetic deflection and electrostatic deflection.

Magnetic Deflection#

This is the method used in most TVs and computer monitors because it allows for wider deflection angles (making the tube shorter from front to back for a given screen size) and can handle higher beam currents needed for brighter images.

Magnetic Deflection: Using magnetic fields to bend the path of an electron beam.

Around the narrow neck of the CRT, near the electron gun, is a set of coils called the deflection yoke. This yoke contains two pairs of coils: one pair creates a magnetic field that deflects the beam horizontally (left and right), and the other pair creates a field that deflects it vertically (up and down).

Deflection Yoke: An assembly of electromagnetic coils placed around the neck of a CRT that generates magnetic fields to steer the electron beam horizontally and vertically across the screen.

To draw an image, the currents flowing through these coils are constantly changed by deflection circuits. For a standard TV or computer monitor display (a raster scan), the horizontal coils are driven by a sawtooth-shaped signal that sweeps the beam rapidly from left to right across the screen. At the end of the line, the beam is quickly brought back to the left side (horizontal retrace). At the same time, the vertical coils are driven by a slower sawtooth signal that moves the beam down the screen line by line. When the beam reaches the bottom, it’s quickly returned to the top (vertical retrace). This sweeps the entire screen area repeatedly in a fixed pattern called a raster.

Raster: The pattern of scanning lines used to create an image on a CRT screen, typically involving sweeping the electron beam horizontally across the screen while simultaneously moving it vertically downwards. Horizontal Scan Rate: The frequency at which the electron beam completes one horizontal sweep (left to right and back again) across the screen. Much higher than the vertical scan rate. Vertical Scan Rate (Refresh Rate): The frequency at which the electron beam sweeps the entire screen area (from top to bottom and back to the top), typically measured in Hertz (Hz).

Magnetic deflection needs significant power, especially for large tubes or high refresh rates/resolutions, as more current is needed to bend the beam further and faster. These coils can generate noticeable heat. The high frequency of the horizontal deflection circuit (often 15 kHz or more, up to over 100 kHz for high-resolution monitors) means energy needs to be recycled efficiently using capacitors in the circuit. In old TVs, the switching noise from the flyback transformer operating at this horizontal frequency can sometimes be heard as a high-pitched whine, especially by younger people.

Electrostatic Deflection#

This method uses electric fields instead of magnetic fields. It’s typically found in oscilloscopes and some older, smaller displays like radar CRTs or vector monitors.

Electrostatic Deflection: Using electric fields created by charged plates to bend the path of an electron beam.

In these CRTs, there are pairs of metal plates placed inside the tube after the electron gun. One pair is oriented vertically to control horizontal movement, and another pair is horizontal to control vertical movement. By applying different voltages to these plates, you create an electric field between them. The negatively charged electron beam is repelled by the negative plate and attracted to the positive plate, causing it to bend.

The cool thing about electrostatic deflection, especially for oscilloscopes, is that you can apply the raw input signal (after amplification) directly to the deflection plates. This lets you instantly see the waveform of the signal drawn on the screen in real-time.

Electrostatic deflection is great for speed and direct signal control, but it’s harder to achieve large deflection angles compared to magnetic deflection without making the tube very long or the deflection plates so large they block the beam. This is why oscilloscopes often have relatively long tubes compared to their screen size.

The Screen (The Phosphor Coating)#

The front inside surface of the CRT glass is coated with a thin layer of phosphors.

Phosphor: A material that emits light when excited by an electron beam. CRT screens are coated with phosphor particles.

When the high-speed electron beam hits the phosphor particles, it transfers energy, causing the phosphor to glow brightly for a short time. The color of the light depends on the type of phosphor material used.

Monochrome CRTs: Have a single, uniform coating of one type of phosphor, typically white, green, or amber. Early white phosphors sometimes contained beryllium or cadmium, but later ones used safer mixes.
Color CRTs: Have the screen covered in tiny dots or stripes of three different phosphors, one that glows red, one green, and one blue. These are arranged in a precise pattern.

The duration for which the phosphor glows after being hit by an electron is called its persistence.

Phosphor Persistence: The length of time a phosphor continues to emit light after the electron beam stops hitting it.

Different applications need different persistence. Oscilloscopes displaying fast, single events might use long-persistence phosphors so the trace stays visible for a while. TVs and computer monitors need short-persistence phosphors so that images change quickly without excessive smearing or ghosting. However, if the persistence is too short relative to the refresh rate, you might see flicker.

Behind the phosphor layer (facing the electron gun), color and many monochrome CRTs have a thin coating of aluminum. This layer does several important things:

Reflects Light: It reflects light from the glowing phosphors forward towards the viewer, making the image brighter. Without it, light would be emitted in all directions, and much would be lost inside the tube.
Ion Protection: It acts as a barrier to prevent ions (charged atoms left over in the vacuum) from hitting and damaging the phosphor layer. Ions are much heavier than electrons and aren’t deflected as easily, so they could cause a dark spot in the center of the screen (“ion burn”) on older, non-aluminized tubes that relied on magnetic ion traps in the electron gun.
Conducts Charge: It provides a conductive path for the electrons after they hit the phosphors and lose energy. These electrons are then collected by the anode voltage, preventing negative charge buildup on the screen that would repel the incoming electron beam and reduce brightness.

Making Color Work (The Tricky Part)#

Color CRTs are significantly more complex than monochrome ones because you need to make each of the three electron beams (one for red, one for green, one for blue) only hit the phosphor dots or stripes of its intended color, no matter where the beam is directed on the screen. This requires precise alignment and components like the shadow mask or aperture grille.

Color CRTs typically have:

Three electron guns, one for each primary color (red, green, blue). These are often grouped together in the neck. Early color tubes (delta-gun) had the guns in a triangle shape; later ones used an “in-line” arrangement (all three in a row), which simplified convergence.
The screen is patterned with dots or stripes of red, green, and blue phosphors. These patterns match the arrangement of the electron guns.
A metal sheet called a shadow mask or a grid of vertical wires called an aperture grille is placed just behind the screen.

Shadow Mask: A perforated metal screen located just behind the phosphor-coated screen in a color CRT, with tiny holes or slots precisely aligned with the phosphor dots/stripes. It ensures that each electron beam only hits phosphors of its intended color. Aperture Grille: A grid of tensioned vertical wires located just behind the screen in some color CRTs (like Sony Trinitron). It serves the same purpose as a shadow mask, allowing electron beams to hit only the correct phosphor stripes.

Here’s the magic: The shadow mask or aperture grille acts like a stencil. The three electron guns are positioned slightly differently in the neck. Because of this angle and the position of the mask/grille, the electron beam from the ‘red’ gun can only pass through the holes/slots from a specific angle to hit the red phosphors. Electrons from the ‘green’ gun come from a different angle and can only hit the green phosphors through the same holes/slots, and same for blue. The mask or grille physically blocks electrons that are heading towards the wrong color phosphor.

If an electron hits the shadow mask instead of passing through a hole, its energy turns into heat and X-rays. This heating can cause the mask to warp slightly, especially in bright areas, leading to color errors (called doming). Shadow masks made of Invar (a low-expansion alloy) helped reduce this. Aperture grilles are often brighter because they block less of the electron beam, but they need horizontal support wires that are sometimes visible.

Convergence and Purity#

Even with the shadow mask or aperture grille, getting perfect color display is tricky due to tiny manufacturing variations. Two key challenges are:

Purity: Making sure that each individual electron beam (red, green, or blue) only hits phosphors of its own color. If the red beam hits some green or blue phosphors, areas that should be pure red will look reddish-magenta or orange, for example.

Color Purity: In a color CRT, ensuring that each electron beam excites only the phosphor dots/stripes of its intended color (red, green, or blue).
Convergence: Making sure that the three electron beams (red, green, and blue) all meet or converge at the same point on the screen as they pass through the shadow mask or aperture grille hole/slot. If they don’t converge properly, you’ll see color fringing or shadows around objects on the screen.

Color Convergence: In a color CRT, ensuring that the three electron beams (red, green, blue) meet at the same point on the screen, typically after passing through a single aperture in the shadow mask or aperture grille.

There are static (center of the screen) and dynamic (edges and corners) aspects to both purity and convergence. Static adjustments are often made at the factory using small, adjustable magnets (purity and convergence rings) around the neck of the tube to slightly tweak the electron beam paths. Dynamic adjustments, especially important for flatter screens and wider deflection angles, require complex electronic circuits that change the deflection or introduce corrective fields based on where the beam is on the screen. Early CRTs, especially color ones, had very curved faces partly because it made achieving good dynamic convergence passively easier.

Degaussing#

The shadow mask or aperture grille is made of metal and can become magnetized. If it does, its magnetic field interferes with the electron beams, causing color purity problems (patches of incorrect color).

To fix this, most color CRTs (and some monochrome ones with magnetic shields) have a degaussing system. This is a coil of wire around the front perimeter of the tube. When you turn the display on, a circuit sends a decaying alternating current through this coil. This creates a strong alternating magnetic field that sweeps through the tube and mask, effectively randomizing the magnetic domains in the metal and removing any magnetization. The field then slowly fades away to zero. This process is called degaussing (or demagnetizing).

Degaussing: The process of demagnetizing the shadow mask or aperture grille in a CRT to restore color purity.

Some displays degauss automatically every time they are turned on. If a display gets strongly magnetized (maybe by putting a magnet near it or moving it after it’s been in one spot for a long time), you might need to run the degauss cycle manually (if available) or even use a stronger external degaussing coil.

Different Flavors of CRTs#

CRTs weren’t just for TVs and monitors. Engineers adapted them for various applications, leading to different types:

Picture Tubes (CPTs): Used in TVs. Designed for standard broadcast formats. Often had a fixed amount of “overscan” (image area extending beyond the visible screen edge) to hide signal imperfections near the borders.
Display Tubes: Used in computer monitors. Generally higher resolution, often had features like underscan (showing the full image, including edges) and could handle a wider range of refresh rates and resolutions (“multisyncing”).
Monochrome CRTs: Simple, single electron gun, single phosphor color. No shadow mask or aperture grille. Found in older computer terminals, security monitors, and early oscilloscopes. Often had better sharpness than color CRTs for a given resolution as there was no mask to limit detail.
Projection CRTs: Small (7-9 inches), bright, monochrome CRTs used in CRT projectors and rear-projection TVs. Three tubes were used, one for each color (red, green, blue), and their images were projected and combined on a large screen. They used very high anode voltages and special cathodes to produce enough light for projection and often needed liquid cooling.
Oscilloscope CRTs: Use electrostatic deflection (as discussed above). Often have features like Post Deflection Acceleration (PDA) to boost brightness after deflection, improving sensitivity. Some included features like internal graticules (measurement grid marked on the glass) to avoid parallax error, or Microchannel Plates for displaying very fast, faint signals more brightly.
Image Storage Tubes: Used in analog storage oscilloscopes. Could hold a trace on the screen for minutes or hours using a special mesh grid and a “flood gun” to constantly refresh the image written by the main electron beam.
Vector Monitors: Used in some older arcade games (like Asteroids) and CAD systems. Instead of scanning a raster, they could draw lines point-to-point by directly controlling the deflection coils/plates. They could use monochrome or color tubes.
Data Storage Tubes (Williams Tube): An early type of computer memory using a CRT. Data was stored as patterns of static charge on the screen, read by an electrode. Not a display device in the traditional sense.
Charactrons: Special CRTs used in some early computers for displaying text very quickly. They had a metal stencil mask with character shapes inside. The electron beam would pass through a shape, forming a character, and then a second deflection system would move the shaped beam to the correct position on the screen.
Nimo Tubes: Small, specialized CRTs with multiple electron guns shaped like digits (0-9) to display numbers directly. Used in some early digital displays.
Flat CRTs: An attempt to make CRTs shallower or truly flat. Flat screens faced major engineering challenges, particularly with dynamic convergence and deflection power, requiring complex compensation circuits. LG’s Flatron line was notable for having a truly flat interior and exterior screen with a tensioned rim band for safety. Some older designs (like the Sinclair TV80 or Sony Watchman) achieved flatness or shallow depth by placing the electron gun to the side of the screen.

Putting Them Next to Other Tech#

Compared to modern displays like LCDs and OLEDs, CRTs have some distinct technical characteristics:

Pros (where CRTs often excelled):
- Color Reproduction: Many high-end CRTs could display a wider range of colors accurately.
- Motion Clarity: Since the phosphors glow for a very short time, there’s virtually no motion blur caused by the display itself (though phosphor persistence could cause smearing if too long). The image is essentially drawn instantaneously pixel by pixel.
- Input Lag: Generally very low input lag, as the analog signal goes directly to the electron gun/deflection system with minimal processing. Great for fast-paced gaming.
- Sharpness at various resolutions (Multisync): Many computer monitors could display sharp images at different resolutions because the analog signal and beam size could be adjusted. Flat panels often look blurry if not used at their native resolution.
- Contrast: High-end CRTs could offer very good contrast with true black levels (simply turning the electron beam off).
Cons (why they were replaced):
- Bulk and Weight: The thick glass tube needed to hold the vacuum makes them very heavy and deep.
- Power Consumption: They use significantly more electricity than flat panels.
- Heat Generation: They produce a lot of heat.
- Flicker: At lower refresh rates (like 50 or 60 Hz), the repeated scanning can cause visible flicker, especially in peripheral vision. Higher refresh rates (75 Hz+) on computer monitors largely eliminated this.
- Blooming: Bright areas can sometimes expand slightly and affect focus due to voltage fluctuations or increased beam current.
- Geometric Distortion: It’s harder to get perfectly straight lines and accurate geometry, especially at the edges and corners, compared to digital displays.
- Screen Size Limit: The weight and difficulty of maintaining the vacuum in larger tubes limited practical size.
- Burn-in: Static images displayed for long periods could permanently damage the phosphors.
- High Voltage: Requires dangerous high voltages internally.

Safety and Environmental Considerations#

Working with or disposing of CRTs involves some important safety points:

High Voltage and Shock Risk: CRTs operate with voltages (thousands to tens of thousands of volts) that are lethal. Capacitors and the tube itself can store this charge for a long time after being unplugged. Proper discharge procedures are essential before working on CRT-based equipment.
Implosion Risk: The vacuum means the tube is under immense external pressure. If the glass envelope is cracked or broken, it can implode violently. Modern CRTs usually have protection like tensioned metal bands or bonded faceplates to contain the glass if this happens, but they still need careful handling.
X-ray Emission: While modern CRTs are designed to meet strict regulations (like FDA limits in the US) and the radiation emitted is generally considered negligible at typical viewing distances, the high-speed electrons do generate X-rays. The glass shielding helps, but older or faulty tubes might emit more. Regulations came about in the 1960s after some TVs were found to emit higher levels.
Toxicity and Recycling: The glass contains significant amounts of lead (especially in the funnel and neck) for X-ray shielding. Some older phosphors contained toxic materials like cadmium or beryllium. This makes CRTs hazardous waste, requiring special recycling to prevent lead and other materials from polluting the environment. Disposing of them in regular landfills is often prohibited. Recycling involves separating the glass types (leaded vs. unleaded screen glass) and recovering metals.

Conclusion#

The CRT was a foundational technology for electronic displays, driving the development of television, computing, and scientific instruments for decades. While superseded by newer flat-panel displays, its underlying principles of electron beam control using electric and magnetic fields remain relevant in various areas of physics and engineering. Understanding how CRTs work provides valuable insight into vacuum tube technology, high voltage systems, and the fundamental physics of electron behavior, making them a fascinating topic for electrical engineers.