Display Technology Fundamentals: CRT, Plasma, and Matrix Screens

1. Does the Cathode Ray Tube (CRT) Use an Electron Gun?

Yes, the picture tube television (CRT) relies on an electron gun to generate the image.

2. Types of Cathode Ray Tubes (CRTs)

The main types of CRTs encountered are:

• Delta Tube
• In-line Tube
• Trinitron Tube
• Precision In-Line (PIL) Tube

3. LCD vs. TFT: Passive and Active Matrix Differences

How is an LCD screen different from a TFT screen?

Traditional LCD screens often utilize a passive matrix structure, whereas TFT (Thin-Film Transistor) screens employ an active matrix structure, which allows for faster response times and better image quality.

4. CRT Degaussing: Purpose and Process

Reasons for Degaussing

The movement of electron beams within the CRT is based on magnetic fields generated by deflection coils. External magnetic sources, including the Earth's magnetic field, can influence the electron beam's trajectory. If the tube becomes magnetized, it leads to color distortion and purity issues.

The Degaussing Process

To avoid these problems, televisions incorporate an automatic degaussing circuit that activates every time the main power switch is pressed.

Upon receiving tension, a variable temperature resistance (a PTC thermistor) applies current across one or two coils surrounding the cone of the CRT. As the PTC heats up, its resistance increases rapidly, causing the current reaching the coils to gradually reduce to zero. This process demagnetizes the tube.

The entire process occurs in only a few tenths of a second, after which the TV is ready to function.

Important Conditions for Degaussing

Two important conditions must be met:

1. The TV must be connected and switched on from the main power switch (not just standby).
2. The PTC resistance must be initially cold to start the process effectively.

5. Understanding Static Convergence in CRTs

What is static convergence, and where can we find it?

Static convergence refers to the precise alignment of the three electron beams (Red, Green, Blue) at the center of the screen. When we act on the convergence magnets, we observe the beams separating into three independent grids (RGB) if misaligned. When correctly converged, the three beams meet to form a single white image. This correct positioning is typically found in Precision In-Line (PIL) cathode ray tubes.

6. How Plasma Display Panels (PDP) Operate

Plasma Generation

If noble gases are subjected to high temperatures or an electric shock, their atomic arrangement is altered, turning the gas into plasma. This process releases energy in the form of ultraviolet (UV) rays, which are not visible to the human eye.

Light Emission Mechanism

For the display to work, the gas is placed inside cavities whose walls are coated with colored phosphor compounds. After an electrical burst through the electrodes, the gas becomes plasma, releasing UV rays. This radiation strikes the phosphor layer, causing it to illuminate and generate visible light.

This visible light has one of the three basic colors (Red, Green, or Blue), depending on the phosphor compound used in that specific cell. The light is then projected outward through the transparent electrode located on the front of the panel.

PDP Structure

The Plasma Display Panel (PDP) uses a ceramic substrate. Cavities pierced into this substrate form the individual cells that create the image. At the base of each cell, an electrode is placed (acting as the cathode) and coated with phosphorus compounds that generate red, green, and blue light in adjoining cells, forming the triads of the image.