Cathode-Ray Tube
Construction and operating principle of the CRT display
Introduction
A cathode-ray tube (CRT) converts electrical signals into visible images. Karl Ferdinand Braun developed an early cathode-ray display tube in 1897. CRT technology was later used in televisions, radar displays, oscilloscopes and computer monitors.
The earliest tubes were relatively simple and contained a fluorescent screen and electrodes, with the beam deflected by a magnet. The first practical colour television systems appeared in the middle of the twentieth century.
Basic Parts of a CRT
The visible image consists of many thousands of small illuminated points. The principal components of a CRT include:
- the electron gun,
- the shadow mask in a colour CRT,
- the phosphor layer,
- the deflection coils,
- the high-voltage accelerating anode,
- and the evacuated glass envelope.
Electron Gun
The electron gun is a metal electrode assembly that generates a very narrow electron beam. Monochrome televisions, radar displays and oscilloscopes generally use one beam, while a conventional colour CRT uses three beams corresponding to red, green and blue.
A heated cathode—historically often based on a tungsten heater with an emissive coating—releases electrons by thermionic emission. Surrounding control, accelerating and focusing electrodes regulate brightness, contrast, beam current and beam shape.
Changes in the cathode and grid voltages alter the beam current. The final focusing and accelerating electrodes form a narrow beam before it leaves the gun assembly.
Deflection Coils and Plates
A television or monitor commonly uses two sets of copper deflection coils wound on a ferrite core. One set produces horizontal deflection and the other vertical deflection.
Without deflection, the beam would strike only one point near the centre of the screen. Oscilloscopes and some radar tubes often use electrostatic deflection plates instead of magnetic coils.
Phosphor Layer
The inside of the front glass is coated with a phosphor. When the electron beam strikes it, the phosphor emits visible light.
A monochrome tube uses one phosphor colour, while a colour CRT uses separate red, green and blue phosphors. Examples historically listed in the original article include:
- ZnS:Ag — silver-activated zinc sulphide, used in some blue or blue-white screens.
- Y₂O₂S:Eu — europium-activated yttrium oxysulphide, used as a red phosphor.
- ZnS:Cu — copper-activated zinc sulphide, used for green emission in some oscilloscope screens.
- Mn-activated materials used in long-persistence radar displays.
The exact phosphor composition depends on colour, persistence, efficiency and application.
Complete Operating Sequence
First, the cathode heater reaches operating temperature. Electrons are emitted from the cathode and accelerated by nearby electrodes. Brightness-control and focusing electrodes regulate the number and shape of electrons in the beam.
The beam is then accelerated by the high-voltage anode coating on the inside of the glass envelope. Typical CRT accelerating voltages range from roughly 10 kV to more than 30 kV, depending on tube size and application.
The deflection system directs the beam horizontally and vertically across the phosphor screen. When the beam strikes a particular point, that point emits light. Varying the beam current changes the brightness.
In a colour tube, three beams pass through a shadow mask or aperture grille so that each beam reaches the correct red, green or blue phosphor dots or stripes.
Image Scanning
Traditional analogue television draws an image line by line. Interlaced scanning divides each frame into two fields containing alternating odd and even lines.
The CRT receives information controlling beam intensity, horizontal return and vertical return. In European 625-line television systems, the horizontal line frequency is approximately 15.625 kHz and the field frequency is 50 Hz. Computer monitors and other television standards use different frequencies and resolutions.
The original page also included macro photographs of a monitor displaying white and of a television screen displaying white and black, showing the individual phosphor pattern.
References
The original article referred to additional CRT and phosphor information:
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