Picture Tube Technology: Black-and-White and Color CRTs

Picture Tubes in Black and White

This device is responsible for converting luminance information, carried as an electrical signal, into a two-dimensional image on the screen.

Basic Components of a Picture Tube

The blocks that make up a picture tube include:

• Electron gun: continuously emits a thin beam of electrons that varies according to the scanned image.
• Deflection coils: permanently deflect the electron beam horizontally and vertically.
• Luminescent screen: produces light when struck by the electron beam.

Deflection System

To ensure the emitted electrons strike the correct point on the screen, the beam must be moved over the screen surface. There are two ways this process is implemented: electrostatic deflection and magnetic deflection.

Electrostatic deflection uses the force between electrons and two conductive plates with opposite electric charge or potential. Magnetic deflection is the deviation of electrons through the magnetic field generated by two (or more) coils through which current flows.

Electrostatic and Magnetic Deflection

Electrostatic systems are based on charged plates, while magnetic systems use current through coils to create a magnetic field that steers the beam.

Deflection Currents

The currents driving the deflection coils should have a sawtooth waveform so that the deflection reaches a maximum value and then returns quickly to its initial value for the next scan line or frame.

Contrast and Brightness Settings

Contrast control adjusts the amplitude of the input video signal, providing more or less difference between light and dark tones in the image.

Brightness control adds a DC offset (a level of voltage) to the video signal, which increases or decreases the overall brightness of the image.

Other TV Settings

Other settings include horizontal centering and vertical centering, which let you frame the image correctly on the screen.

The Color Picture Tube

Color tubes use three electron beams, one for each primary color. The operating voltages for different electrodes are typically in the following ranges:

• Cathode: 100–150 V
• Control grid (G1): ~30 V
• First anode (G2): 500–1,000 V
• Focus grid (G3): 2–7 kV
• Final anode (G4): 25–30 kV

Luminescent Material

The image is formed on a phosphor layer located on the screen. The phosphor layer consists of combinations of the three primary phosphor colors: red, green, and blue. In some designs a white-emitting phosphor layer or mixed phosphors may be used in specific regions.

The number of luminescent points (pixels) on the screen—often on the order of hundreds of thousands—determines the resolution of the display. Initially, luminescent points were arranged as triads; modern designs may use narrow strips or other arrangements.

Types of Color Picture Tubes

Common arrangements of guns and phosphors include:

• Delta (triad) tubes: the three electron guns are mounted at the corners of an equilateral triangle, forming a delta configuration.
• In-line guns: improvements in gun alignment led to a generally aligned, in-line system used widely today.
• Trinitron tubes: use a single-folded or single-barrel arrangement with three cathodes aligned in-line and a special aperture grille.

Autoconvergent Picture Tubes

A simplification of convergence adjustments resulted in autoconvergent picture tubes. Purity and convergence adjustments are corrected statically during manufacturing, so no further adjustments are usually needed by the user.