Chapter 8: Electronic Displays

The cathode-ray tube (CRT)

The CRT drove the first generation of glass cockpits and weather radar displays, and is still found in older aircraft. It is an evacuated glass tube in which a beam of electrons is fired at a phosphor-coated screen.

• Electron gun: a heater warms the cathode, which releases electrons by thermionic emission. The control grid, a metal cylinder held negative with respect to the cathode, controls how many electrons pass — the beam current and therefore the brightness (intensity). Anodes at high positive voltage accelerate the electrons, and a focusing anode (or focus coil) shapes them into a fine beam that makes a sharp spot.
• Deflection steers the beam. Electrostatic deflection uses two pairs of plates — the Y plates move the beam vertically and the X plates horizontally — and is used in small tubes such as oscilloscopes. Electromagnetic deflection uses coils in a yoke around the neck and is used in larger display tubes, such as EFIS and television.
• Phosphor screen: the inside of the screen is coated with phosphor that glows where the beam strikes. The phosphor's persistence (afterglow) and the refresh rate together give a steady picture.
• A conductive graphite (aquadag) coating on the inside of the funnel, connected to the final anode, collects electrons knocked off the screen and completes the circuit.
• The accelerating voltage — the EHT (extra-high tension) — is typically 10–25 kV.

• Raster scan: the beam sweeps the whole screen in horizontal lines from top to bottom, switching its intensity to form the image — like a television. Good for filled areas such as weather radar returns.
• Stroke (cursive or vector) writing: the beam traces the outline of each symbol directly, producing very bright, sharp lines. Many EFIS CRTs combined both, writing stroke symbology over a raster weather picture.
• Colour CRT: three electron guns (red, green and blue) and a shadow mask — a metal sheet with tiny holes just behind the screen — arranged so that each beam can only strike its own colour of phosphor dot. Mixing the three colours makes every other colour.

Light-emitting diodes (LED)

• An LED is a PN junction diode made from a compound semiconductor. When it is forward-biased, electrons crossing the junction recombine with holes and release energy as light (electroluminescence). Reverse bias produces no light.
• The colour depends on the semiconductor material: gallium arsenide gives infrared, gallium arsenide phosphide red, orange and yellow, gallium phosphide green, and indium gallium nitride blue. White LEDs are blue LEDs with a yellow phosphor.
• Forward voltage is about 1.5–3.5 V depending on colour, and a series resistor (or a constant-current driver) limits the current to a few tens of milliamps.
• Seven-segment displays show digits using segments a to g (plus a decimal point), driven by a BCD-to-seven-segment decoder. In a common-anode display all anodes go to +V and a segment lights when its cathode is pulled low; in a common-cathode display the reverse. Dot-matrix displays (for example 5 × 7 dots) show letters; bar graphs show levels.
• Advantages: robust, solid-state, very long life, fast response, low voltage, work well in the cold. Disadvantages: more current than an LCD, can be washed out in bright sunlight unless driven hard, and many digits need multiplexed drive.
• OLEDs (organic LEDs) make each pixel its own light source — thin, high contrast, wide viewing angle, no backlight — and are appearing in newer aircraft and cabin displays.

Liquid crystal displays (LCD)

A liquid crystal is a substance that flows like a liquid but whose long molecules line up like a crystal. Their alignment can be set by grooved alignment layers on the glass and changed by an electric field, and the aligned molecules rotate the plane of polarised light passing through them.

• Twisted nematic (TN) cell: the liquid crystal is sandwiched between two glass plates with transparent electrodes, between two polarisers crossed at 90°. With no voltage the molecules form a 90° twist that rotates the light so that it passes the second polariser — bright. With voltage applied the molecules align with the field, the twist is lost and the light is blocked — dark. (Reversing one polariser gives the opposite, "normally black" display.)
• LCDs are driven with AC — a steady DC voltage would damage the liquid crystal by electrolysis.
• The crystal does not emit light. A transmissive LCD needs a backlight; a reflective LCD (a watch or calculator) uses room light with a mirror behind; transflective displays do both.
• Passive matrix: rows and columns of electrodes switch pixels one row at a time — cheap, but slow, low contrast and prone to crosstalk between pixels. Used for simple readouts.
• Active matrix (AMLCD / TFT): a thin-film transistor and a small storage capacitor at every sub-pixel switch and hold its voltage, giving fast, high-contrast, full-colour pictures. Colour filters make each pixel from red, green and blue sub-pixels. This is the display in modern PFDs, NDs and MFDs.

• Aircraft AMLCDs use very bright backlights — formerly cold-cathode fluorescent tubes, now LEDs — for sunlight readability, plus optical films that widen the viewing angle so both pilots can read the centre displays.
• Advantages: thin, light, low power and low heat, low voltages, long life, high resolution, no implosion hazard, unaffected by magnetic fields. Disadvantages: needs a backlight, limited viewing angle, slow response and poor contrast when cold (some units have heaters) and damage at extreme temperatures, and possible pixel defects (stuck or dead pixels are judged against limits in the component manual).

Comparing the technologies

• Display terms: pixel (smallest picture element), resolution (number of pixels), luminance (brightness, in cd/m² or foot-lamberts), contrast ratio, viewing angle, response time and refresh rate.
• Other types: electroluminescent panels (panel lighting and some displays) and head-up displays, which project symbology onto a transparent combiner in the pilot's line of sight.