Electronic Displays Chapter 5.11

Electronic Displays (5.11)

• Learning objectives include describing cathode ray tubes, light-emitting diodes (LEDs), and liquid crystal displays (LCDs) in modern aircraft (Level 2).

LED Fundamentals

• Light-Emitting Diodes (LEDs) are optoelectronic devices used to replace fragile incandescent bulbs for on/off indications.
• LEDs produce visible light when forward biased, in various colours (red, orange, yellow, green, blue, white, ultraviolet, and infrared).
• LEDs are valuable in infrared detection applications.
• Typical forward voltage is about 1.6 V.
• Typical operating current is about 10 mA.
• LEDs have a long lifespan, exceeding 100,000 operating hours.
• LEDs are susceptible to damage if connected to voltage supplies exceeding 2 V; resistors are needed to limit current at higher voltages.

Peak Wavelength Single-Coloured (Monochromatic) LEDs

• Light colour perception is related to wavelength.
• LEDs emit light over a specific narrow band of the spectrum, unlike incandescent lamps which produce a wide spectrum.
• Peak wavelength defines the colour of emitted light.
• Examples of peak wavelengths include 450nm (blue), 535nm (green), 585nm (yellow), 620nm (orange), 700nm (red), and 950nm (infrared).
• Light intensity distributions are peaked at the specified wavelengths.

Multi-Coloured and Bi-Coloured LEDs

• Multi-coloured LEDs contain multiple reverse-parallel semiconductor chips, each emitting a single colour.
• Only one chip emits light at a time, the choice dependent on current direction.
• A single series resistor is suitable for calculating the required resistance.
• Bi-coloured LEDs combine red and green chips to generate yellow light. Alternating current operation results in seemingly constant yellow light.

Tri-Coloured LEDs

• Tri-coloured LEDs combine three semiconductor chips emitting different colours to form a three-terminal component.
• Common anode and common cathode types are available.
• Switching voltages between the two semiconductor chips allows selection of a two-colour light source.

Seven-Segment LED Display

• Seven-segment LEDs are common in calculators and digital voltmeters as indicators or displays.
• The seven segments, represented by letters A-G, can be combined to form numbers 0-9.

Common Anode/Common Cathode Displays

• Common anode displays have all anodes connected together.
• Common cathode displays have all cathodes connected together.
• Appropriate voltage/ground applications, are required to illuminate segments.

Alphanumeric LED Display

• Alphanumeric displays function similarly to seven-segment displays, often employing 16 segments.

Dot Matrix LED Display

• A more flexible display choice for alphanumeric representations using an array of LED dies (typically 7x5 arrangement).
• Offers greater application possibilities compared to seven-segment displays.

Organic LEDs (OLEDs)

• OLEDs are organic light-emitting diodes, formed by placing organic thin films between conductors.
• Electrical current triggers light emission.
• OLEDs do not require backlights, thus making them thinner and more efficient than LCDs.

OLED vs LCD

• OLED displays generally exhibit lower power consumption, faster refresh rates, better contrast, and greater brightness.
• Also offering better durability in a wider operating temperature range, and lighter weight.

Liquid Crystal Displays (LCDs) - Polarisation

• Light is composed of electromagnetic waves that can travel in any orientation.
• A polarised filter allows light waves of a specific orientation to pass
• Polarisation restricts light waves to vibrate in a single plane.
• Using cross-polarising lenses can block light completely.

Liquid Crystal (LC)

• Liquid crystals exhibit characteristics between solid crystals and liquid, within specific temperature ranges.
• They display a crystalline structure.
• Refraction, depending on the crystalline state, shows variations.

Liquid Crystal Reorientation

• LC molecules align with the direction of an applied electrical field.
• This field causes the molecules to align parallel to the field.

LCD Construction

• LCDs consist of two glass plates with a layer of liquid crystal fluid separating them.
• Transparent-conductive electrodes, which shape segments, pixels, or symbols, are deposited onto the inner glass surfaces of the LCD.
• Polarising films are layered perpendicularly onto the outer surfaces of the glass plates.

LCD Operation

• Applying voltage between the segment and backplane causes the LCD segment to appear dark. With no voltage, the LCD appears clear.
• Usually employs AC voltage for operation.

Backlit LCDs

• Backlit LCDs, as opposed to reflective LCDs, employ internal light sources (like fluorescent tubes) to illuminate the display.
• White diffusion panels behind the LCD are used to spread light evenly.

Greyscale LCDs

• Greyscale displays use varying levels of voltage supplied to components, to control the light level and transition through shades.

Colour LCDs

• Colour LC displays utilise sub-pixels (red, green, and blue) with colour filters.
• Varying voltages produce varied shades within each sub-pixel to create combinations of colours.

Additive Colour Mixing

• This method combines coloured light to form other colours.
• Using red, green, blue (RGB) combinations results in various shades.

Cathode Ray Tube (CRT) - Thermionic Emission

• Thermionic emission describes the escape of electrons from a heated material (such as a filament) in a vacuum.
• This effect was discovered by Edison.
• The released electrons move from the heated filament to the positively charged plate.
• Materials with high temperatures release electrons more readily.

CRT - Electron Gun

• The cathode is a small-diameter nickel cap coated with emitting material.
• Used to produce an electron beam, which is highly concentrated.
• A grid is used to concentrate and focus the emitted electrons into a beam.
• Additional accelerating anodes (electrodes) ensure the electron beam reaching the screen is focused and high speed.

CRT - Screen

• CRT screens comprise a phosphor-coated interior.
• Phosphors give off light when struck by electrons.
• A coating of aquadag, sometimes present on the screen, mitigates secondary electron emission effects.
• Multiple phosphors within the display generate colour.

CRT - Operation Review

• CRT elements (cathode, grid, focusing and accelerating anodes, deflection plates, aquadag-coating, and screen) work to control electron beam movement for creating the image.
• Various voltages control the brightness of each spot or area.
• Deflection coils (or plates) are used to control horizontal and vertical beam movement across the screen.

CRT - Deflection Coils/Plates

• Deflection coils/plates function similarly, controlling the electron beam for horizontal and vertical sweeps across the screen.

###CRT Handling and Disposal Safety

• Specific safety precautions are essential for handling CRTs.
• Risk of implosion when damaged or broken.
• The internal phosphor-coatings are often hazardous.
• Specific precautions to minimize damage during removal or handling of damaged units.
• Proper disposal procedure is crucial.

Simultaneous Picture Formation

• Light-sensitive devices capture the image; the data is processed, amplified, and sent to the output displays.
• Signal strength corresponds to light intensity.
• System uses an array of amplifiers to ensure the quality and consistency in the output.

Sequential Scanning

• Eliminates the drawbacks of simultaneous scanning by sequentially sending signals in rapid succession, providing a complete image.
• Scanning techniques use the phenomenon called persistence of vision.
• Sending all the odd-numbered lines first, then all even-numbered lines, this method creates a fluid image or raster.

Scanning Raster

• The spot/electron beam moves across the screen in a predetermined sequence of horizontal lines and rows.
• -The spot then rapidly returns to the starting point for each subsequent line.
• The flyback period (returning the spot) is invisible.

Interlace Scanning

• Images are broken into two fields (odd and even lines), which are consecutively displayed, creating a smoother image for viewers.
• Allows for more video channels to be accommodated with a lower bandwidth.
• The persistence of vision blurs the transition between the two fields.