1 Chapter 1 1. Types of Electronic displays 2. LCD displays ITECC/TP/ECE/SimBN EC5103PA_Apr2008

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Chapter 1

1. Types of Electronic displays2. LCD displays

ITECC/TP/ECE/SimBNEC5103PA_Apr2008

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Types of Electronic displays

1 Emissive

2 Non emissive

(Based on light control)

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Types of Emissive Displays

1. Neon light

- used in advertising and commercial signage.

- long, narrow glass tubes, often bent into all sorts of shapes. can spell out a word.

- emit light in different colours.

- Inside the glass tube there is a gas like neon, argon or krypton at low pressure. At both ends of the tube there are metal electrodes. When you apply a high voltage to the electrodes, the neon gas ionizes, and electrons flow through the gas. These electrons excite the neon atoms and cause them to emit light that we can see.

- Neon emits red light when energized. Other gases emit other colours.

http://en.wikipedia.org/wiki/Image:NeTube.jpg

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2. Cathode ray tube (CRT)

A CRT monitor contains millions of tiny red, green, and blue phosphor dots that glow when struck by an electron beam that travels across the screen to create a visible image.

A: Cathode D: Phosphor-coated screen

B: Conductive coating E: Electron beam

C: Anode F: Shadow mask

Fig 1 Inside a CRT

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3. Light emitting diode (LED)

- a semiconductor diode that emits incoherent narrow-spectrum light when electrically biased in the forward direction of the p-n junction.

- often used as small indicator lights on electronic devices and increasingly in higher power applications such as flashlights and area lighting.

- The colour of the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible, or ultraviolet.

- LEDs can also be used as a regular household light source. Besides lighting, interesting applications include sterilization of water and disinfection of devices.

http://en.wikipedia.org/wiki/Image:RBG-LED.jpg

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4. Electro-luminescent display (ELD)

- A technology used to produce a very thin display screen, called a flat-panel display, used in some portable computers.

- works by sandwiching a thin film of phosphorescent

substance between two plates. One plate is coated with vertical wires and the other with horizontal wires, forming a grid. When an electrical current is passed through a horizontal and vertical wire, the phosphorescent film at the intersection glows, creating a point of light, or pixel.

- light and small making them ideal for vehicle dashboards and various military applications.

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5. Gas-Plasma display (GPD) (PDP)

- A type of flat-panel display that commonly used for large TV displays (typically above 940 mm).

- Theatre, theme park, sale display, home entertainment system.

- 21 to 63 inches, 3 to 6 inches deep, hang on wall.

- 16 millions colours & 160 degree-viewing angle.

- VGA, SVGA, XGA, SXGA, UXGA.

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What is Plasma Display

• Plasma display or Gas-Plasma Display (GPD) works by sandwiching neon gas between two plates.

• Each plate is coated with a conductive print.

• The print on one plate contains vertical conductive lines and the other plate has horizontal lines. Together, the two plates form a grid.

• When electric current is passed through a horizontal and a vertical line, the gas at the intersection glows, creating a point of light, or pixel.

• You can think of a gas-plasma display as a collection of very small neon bulbs. Images on gas-plasma displays generally appear as orange objects on top of a black background.

Fig 9

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Advantages and disadvantages of Plasma Display

Advantages

• The main advantage of plasma display technology is that it can produce a very wide screen using extremely thin materials.

• Each pixel is lit individually, the image is very bright and looks good from almost every angle.

• The image quality isn't quite up to the standards of the best cathode ray tube sets, but it certainly meets most people's expectations.

Disadvantages

• The biggest drawback of this technology has to be the price.• Consumes high power.

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6. Vacuum fluorescent display (VFD)

- used commonly on consumer-electronics equipment such as video cassette recorders, car radios, and microwave ovens.

- emits a very bright light with clear contrast and can easily support display elements of various colours. The technology is related to both the cathode ray tube and the nixie tube.

http://en.wikipedia.org/wiki/Image:Vacuum_flourescent_2.jpg

http://en.wikipedia.org/wiki/Image:Vacuum_fluorescent_1.jpg

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7. Field emission display (FED)

- a type of flat panel display using field emitting cathodes to bombard phosphor coatings as the light emissive medium.

- very similar to cathode ray tubes, however they are only a few millimeters thick.

- uses a large array of fine metal tips or carbon nanotubes (which are the most efficient electron emitters known), with many positioned behind each phosphor dot, to emit electrons through a process known as field emission.

- FEDs are energy efficient and could provide a flat panel technology that features less power consumption than existing LCD and plasma display technologies. They can also be cheaper to make, as they have fewer total components.

- Nano-emissive display is the name given by Motorola for field emission display.

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8. Organic LED (OLED)

• solid-state devices composed of thin films of organic molecules that create light with the application of electricity.

• no backlight, • brighter, thinner, faster & lighter than LCDs• use less power, offer high contrast & cheaper to manufacture.• used in PDAs, cellular phones, television screens, computer displays, portable

system screens, advertising, information and indication.

Fig 2 (a) OLED display for Sony Clie (b) Sony 11-inch OLED

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8. Organic LED (OLED)

• biggest technical problem is the limited lifetime of the organic materials. In particular, blue OLEDs historically have had a lifetime of around 14,000 hours when used for flat-panel displays, which is lower than typical lifetime of LCD, LED or Plasma display technology – each currently rated for about 60,000 hours, depending on manufacturer and model.

• The intrusion of water into displays can damage or destroy the organic materials. Therefore, improved sealing processes are important for practical manufacturing and may limit the longevity of more flexible displays.

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1. Liquid crystal display (LCD)

- composed of liquid crystal suspended between two transparent sheets.

- can be found in digital watches, hand held displays, laptop computers, computer projectors, calculators, stereo equipment and a host of other electronic equipment today.

Types of Non Emissive Displays

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2. Digital micromirror device(DMD)

- Digital Light Processing (DLP), a new technology developed by Texas Instruments used for projecting images from a monitor onto a large screen for presentations.

- DLP uses tiny mirrors housed on a special kind of microchip called a Digital Micromirror Device (DMD). The result is sharp images that can be clearly seen even in a normally lit room.

- The number of mirrors corresponds to the resolution of the screen. DLP 1080p technology delivers more than 2 million pixels for true 1920x1080p resolution, the highest available.


Digital Micromirror Device (DMD) DLP chip

http://upload.wikimedia.org/wikipedia/commons/b/b2/DLP_micromirrors.gif

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3. Electrophoretic ink (e-ink)

- a specialized type of electronic paper (e-paper) that combines the uses and advantages of a computer display and paper. - e-paper displays are extremely thin, use minimal amounts of power and provide a high-contrast viewing surface like paper, but can be easily updated like a monitor. http://www.sonystyle.com/webapp/wcs/stores/servlet/CategoryDisplay?catalogId=10551&storeId=10151&langId=-1&categoryId=16184


Fig 3 (a) Sony reader (b) Seiko watch (c) 2mm thick flexible active-matrix display

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• EPDs are a technology enabled by electronic ink (e-ink) that carries a charge enabling it to be updated through electronics.

• is a reflective technology which requires no front or backlight, is viewable under a wide range of lighting conditions, including direct sunlight, and requires no power to maintain an image.

• http://en.wikipedia.org/wiki/E-paper• http://www.electronista.com/articles/07/11/16/

seiko.ultra.dense.e.paper/


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• There are several different technologies to build e-paper, some of which can use plastic substrate and electronics, so that the display is flexible. It is considered more comfortable to read than conventional displays. This is due to the stable image which does not need to be constantly refreshed, the large viewing angle, and the fact that it uses reflected ambient light. It has a similar contrast ratio to that of a newspaper and is lightweight and durable, however it still lacks good color reproduction.

• Applications include e-book readers capable of displaying digital versions of books and e-paper magazines, electronic pricing labels in retail shops, time tables at bus stations, and electronic billboards.


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•is designed to appeal to the low-end market and developing countries, making it less functional, but also less expensive than most phones.

•the first mobile phone to use electronic paper in its screen.

•Motorola uses the term ClearVision to describe the new display, which is manufactured using E Ink's electrophoretic imaging film.

•The electronic paper main display allows for the phone's thinness (no glass), longer battery life, and outdoor viewability (paper-like reflectivity).

• It has a backlight for the keypad and a slit that projects the backlight onto the screen so the display can be seen in darkness.

Motorola F3/F3c (MOTOFONE)

Screen electronic paper (ClearVision Display)

Ringtone Polyphonic

Networks GSM / CDMA

Physical size

47 x 114 x 9.1 mm

Weight 68 g

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What is LCD Displays

- most common type of flat panel display.

- applications: digital watches, notebook PC, handheld computers, calculators, televisions, digital camera, video cameras, monitors.

- use two sheets of polarising material with a liquid crystal solution between them.

- composed of pixels or other shapes which can be turned on or off with electrical stimulation.

- typically a light is passed through the LCD to illuminate the pixels.

- electric current passed through liquid causes liquid crystals (rod-shaped molecules) to align so that light cannot pass through them.

- each crystal is like a shutter, either allowing light to pass through or blocking the light.

- http://en.wikipedia.org/wiki/Liquid_crystal_display

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Types of LCD Display

• Passive matrix LCD Display (PMLCD)– TN (twisted nematic)– STN (supertwisted nematic) – DSTN (dual scan twisted nematic)

• Active matrix LCD Display (AMLCD)– TFT (Thin Film Transistor)

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Consist of a grid of horizontal & vertical wires. At the intersection of each grid is an LCD element that constitutes a single pixel.

Advantages - use less power and cheaper than AMLCDs.

Disadvantages - slow response time, smearing occurs when the display

cannot keep up with changes of content. - causes ghosting, an effect whereby an area of "on" pixels

causes a shadow on "off" pixels in the same rows and columns.

- narrow viewing angle.

Passive matrix LCD displays

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AMLCD displays or called TFT (thin film transistor) displays. Transistor are built into each pixel to switch each one on or off within the screen.

Advantages- sharper, broader viewing angle than passive matrix. - thinner and lighter.- faster response time.

Disadvantages - consume more power and more expensive.

http://en.wikipedia.org/wiki/TFT_LCD

Active matrix LCD displays

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The first principle of a LCD

• Consists of sandwiching liquid crystals between two finely grooved surfaces, where the grooves on one surface are perpendicular (90°) to the grooves on the other, see Fig 1.

• If the molecules at one surface are aligned north to south, and the molecules on the other are aligned east to west, then those in-between are forced into a twisted state of 90 °.

• Light follows the alignment of molecules, therefore is also twisted through 90 ° as it passes through the liquid crystals.

• When a voltage is applied to the liquid crystal, the molecules rearrange themselves vertically, allowing light to pass through untwisted.

Basic Operating Principles of LCD Display

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The second principle of an LCD

• It relies on the properties of polarising filters and light itself.

• A polarising filter is a set of incredibly fine parallel lines. These lines act like a net, blocking all light waves apart from those (coincidentally) orientated parallel to the lines.

• A second polarising filter with lines arranged perpendicular (at 90 °) to the first would therefore totally block this already polarised light.

• Light would only pass through the second polariser if its lines were exactly parallel with the first, or if the light itself had been twisted to match the second polariser.

Basic Operating Principles of LCD Display

Light

Polarisingfilter

Polarisingfilter

Liquid crystal(rod-like molecules)

Fig 1

0°

90°

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Fig 2 (a) 0v transmitting state (b) >5v non-transmitting state

Twisted nematic (TN) LCD at normally white mode

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Normally White and Normally Black Mode

• Normally White Mode - no voltage applied equals light passing through, see Fig 2a.

- while applied voltage equals no light emerging at the other end, see Fig 2b.

• Normally Black Mode - when voltage applied equals light passing through,

- while no voltage equals no light emerging at the other end.

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No power is applied

DSTN (dual scan twisted nematic) display

Fig 3 normally white mode

The orientation of alignment layers varies between 90 degrees and 270 degrees, depending on the total rotation of the liquid crystals between them.

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Backlight and Colour pixel

Backlight- cold-cathode fluorescent tubes mounted along the top

and bottom edges of the panel, the light are distributed across the panel using a plastic light guide or prism.

- the image which appears on the screen is created by this light as it passes through the layers of the panel.

Colour pixel- In a colour monitor, each pixel is made out of 3 subpixels that

have either red, green, or blue colour filters.

- Each subpixel is energized with different intensities, creating a range of colors perceived as the mixture of these dots.

- Fig 5 shows that additional red, green and blue coloured filters are used to create a single multi-coloured pixel.

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Structure of a LCD monitor

Fig 4

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TFT (Thin Film Transistor) Displays

- In a TFT screen, also known as active matrix, an extra matrix of transistors is connected to the LCD panel,

- one transistor for each colour (RGB) of each pixel.-

- these transistors drive the pixels, eliminating the problems of ghosting and slow response speed.

Fig 5 Simple TFT Active Matrix Array

Row select(Gate)

Column select (Data source)

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Main reason for expensive TFT displays

- VGA screens need 640 x 480 x 3 = 921,000 transistors,

- XGA screens need 1024 x 768 x 3 = 2,359,296 transistors,

- The complete matrix of transistors has to be produced on a single, expensive silicon wafer and more than a couple of impurities means that the whole wafer must be discarded.

- This leads to a high wastage rate and is the main reason for the high price of TFT displays. It's also the reason why in any TFT display, there are liable to be a couple of defective pixels where the transistors have failed.

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Two phenomenon to define a defective LCD pixel

1. A "lit" pixel, which appears as one or several randomly-placed red, blue and/or green pixel elements on an all-black background.

2. A "missing" or "dead" pixel, which appears as a black dot on all-white backgrounds.

Note: A “lit” pixel is more common and is the result of a transistor occasionally shorting on, resulting in a permanently "turned-on" (red, green or blue) pixel.

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No. of defective pixels accepted per LCD panel

•LCD manufacturers set limits based on user feedback and manufacturing cost data - as to how many defective pixels are acceptable for a given LCD panel.

•The goal is to maintain reasonable product pricing while minimising the degree of user distraction from defective pixels. -For a XGA panel 1024x768x3 = 2,359,296 pixels

= 2,359,296 transistors

-If 20 defective pixels, pixel defect rate = (20/2,359,296)*100 = 0.0008%

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a-si (amorphous silicon) and p-si (polysilicon) TFT LCD panel

Fig 6

p-Si (polysilicon) technology

- allows driver circuitry and peripheral electronics to be made as an integral part of the display.

- thinner, brighter panels with better contrast ratios, and tougher than a-si panels. It allows larger panels to be fitted into existing casings.

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Two specifications for CRT screen size:

1) CRT size (the actual size of the picture tube).

2) viewable screen size (the usable screen area). - the CRT picture tube is enclosed in the plastic

casing, the viewable screen size is smaller than the overall CRT size.

http://bravotech.us/info/crt-lcd.htm

Comparison between CRT & LCD Displays

Flat Panel size CRT sizeTypical

resolution

13.5in 15in 800x600

14.5in to 15in 17in 1024x768

18in 21in1280x1024 or

1600x1200

Table 1

1. Size

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2. + LCD monitor is thinner and lighter than CRT monitor.

3. + Unlike CRT, the LCD monitors have no convergence problems, because each cell is switched on and off individually.

4. + LCD monitors are also called "soft" screens. The image does not flicker thus causing less eye strain.

5. - It is possible for one or more pixels on LCD panel to be flawed. There's only a slim chance that all pixels will be perfect; some will be stuck on (a "bright" defect) or off (a "dark" defect).

6. - LCD panels are lit through the backlight; sometimes, will exhibit brighter lines in some parts of the screen. Ghosting may occur where a particularly light or dark image can affect adjacent portions of the screen.

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7. -Viewing angle problems on LCDs occur because the technology using transmissive method, which works by modulating the light that passes through the display. CRTs are emissive displays, the light is emitted at the front of the display, which is easily viewed from greater angles.

8. CRT require an analogue signal to produce a picture and the LCD require a digital signal. • The graphics signal is generated digitally inside the PC,

converted by the graphics card to an analogue signal, then fed to the LCD panel where it has to be converted back into a digital signal.

• This process limits the display's performance and compromise the image quality.

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Fig 7Interface between LCD monitor and PC using (a) digital interface (b) analogue interface

(a)

(b)

DVI : Digital Visual InterfaceVGA : Video Graphics Adapter

Fig 8 A graphics adapter with both analogue (VGA) and digital (DVI) connectors

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Display

TypeViewing Angle

Contrast Ratio

Response Speed

Bright-ness

Power Consumpt

-ion

Life

PMLCD49-100

degrees40:1 300ms 70 - 90 45 watts

60K hours

AMLCD> 140

degrees140:1 25ms 70 - 90 50 watts

60K hours

CRT> 190

degrees300:1 n/a

220 - 270

180 watts Years

9. Table 2 compares the difference between a 13.5in PMLCD, AMLCD and a 15 in CRT monitor.


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10. Contrast ratio is a measure of how much brighter a pure white output is compared to a pure black output. The higher the contrast the sharper the image and the more pure the white will be. When compared with LCDs, CRTs offer by far the greatest contrast ratio.

11. Response time is measured in milliseconds and refers to the time it takes each pixel to respond to the command it receives from the panel controller.

12. An AMLCD has a much better response time than a PMLCD. Response time doesn't apply to CRTs because of the way they handle the display of information (an electron beam exciting phosphors).

13. The higher the level of brightness the brighter the white displays.

14. The life span of an LCD, is the mean time between failures for the flat panel. If it runs continuously it will have an average life of 60,000 hours (about 6.8 years) before the light burns out. LCDs simply burn out, CRT's get dimmer as they age.

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Review Questions on LCD

1. What are LCD monitors and how do they work?

2. Name four applications of LCD displays.

3. Give the full name for the following:i) AMLCDii) PMLCD iii) TFT iv) DSTN

v) DVI

4. Give two advantages of an AMLCD as compared to PMLCD.

5. State two types of PMLCD technology.

6. What is the function of the backlight of a LCD panel?

7. What are the two phenomenons which define a defective LCD pixel?

8. State four disadvantages of CRT monitor as compared to LCD monitor.

Documents

1 Chapter 1 1. Types of Electronic displays 2. LCD displays ITECC/TP/ECE/SimBN EC5103PA_Apr2008