LCD Technology

Liquid Crystal : A liquid crystal display consists of an array of tiny crystals which can be manipulated to present data, such as, letters, numbers and even graphics.

Liquid Crystal Display Formation : A combination of polarizing filters and twisted liquid crystal creates a liquid crystal display.

Fig i

Liquid crystals are sandwiched between upper and lower plates of glass and they line up in groove pointing in direction 'a' and 'b' as in figure (i). If direction b is perpendicular to the direction a, then, two layers of liquid crystals form a 90° twist pattern. The light can pass through these crystals but because molecular structure arrangement is twisted 90°, the light also twists 90° as it passes through the crystal.

Fig ii

Fig iii

When voltage is applied to liquid crystals structure, the crystals rearrange themselves in the manner shown in the figure (ii) and light passes through, straight, along the arrangement of molecules.

This figure (iii) and description depicts the principle behind typical twisted nematic (TN) liquid crystal displays. When two polarizing filters are arranged along perpendicular polarizing axes, light entering from above is re-directed 90° along the helix arrangement of the liquid crystal molecules so that it passes through the lower filter. When voltage is applied, the liquid crystal molecules straighten out of their helix pattern and stop redirecting the angle of the light, thereby preventing light from passing through the lower filter.

In a TN type LCD, liquid crystals in which the molecules form a 90-degree twisted helix, are sandwiched between two polarizing filters. When no voltage is applied, light passes; when voltage is applied, light is blocked and the screen appears black. In other words, the voltage acts as a trigger causing the liquid crystals to function like the shutter of a camera.

Types of Displays : The displays can either be of positive type or negative type.
Positive types are used with ambient light and reflective type of configuration as the background is light and the display is dark. These are mainly used in Calculators and digital watches.

Negative type is opposite of the positive type with dark background and light display. It is most applicable for backlighting system and is capable of multi-colour displaying. This type is applicable in medical equipments, automotive, audio, etc.

Display Technologies : Displaying letters, numbers and graphics are based on the following display methods:

Fig vi Fig vii Fig viii

Segment system : Long display units are arranged to form a figure '8' to display numbers, as in fig (vi)

Dot Matrix Display (Character Display) : The display units are arranged in rows and columns to form characters as in fig. (vii). The typical character size is 5X7 digits. The display is characterised by number of rows and number of characters in each row.

Dot Matrix Display (Graphic Display) : The display units are arranged in rows and columns to depict graphics as in fig (viii). The display is characterised by number of rows and number of dots in each row.

Display Configurations : The light passing through the liquid crystals is merely natural or artificial ambient light. The configuration of the display is categorized by the relative position of the light source. There are three types:

Reflective Type : A "Reflective" type of display needs to have a light source in front of the it in order to be read, fig (ix). Reflective technology is most commonly found on calculators and some inexpensive digital wristwatches.

Transmissive Type : Transmissive technology is the opposite of the Reflective technology. This technology requires a backlight in order to see the characters. Most transmissive displays are negative mode, meaning that the text will be a light color and the background of the display is dark colored. With this technology, the light source must be behind the LCD panel, so that the light shines through the display and the pixels that are activated will allow the light to pass, fig (x). Transmissive technology can be found in Medical devices, test and measurement instruments, automotive audio, laptop computers etc.

Fig xi

Transflective Type : Transflective technology is the most commonly used polarization types in LCDs. Transflective is the configuration bonded to the rear polarizer, which enables light to pass through the back as well as reflecting light from the front, fig (xi). Most cellular phones, PDA, GPS, etc. use this type of polarization.

Electro-Optical Characteristics : The LCDs have the following Characteristics :

Viewing Angle (Ø1-Ø2): When one looks at an oblique way on the display, the colour and brightness seem to vary. The angle w.r.t. the normal on the screen, from where one gets the best view is known as the Viewing Angle. Viewing direction is specified in terms of the clock direction, refer to the figure below :

Contrast Ratio (K): This is defined as the ratio of brightness of non-selected part to the brightness of the selected part.

Backlighting : Backlighting is a technique used to make LCD easier to read. A backlit display is illuminated so that the foreground appears sharper in contrast with the background. The common backlight technologies used for the LCDs are:

EL (Electroluminiscent Lamp)

LED (Light Emitting Diode)

CCFL (Cold Cathode Fluorescent Lamp)

Characteristics	EL	LED	CCFL
Type	Thin Flat Panel	Edge and Array	Direct and Edge Lighting
Power Requirements	100~130 VDC 400Hz, Needs a DC/AC inverter, Low power consumption	4.2 VDC High power consumption in array type	600~1,000 VAC, 30 Hz, Need DC/AC inverter, Low power consumption.
Brightness ( CD/M2 )	70~200	15~80	200~600
Life Expectancy (Hours)	2,000 ~ 5,000	50,000 ~ 100,000	10,000 ~ 20,000
Thickness	1.5 mm max.	Array : 5 mm Edge : 1.3~4 mm	Direct : 15.0 mm Edge : 3.0 mm
Colour	Blue/Green, White, Yellow/Green	Yellow/Green, Amber, Red, Orange, Green	White

Display Types : TN (Twisted Nematic) has become the standard today. Almost all active matrix drive displays use TN type LCDs. The use of TN type LCDs in simple matrix drive displays causes the contrast to drop as the number of scan-lines of the image displayed is increased. The compensation for this drawback, led to the development of STN (Super Twisted Nematic) type LCDs, which offer high contrast, even on large screens; and TSTN (Triple STN) and FSTN (Film STN) LCDs, which feature a lightweight and thin body design that are optimal for large black-and-white LCDs and precise color imaging when equipped with a color filter.

Type	TN	STN	TSTN
Full Name	Twisted Nematic	Super Twisted Nematic	Triple Super Twisted Nematic
Structure	Twists nematic crystals 90°.	Twists nematic crystal about 260° (opposing twist directions)	Replaces DSTN compensation cell with plastic film
Color	Black/white	Yellow-green/dark blue	Black/white, multicolor
Features	Low power consumption Thin, lightweight Low cost	Large capacity display Thin, lightweight Low power consumption High contrast	Large capacity display Thin, lightweight Low power consumption Color display High contrast
Problems or Advantages	Cannot handle a large capacity	Black/white display not possible (therefore, color display not possible)	High contrast and high speed
Main Applications	Calculators, electronic organizers	Word processors (monocolour)	Word processors, laptop computers

LCDs in everyday life : LCDs are finding a growing variety of applications in televisions, word processors, personal computers, and in other electronic office equipment products. LCDs are widely used in imaging and information systems that are found in railway systems and other modes of transportation.

Precaution for handling LCD Panels and Modules :

	Keep the temperature within specified range for use and storage. Polarization degradation, bubble generation or polarizer peel-off may occur with high temperature and high humidity.
	Since LCM has been assembled and adjusted with a high degree of precision, avoid applying excessive shocks to the module or making any alterations or modifications to it.
	Do not scrub hard to avoid damaging the display surface. Avoid dropping or jarring.
	Wipe off saliva or water drops immediately. Contact with water over a long period of time may cause deformation or color fading.
	Do not put or attach anything on the display area to avoid leaving marks on.
	As glass is fragile, it tends to become or chipped during handing especially on the edges.
	Do not make extra holes on the printed circuit board, modify its shape or change the positions of components to be attached.
	Do not damage or modify the pattern wiring on the printed circuit board.
	Absolutely do not modify the zebra rubber strip (conductive rubber) or touch it with another object.
	Except for soldering the interface, do not make any alterations or modifications with a soldering iron.
	Do not drop, bend or twist LCM.

Storage : When storing LCDs as spares for some years, the following precautions are necessary.

Store them in a dark place, do not expose to sunlight or fluorescent light.

Keep the temperature between 0°C and 35°C.

The polarizer surface should not come in contact with any other object. (we advise you to store them in the container in which they were shipped).

Exposure to high humidity and temperature :

	Do not leave them for more than : 160 hours at 60°C
	These should not be left for more than 48 hours at -20°C.

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