Liquid Crystal Displays for Laptops and TV

What they are and how they work

Liquid Crystal Display (LCD) for TV or laptop

Introduction• In 1980, flat panel TV & laptop displays didn’t exist. LCDs made them possible.• An LCD controls light from a backlight, or from around us: it does not emit light• It does this independently for each of many tiny areas (pixels) on the screen• In a colour LCD, each pixel is comprised of a red, green and blue subpixel, each with

its own colour filter• Colours are produced by applying a small voltage at each subpixel to allow more, or

less, red, green, or blue light to pass through it• This causes a colour picture to be displayed on the LCD• Subpixels can change fast enough to allow moving pictures to be displayed

Where do all the colours come from?• Each row of subpixels has its own narrow colour filter stripe - some red, some green

and some blue– You can see these with a x30 magnifier on a laptop, or TV, LCD screen– If all the subpixels in an area are bright, then we see white there. If they are dark, we see black– Screw up your eyes slightly and look at this picture

• Subpixels are too small to see individually by eye, so their colours mix • We see colours that depend on the brightnesses of adjacent R, G & B subpixels

• Each LCD subpixel is controlled by a voltage (applied using conductors you can see through) to create the right amounts of R, G & B for each part of the display

Liquid Crystal Display (LCD) for TV or laptop

What is a liquid crystal?• All materials are comprised of vast numbers of tiny

particles (molecules)• A liquid crystal has rod-like particles that all point in

the same direction (more or less)• It is a milky liquid that forms when certain solids melt

and becomes a normal clear liquid at some higher temperature– In an LCD, the thin layer of material used will remain a liquid

crystal over a wide temperature range (typically from -20oC to +80oC)

– Note that a liquid crystal is not solid, liquid or gas. It is an additional phase of matter.

– It has unusual optical properties, but these can normally only be seen using Polarisers

Liquid Crystal Display (LCD) for TV or laptop

What are polarisers (often called Polaroid)?• Polarisers are thin plastic sheets used for the lenses of Polaroid sunglasses• The sheet has a preferred direction (created by stretching it when it is made)• If two sheets have their preferred directions parallel, they will allow light to pass

– Demonstrate this using the two polarisers.• If their preferred directions are crossed at a right angle, they will block light and look

black, or dark blue– Demonstrate this using the two polarisers. What happens if they are at 45 degrees?– What happens when you look at a laptop, or LCD TV, through a polariser and turn the sheet?

Parallel 45 degrees Crossed

• In an LCD, it is used in a thin layer (~0.005 mm thick) between glass plates, when it appears transparent

• Surface coatings on the plates make its rod-like particles twist by 90o through the layer

• The LCD changes its optical properties when a small voltage is applied to clear conducting layers (made of Indium Tin Oxide, ITO), on the inside surface of each plate. This voltage is always AC, since DC voltage (e.g. from a battery) would damage the LCD.

• Applying the voltage realigns the tiny particles perpendicular to the plates

• The resulting optical change can only be seen using polarisers, as shown

Liquid Crystal Display (LCD) for TV or laptop

How is liquid crystal used in an LCD?

Liquid Crystal Display (LCD) for TV or laptopSo how does an LCD work?• An LCD between crossed polarisers “uncrosses” them and allows light to

pass– See Figure 1

• An LCD between parallel polarisers “crosses” them and light is blocked– See Figure 2

• This property is temporarily destroyed on applying a small voltage to its conductors

– An LCD placed between crossed polarisers on a light box allows light to pass.– If ~3Vac is applied to its conductors. The LCD becomes black. See Figure 3– When the 3Vac is removed, the LCD becomes clear after a short delay.– If the polarisers are parallel the LCD goes from black to clear. See Figure 4– Between 0V and 3Vac, a graph of the light intensity against voltage shows that greys

are observed above ~ 1V threshold. See Figure 5– If a coloured filter is placed beneath the LCD and the voltage varied, it goes from

coloured to black, or vice versa– If you prod the LCD in the ON state, you can see waves in the LC layer. – If Cellophane is placed between the LCD and one of the polarisers, it changes

between two colours, since the Cellophane rotates different colours by different amounts. Overlapping shards of Cellophane will give a changing coloured pattern.

– If you drive this LCD with an audio voltage (~5Vac) from an amplifier the colours change in direct response to the music/speech.

Liquid Crystal Display (LCD) for TV or laptop

So how does a simple LCD on a clock, or a watch, work?• In a simple LCD the electrodes are patterned so that different

areas can be switched ON and OFF independently

• Applying a small voltage to the electrodes for selected areas causes a numeral to be displayed

• A reflector is often included so that the LCD can use the light from its surroundings, daylight for example

• Such reflective LCDs only use a backlight in the dark and, in daylight, or room light, they take only a tiny amount of electrical power: about one millionth of a Watt

• This is one reason why they are used in clocks, watches and many other battery operated devices.

Liquid Crystal Display (LCD) for TV or laptop

So how does a laptop, or TV, LCD work?• In a laptop, or TV, display there may be a million, or

more, subpixels: too many to connect externally

• A thin layer containing millions of tiny electronic switches (called Thin Film Transistors, or TFTs) is used, in contact with liquid crystal in the LCD, to route appropriate drive voltages from the edge of the display to the corresponding subpixels, in order for them to form the image – click here to see illustration.

• An external electronic circuit controls the switching of the TFTs to apply the correct voltage to each subpixel.

• The polarisers are usually mounted at ±45o to improve the viewing angle of the LCD and let it be viewed wearing Polaroid sunglasses.

• Similar, but much smaller, LCDs are used in data projectors, mobile phones & camera viewfinders

• Since a liquid crystal is an insulator, LCDs consume little current, or energy, but the backlight takes quite a lot. This limits the operating time for a laptop, or other battery operated product.

SUMMING UPImportant Points:

• Liquid crystal materials are made of rod-like tiny particles (molecules) whose arrangement and directions define their optical properties

• Liquid crystals can exist over wide temperature ranges (e.g. -20oC to 80oC)• Liquid crystals are different from solids, liquids and gases• Low AC voltages can change a thin layer of liquid crystal, so that the

amount of light passed through the layer and polarisers is varied• Liquid crystal displays are flat panels that can be used to present

information, colour pictures and movies • Reflective LCDs take almost no power: most of the power in a laptop/TV

LCD is consumed by its backlight• In simple LCDs, each segment can have its own connection• In complex LCDs, millions of Thin Film Transistors (TFTs) are used as on-

board switches to control the voltage applied to each subpixel• Each subpixel has its own colour filter and almost every colour can be

displayed using different combinations of voltages on the subpixels• Laptops not possible without LCDs and TVs are no longer large boxes

RED GREEN BLUE

BLUE+GREEN RED+BLUE RED+GREEN

RED+GREEN +BLUE

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Construction of a Liquid Crystal Display (LCD) for TV or laptop

By kind permission of Merck KGaA, Liquid Crystal Division, D-64271 Darmstadt, Germany

www.licristal.comCLICK TO RETURN

LC Shutter

OFF

ON

LC Shutter between Polarisers

ParallelCrossed

Figure 2Figure 1

Figure 4Figure 3

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LC Shutter

OFF

ON

LC Shutter between Polarisers

ParallelCrossed

Figure 2Figure 1

Figure 4Figure 3

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LC Shutter

OFF

ON

LC Shutter between Polarisers

ParallelCrossed

Figure 2Figure 1

Figure 4Figure 3

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LC Shutter

OFF

ON

LC Shutter between Polarisers

ParallelCrossed

Figure 2Figure 1

Figure 4Figure 3

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LCD Voltage0 3V1V

Transmitted Light Intensity (Lux)

white

black

Graph of Transmitted Light Intensity against Voltage applied to an LCD

with Crossed Polarisers

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2V