Monitor technology

Published on 12/06/2015 by admin

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Last modified 22/04/2025

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5 Monitor technology

Terminology

Aspect Ratio The ratio of the width of a display screen to the height, e.g. 4 : 3
Bezel The plastic or metal frame round a display screen
Brightness (Luminance) The amount of light a LCD monitor produces in candela (cd) per square metre (m2), e.g. 250 to 350 cd/m2
Colour Depth The number of bits used to give the colour of one pixel, and gives the number of different colours that can be displayed at one time, e.g.

• 1 bit depth gives 2 colours (black and white)
• 16 bit depth gives 65 563 colours that can be displayed
• 24 bit depth gives 16.8 million colours that can be displayed
Contrast Ratio The difference in intensity between the black and white on an LCD screen

• The higher the contrast ratio the better the detail
Cursor A flashing marker on the screen which indicates where the next character is to be inserted
Dot Pitch A measure of the sharpness or resolution of a screen

• DPM – dots per millimetre
• The higher the figure the better the resolution of the screen as the dots are closer together
LCD Liquid Crystal Display monitor
Monitor A device very similar to a television, but which receives video signals directly from the computer
Native Resolution The optimum resolution of a LCD monitor. If this is changed the image quality diminishes
Examples of native resolution:

• 17 inch screen = 1024 × 768
• 20 inch screen = 1600 × 1200
Pixel Picture cell. A pixel is the smallest number of dots which can be used by a character on the display screen
Refresh Rate Number of times the monitor is scanned by the electron beam per second

• The higher the refresh rate the less screen flicker
• A refresh rate of 85 Hertz means the screen is scanned 85 times each second
Resolution The number of ‘dots’ on a monitor screen defines the resolution of a system by describing the number of pixels horizontally and vertically, e.g. 1200 × 1200 gives a resolution of 1200 separate points horizontally and vertically
RGB Input The colour input on a monitor. The signal from the computer is taken by the monitor as a basic Red, Green and Blue input
Screen Size Cathode ray tubes

• The diagonal measurement from the outer corners of the casing

Liquid Crystal Display screens

• The diagonal measurement from the corners of the screen
Scrolling The movement of text or data on the display screen. Scrolling can be upwards, downwards or sideways
Thermionic Emission The release of electrons when a substance is heated

VDU Visual display unit
Viewing Angle The maximum horizontal and vertical angle that a monitor screen can be viewed at to give a clear image with accurate colours, e.g.

• 120°
• 170°
Voxel A three-dimensional pixel
Interlacing Consider one still picture on a cathode ray monitor (Figs 5.1, 5.2)

• The picture is made up of two fields which consist, in a 625 line system, of 312.5 lines each
• Each horizontal line is made up of minute dots (pixels)
• The picture is constructed by an electron beam scanning the tube phosphor from top left to bottom right in a series of horizontal lines
• First the odd lines are scanned, the beam jumps back to the top left and then the even lines are scanned
• This system of constructing the frame is called ‘double interlacing’
• ×4 and ×8 interlacing will produce images which, are virtually line free
Principle of Operation
• Electricity can either be on or off
• The signal is received by the visual display unit in the form of a stream of electrons (on) and gaps (off)
• The stream of electrons and gaps scan across the screen from side to side, slowly moving down (see interlacing)
• When an electron hits a pixel on the screen the pixel lights up
• When a ‘gap’ is present the pixel does not light
• Therefore a black and white image is formed on the screen
Cathode Ray Tubes Composed of:

• A negative cathode

image When heated produces a stream of electrons by thermionic emission

• A grid

image To focus the electron beam

• A positive anode

image Attracts the electron beam from the cathode

• Deflecting coils

image Focus the electron beam
image Move the beam across the fluorescent screen from side to side

• A fluorescent screen

image Coated with phosphors which have a characteristic light emission
image The phosphors are given numbers, e.g.

– P45, P4 and P40 emit white light (a mixture of blue and green)
– P11 emits blue light
– P31 and P24 emit green light
In colour monitors:

• Red, green and blue emitting phosphors are used
• Phosphors are arranged in groups of either dots or strips
• A shadow mask

image A perforated metal plate next to the fluorescent screen
image Allows accurate focusing of the electron beam
image The perforations match the phosphor dots

• An aperture grill

image Strips of wire
image Allows accurate focusing of the electron beam
image The gaps between the wires match the phosphor strips

• A slot mask

image Combines a shadow mask and an aperture grill
image Uses vertical slots
image Creates a brighter image
image

Fig. 5.1 Odd, then even lines are scanned.

image

Fig. 5.2 Multiple interlacing. (A) ×2 Interlacing. (B) Raster lines almost disappear.

Flat Screen Monitors

Gas Plasma Monitors Construction

• Front glass plate

image Protects and supports the screen

• Display electrode

image Arranged in horizontal rows

• Dielectric layer

image Provides insulation

• Magnesium oxide layer

image Protective layer

• Pixel layer

image Each pixel contains neon gas and is placed between two electrodes
image Each pixel made of three sub pixels – green, red and blue
image By varying the current pulse different combinations can be illuminated

• Phosphor

image Coated on the inside of each pixel cell

• Address electrode

image Arranged in vertical columns
image Form a grid pattern with the display electrodes

• Address protective layer

image Protects electrodes

• Rear glass plate

image Supports and protects the screen
Principles of operation

• The computer charges the electrodes at the point where they cross
• An electric current is formed in the pixel gas at that point
• The charged particles in the gas cause the release of ultraviolet photons
• The photons react with the phosphor coating causing an electron to be raised to a higher energy level
• When the electron returns to its normal energy band it releases a visible light photon
• The image can then be viewed on the monitor
• By varying the current from the computer the brightness and colour combination of the image can be varied
Liquid Crystal Displays (Active) Construction

• Backlight

image Illuminates the substrate

• Substrate

image Polarised glass

• Liquid crystal material

image Forms a thin layer trapped between the substrate

• Thin film transistor (TFT)

image A matrix of transistors and capacitors

• Substrate

image Polarised glass
Principles of operation

• Works by blocking light
• The rear substrate is back lit
• The computer sends a signal to a specific pixel

image In the TFT the specific row is switched on and a charge is sent down the specific column
image The capacitor aligned to the pixel receives a charge
image The charge causes the liquid crystal molecules to block some of the light
image This gives the colour and image on the second substrate

• The capacitor remains charged until the next refresh cycle
Liquid Crystal Displays (Passive)
• Work as active LCD
• Grid of conductive metal replaces TFT
• Rarely used due to:

image Slow response times
image Poor voltage control

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