Vision & Visual Display Units (VDU)

Question 1

Describe and list Visual Display Devices/Units?

Answer 1

• Primary user hardware for displaying visual media e.g. graphics, text, images
• Hardware: monitor, video adapter card, video adapter cable (most got rid of now)
• Examples of VDU:
  o Cathode Ray Tubes (CRT) – earliest VDUs
  o Colour CRT
  o Direct View Storage Tube (DVST: resembles CRT as it uses an electron gun to draw a picture and has phosphor coated screen to display it)
  o Flat Panel Displays (Liquid Crystal Display & Plasma)
  o Light Emitting Diode Displays (LED)
  Monitor = display = screen = visual display unit

Question 2

Describe the monitor in VDU?

Answer 2

• Electronic visual display for computers
• Originally, computer monitors were used for data processing & television receiver were used for entertainment
• Quality of picture depends on:
  o Monitor quality
  o Video controller
• From 1980s, computers (& their monitors) have been used for data processing & entertainment
• Mobiles use VDU technology -> cheaper phone – worse graphics – image quality ↓
• Varied techniques have been used to display images on computer monitors
• Most monitors used to be Cathode Ray Tubes (CRT) – these were phases out for Liquid Crystal Display (LCD) monitors – which are being phased out for more LED type VDUs now
• Categorised by colour output
• Monitor connects to video cars of a computer system & produces the image

Question 3

List the 3 Monitor Types?

Answer 3

Monochrome
Greyscale
Colour

Question 4

Describe the monochrome monitor type?

Answer 4

o Type of CRT computer display commonly used in early days of computing – from 60s-80s before colour monitors became popular
o Still used in visual psychophysics laboratories
o A Green Screen was common name
o Used until early 90s chiefly for experiments

Question 5

Describe the greyscale monitor type?

Answer 5

o Type of monochrome monitor capable o displaying different shades of grey
o Known as black-and-white
o Composed exclusively of shades of grey – vary from black at weakest intensity to white at strongest
o Early greyscale monitors can only show up to 16 different shades of grey

Question 6

Describe the colour monitor type?

Answer 6

o Display monitor capable of displaying many colours
o Colour Monitors work like monochrome ones, except that there are 3 electron beams instead of one
o 3 guns represent additive colours – red, green and blue
o Each pixel includes 3 phosphors – red, green and blue – arranged in a triangle
o When beams of each gun are combined & focused on a pixel, phosphors light up
 CRT type monitors
o Monitors display different colours by combining various intensities of 3 beams
o Mixing of colours

Question 7

How do monitors work?

Answer 7

• Most use a cathode-ray tube as a display device
• CRT: glass tube is narrow at one end and opens to a flat screen at other end
• Electrons travel through vacuum sealed container from cathode (-ve end) to the anode (+ve end)
• Because electrons are -vely charged, they are repelled away from the cathode, & move across the tube to the anode
• Ray can be affected by a magnet because of its relation to +ve & -ve charges

Question 8

Describe Cathode-ray tube (CRT) & CRT Monitors?

Answer 8

• Anode -> +vely charged, ray travels towards this
• Cathode -> -vely charged, ray travels away form this
• Contains millions of tiny red, green & blue phosphor dots that glow when struck by an electron beam
• Electron beams travel across screen to create a visible image
• In CRT monitor tube, cathode is a heated filament
  o Heated filament is in a vacuum created inside a glass tube
  o Electrons are negative & screen gives off a +ve charge making screen glow

Question 9

What is the scanning pattern of CRT electron gun?

Answer 9

• Electron gun scan from L to R and from top to bottom -> refreshing every phosphor dot in zig-zag pattern

Question 10

Describe Basic CRT?

Answer 10

• Electrons excite phosphor to glow
• Electrons fired from the back
• Phosphor is arranged in dots called pixels
• Dot mask ensures proper/appropriate pixel is lit

Question 11

What is a phosphor?

Answer 11

• Semi-conductor which emits visible radiation in response to impact of electrons
  o When it absorbs energy from a source e.g. electron beam, it releases a portion of this energy in form of light
• In response to a sudden change in electron beam (from on to off), light emission does not fall instantaneously, there is a gradual reduction called ‘fluorescence’

Question 12

What are the advantages of CRT?

Answer 12

• Easy to ↑monitor’s brightness using a CRT  by reflecting light
• Produced more colours than the newer LCD or plasma screens
• Therefore, historically, quality of image displayed on CRT used to be superior to that produced by an LCD or plasma monitor
• Contrast features of cathode ray tube monitor considered excellent
• This is historic info – not true anymore

Question 13

What are the disadvantages of CRT?

Answer 13

• Had a big back & took up too much space on desk
• Electromagnetic fields emitted by CRT monitors constituted a health hazard to functioning of living cells
• CRTs emit a small amount of X-ray radiation which may have resulted in health hazard
• Constant refreshing of CRT monitors may have produced headaches in users
• CRTs operated at v high voltage which often overheated & resulted in an implosion
• Strong vacuum that existed within a CRT also had similar effects to overheating – chiefly causing an implosion
• They were heavy to pick up & carry around

Question 14

What is a Liquid Crystal Display (LCDs) Monitor?

Answer 14

• Flat panel display, electronic visual display or video display that uses light modulating properties of liquid crystals (LCs)
• LCs do not emit lightly directly
• Not used as much anymore
• Use a combination of fluorescent-based backlight, colour filters, transistors & liquid crystal to create & illuminate images

Question 15

Difference between CRT & LCD?

Answer 15

• CRT:
  o Bulky, heavy, use vacuum tube technology
  o Using technology developed in 19th century
• LCD:
  o 1st LCD laptop monitors were v small due to manufacturing costs – now available in variety of sizes
  o Light, sleek, energy-efficient, have sharp picture

Question 16

Name the 2 main categories of LCD?

Answer 16

Passive Matrix LCD
Active Matrix LCD

Question 17

Describe Passive Matrix LCD?

Answer 17

o Monochrome passive-matrix LCDs were standard in most early laptops
 Still used for applications less demanding than laptops & TVs  used for alarm clocks
o Consists of grid of horizontal & vertical wires
 At intersection of each grid is LCD element which constitutes a single pixel, either letting light through or blocking it
o Pixels arranged in a grid
o Pixels are activated indirectly
 Row & column activated
o Animation can be blurry

Question 18

Describe Active Matrix LCD?

Answer 18

o Depend on thin film transistors (TFT)
 TFTs are tiny switching transistors & capacitors
o They are arranged in a matrix on a glass substrate
 Each pixel is activated directly
 Pixels have 4 transistors
* One each for red, green, blue
* One for opaqueness
 Animation is crisp & clean

Question 19

What are the advantages of LCD?

Answer 19

• LCD display may be tweaked maximally to give a sharp image
• High peak intensity produces v bright images – best for brightly lit environments
• Screens are perfectly flat
• Thin, w/ small footprint – consume little electricity & produce little heat
• Lack of flicker & low glare reduces eyestrain

Question 20

What are the disadvantages of LCD?

Answer 20

• After a while some of pixels on LCD display will die
• Seen as discoloured spots/black spot/opaque spots on the display
• LCDs are expensive -> cheaper now
• Display response times are slow
• LCD display has a fixed resolution display & cannot be changed
• Viewing angle of LCD display is very limited

Question 21

List other types of monitors?

Answer 21

• Paper-white displays  kindle
  o High contrast between fore & background
• Electro-luminescent displays (ELD)
  o Similar to LCD
  o Uses phosphor to produce light (not liquid crystals)
• Plasma monitor
  o Gas is excited to produce light
  o Heavy

Question 22

List the specifications which monitor is judged by?

Answer 22

Size
Resolution
Refresh rate
Dot pitch

Question 23

Describe size in relation to monitor specification?

Answer 23

 Affects how well can see images
 Larger monitor – objects on screen appear bigger
 Monitors measured diagonally in inches across front of screen from lower left to upper right corner
 CRT monitor’s viewing area was smaller than monitor’s overall size

Question 24

Describe resolution in relation to monitor specification?

Answer 24

 Images seen on monitor made of tiny dots called pixels
 Resolution refers to sharpness & clarity of image
 Monitor resolution determined by number of pixels on screen  expressed as a Matrix
 More pixels a monitor displays, the higher resolution & images will be clearer
* E.g. 640x480 – 640 pixels horizontally & 480 vertically
 Actual resolution determined by video controller
 Most monitors can operate at several different resolutions:
* 640x480
* 800x600
* 1024x768
* 1152x864
* 1280x1024
* 5120x2880
 As resolution ↑, image on screen gets smaller

Question 25

Describe refresh rate in relation to monitor specification?

Answer 25

 Number of times per second a display refreshes its image
 Since movement is displayed by difference between frames, refresh rate places hard cap on frame rate that’s visible
 Not same as frame rate
 Attribute of the monitor, while frame rate is an attribute of monitor – frame rate is attribute of info being sent to it
 If can run computer game at 100 frames per second (FPS), benefit from playing on monitor that can refresh that many times per second
 If watching movie at classic 24 FPS – higher refresh rate monitor won’t make difference
 In CRT, monitor refresh rate was number of times per second electron guns scanned every pixel on screen
* Was important as phosphor dots fade quickly after election gun charges them with electrons
* If screen not refreshed, will appear to flicker
 Measured in Hz or cycles per second -> monitor refresh rate 100Hz means refreshes its pixels 100 times every second

Question 26

What is a Video card/graphic processing unit and describe the evolution of them?

Answer 26

• Interface between computer and a display device
• Central processing unit (CPU), working in conjunction w/ software applications, sends info about image to video card
• GPU &CPU decide how to use pixels on screen to create image
• Then sends that info to monitor through output interface
• IBM introduced 1st video card in 1981 named Monochrome Display Adapter (MDA)
• MDA provided text-only displays of green or white text on black screen
• Similar to CPU but designed specifically to perform complex mathematical & geometric calculations necessary for graphics rendering
• Less congestion on system bus
• Reduction in workload of CPU
• Operations: bitmap transfers, painting, window resizing & repositioning, line drawing, font scaling & polygon drawing etc
• Some GPUs have image enhancement algorithms built-in  all nowadays
• Some of latest GPUs have more transistors than average CPU & produce lot of heat
  o Heatsinking & fan cooling are required
   Don’t have heat-sink fans anymore

Question 27

How did video cards work?

Answer 27

• At most common resolution settings, a screen displays over million pixels, & computer decide what to do with each one to create image
  o Takes binary data from CPU & turns it into picture can see
• Unless computer had graphics capability built into motherboard, that translation took place on the graphics card
• All computers now have GPUs built into them
• CPU working in conjunction with software applications, sends information about image to graphics card
• Graphics card decides how to use the pixels on the screen to create the image
• Then sends that info to monitor through a cable
• It is capable of rendering 3D images – has to be an interaction between CPU & GPU to give 3D images

Question 28

Describe visual ergonomics and monitors?

Answer 28

• Eyestrain:
  o Fatigue of eyes
  o Steps to avoid:
   Choose good monitor
   Place monitor 2-3 feet away (~60-90cm)
   Centre of screen below eye level
   Avoid reflected light/glare
• Electronic Magnetic Fields (EMF):
  o Generated by all electronic devices
  o EMF may be detrimental to health
  o Steps to avoid:
   Keep computer at arm’s length
   Take frequent breaks – “20-20-20 rule”
   Use an LCD monitor

Spectacles can be used - varifocal for young people
Coopervision Energys CL has low power reading addition which they claim ‘soothes’ tired eyes – when looking at digital devices

Question 29

Describe data projectors?

Answer 29

• Video projector – image projector that receives video signal & projects corresponding image/film on a projection screen using a lens system
• Overhead & slide projectors
  o Project image onto wall or screen
  o Use sheets of acetates
• LCD projectors:
  o Most common type of projector
  o Small LCD screen
  o V bright light
  o Require darkened room