Vision & Visual Display Units (VDU) Flashcards
Describe and list Visual Display Devices/Units?
- 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
Describe the monitor in VDU?
- 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
List the 3 Monitor Types?
Monochrome
Greyscale
Colour
Describe the monochrome monitor type?
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
Describe the greyscale monitor type?
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
Describe the colour monitor type?
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
How do monitors work?
- 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
Describe Cathode-ray tube (CRT) & CRT Monitors?
- 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
What is the scanning pattern of CRT electron gun?
- Electron gun scan from L to R and from top to bottom -> refreshing every phosphor dot in zig-zag pattern
Describe Basic CRT?
- 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
What is a phosphor?
- 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’
What are the advantages of CRT?
- 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
What are the disadvantages of CRT?
- 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
What is a Liquid Crystal Display (LCDs) Monitor?
- 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
Difference between CRT & LCD?
- 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
Name the 2 main categories of LCD?
Passive Matrix LCD
Active Matrix LCD
Describe Passive Matrix LCD?
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
Describe Active Matrix LCD?
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
What are the advantages of LCD?
- 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
What are the disadvantages of LCD?
- 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
List other types of monitors?
- 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
List the specifications which monitor is judged by?
Size
Resolution
Refresh rate
Dot pitch
Describe size in relation to monitor specification?
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
Describe resolution in relation to monitor specification?
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
Describe refresh rate in relation to monitor specification?
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
What is a Video card/graphic processing unit and describe the evolution of them?
- 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
How did video cards work?
- 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
Describe visual ergonomics and monitors?
- 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
Describe data projectors?
- 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
