Week 3- Visual display devices Flashcards
Visual display devices facts:-
• Hardware used for displaying visual media using graphics, texts, images
• Consists of components such as monitor, video adapter card, video cable
• Devices include Cathode ray tubes (CRT): Colour CRT, Direct View storage Tube (DVST): resembles CRT
What is a monitor?
•Monitor or VDU (Display), electronic display for computers
- Example: liquid crystal display
• Quality depends on: monitor/video controller
Monitor types:- (3)
• Monochrome
• Greyscale
• Colour
Monochrome monitor:-
• “Green screen”
• Oldest type of CRT display: 1960’s-1980’s
•Still used in visual psychophysics laboratories
Greyscale monitors:-
• “Black and white” monitors
• Special type of monochrome display able to show different shades of grey
• sixteen different shades
Colour monitors:-
• Displays many different colours
• Work like monochrome - except have three electron beams instead of one
• Three guns representing red, green and blue
• Each pixel includes three phosphors: red, green and blue; arranged in triangle
• Beams can combine different combinations lighting up phosphors
How do CRT’s work?
• Using electromagnetic coils in vacuum tubes
• Due to negative charge on particles, the electrons can be manipulated
How do monitors work?
• Cathode ray-tube as display device
• CRT: Glass tube that is narrow at one end, and opens to flat screen at other end
- Electrons travel through vacuum sealed container from cathode (negative) to anode (positive)
• Ray can be manipulated by magnet due to negative charge using COILS
How do cathode ray tube(CRT) monitors work?
• CRT monitors contain millions of tiny red, green and blue phosphor dots that glow when struck by electron beam
• CRT tube: the cathode is a heated filament
• Heated filament is in vacuum inside glass tube
• Electron beams hitting anode screen produce image by making screen glow
Explain Basic Cathode ray tube:-
• Electrons excite phosphor to glow (anode)
• Electrons fired from back (cathode)
• Phosphor is arranged in dots called Pixels
• Dot mask ensures proper pixel is lit
What is phosphor:-
• Semi-conductor which emits visible radiation in response to impact of electrons
• Due to sudden change in electron beam, light emission does not fall instantaneously; there is gradual reduction called “Fluorescence”
What is the scanning pattern of CRT electron gun?
• Electron gun scams beam left to right AND top to bottom
• This refreshes every phosphor in zig zag pattern
Advantages of CRTs:- (Old facts - not true anymore)
• Easy to increase brightness using CRT
• Produced more colours than newer LCD or Plasma screens
• Image quality also of cathode ray also higher than LCD or Plasma Screens
• Contrast feature of cathode ray considered excellent
Disadvantages of CRT (6)
• Large back + takes up space
• Heavy to pick up + move around
• Electromagnetic fields constitute health hazard to living cells
• Also emit x-rays which may institute health hazard
• Refresh rate of CRT produces headaches
• CRT’s operators at high voltage: overheating + exploding
- This is exaggerated due to vacuum seal
Liquid Crystal Display Monitors are:-
• Flat panel displays, electronic or video displays that use light modulating properties of liquid crystals (LCs)
• LC’s don’t emit light directly
• Were offered as replacement to CRT
CRT compared to LCD:-
• CRT
- Bulky, heavy, use vacuum tube technology.
- Using technology that was developed in the 19th century.
• LCD
- First LCD laptop monitors were very small due to manufacturing costs but now are available in a variety of sizes.
- Light, sleek, energy-efficient, have sharp picture.
Two categories of LCD’s:-
• Passive matrix
• Active matrix
How do Passive Matrix LCD work?:-
•Monochrome passive-matrix LCD’s were standard in most early laptops
- Consists of grid of horizontal and vertical wires
- At intersection each grid is LCD element constituting a single pixel, either by letting light through or blocking it
•Passive matrix LCD
- Pixels arranged in grid
- Pixels activated indirectly : rows/columns activated
- Animation can be blurry
Active Matrix LCD’s:-
• Depend on Thin-film transistors
- TFT’s are tiny switching transistors and capacitors
• Arranged in matrix on a glass substrate
• Each pixel is activated directly
- Pixel have 4 transistors, one for red, green, blue
- One for opaqueness
• Animation is crisp and clean
Advantages of LCD:-(6)
• LCD display may be tweaked maximally to give a sharp image.
• High peak intensity produces very bright images.
• Screens are perfectly flat.
• Thin, with a small footprint. little electricity and produce little heat
• Lightweight
• Lack of flicker and low glare reduce eyestrain.
Disadvantages of LCD:- (5)
• After a while some pixels on an LCD display will die
- This is seen as discoloured spots or a black spot on the display.
• LCDs are expensive.
• The display response times are slow.
• An LCD display has a fixed resolution display and cannot be changed.
• The viewing angle of a LCD display is very limited.
Other types of display monitors:-
• Paper-white displays
- High contrast between fore and background
• Electro-luminescent displays (ELD)
- Similar to LCD
- Uses phosphor to produce light
• Plasma monitor
- Gas is excited to produce light
Monitor specifications include:-
Monitor Specifications can be judged by:
- Size
- Resolution
- Refresh rate
- Dot pitch
Monitor size facts:-
•Determines how well images can be seen
• Larger monitor = Larger images
• Monitors measured diagonally
- A 17-inch monitor measures 17 inches from the lower left to the upper right corner.
• In contrast, a CRT monitor’s viewing area was smaller than the monitor’s overall size.
