Output Devices
Lecture-4
Standard Methods of Output
¾
The Monitor
Monitor
¾
Monitors are categorized by the technology they use:
¾
Cathode ray tube (CRT) monitors
¾
Flat-panel displays
¾
And by the way they display colors:
¾
Monochrome – One color on a black background
¾
Grayscale – Shades of gray on a white or off-white
background
¾
Color – From 16 to 16 million unique colors
CRT Monitors
¾ Near the back of monochrome or grayscale monitor housing is an electron gun, which shoots a beam of electrons through a magnetic coil, which aims at the front of monitor.
¾ The back of the monitor’s screen is coated with phosphorous, chemical that glow when they are struck by the electron beam.
¾ The screen’s phosphor coating is organized into a grid of dots. The smallest number of phosphor dots that gun can focus on is called a
pixel (for,picture element).
¾ Actually the electron gun doesn't just focus on a spot and shoot electrons at it. It systematically aims at every pixel on screen, starting at the top left corner and scanning to the right edge. Then it drops down a tiny distance and scans another line.
¾ A color monitor works like monochrome, except that there are 3 electron beams instead of one. 3 guns represent the primary additive colors (red, green and blue), although the beam they emit is colorless.
¾ In a color monitor, each pixel includes three phosphors – red, green and blue, arranged in a triangle.
¾ When the beams of each of these guns are combined and focused on a pixel, the phosphorus lights up.
¾ The monitor can display different colors by combining various intensities of three beams.
Flat Panel Monitors
¾ Most flat-panel monitors use liquid crystal display (LCD)
technology.
¾ LCD monitors create images with a special kind of liquid crystal that is normally transparent but becomes opaque when charged with electricity.
¾ One disadvantage of LCD monitors is that, unlike phosphor, liquid crystal doesn't emit light, so there is not enough contrast between the images and background to make them legible under all conditions.
¾ Another disadvantage of LCD is their limited viewing anglei.e. the angle from which the display’s image can be viewed clearly.
¾ Two main categories of LCD:
¾ Passive matrix LCD uses a transistor for each row and column of pixels, thus creating a grid that defines the location of each pixel. The color displayed is determined by the electricity coming from the transistors at the end of the row and top of column. Less expensive than active matrix but have a narrow viewing angle and don't refresh the pixel very often. If the pointer is moved quickly it seems to disappear an effect known as sub-marining. Animated graphics appear blurry
¾ Active matrix LCD uses a transistor for each pixel on the screen.
¾ Thin-film transistor displays use multiple transistors for each pixel.
