A (Cathode Ray Tube) display unit is used output device. It displays the data or information received from the computer. It can display alphanumeric characters and graphs. Though the CRT is basically an output device it can perform limited input function when used in conjunction with a light pen. The CRT screen is similar to a TV screen. A CRT is a vaccum tube. An electronic beam is produced by the electron gun located at the back portion of the tube. The electron beam is directed towards the front of the CRT (i.e. CRT screen). A coating of phosphor material is made on the inner surface of the screen. The phosphor emits light when it is struck by electron beam. The color of the emitted light depends on the phosphor substance used. A CRT display may be either monochrome (i.e. only one color) or color (i.e. multicolor). Monochrome displays are available in green, blue, orange, yellow, pink, red and white depending on the type of the phosphor material used. Color displays are produced on CRT screen on the same principle as they are produced on color TV screen. To produce color displays three phosphors: red, blue and green are used. The coating of these phosphor materials is made in such a way that dots of these three phosphors in a triangular spot are so close that they appear as a single dot. Three separate electron beams are employed to illuminate the dots of three different phosphors. By varying the intensity of the three electron beams the intensity of red, blue and green dots is varied. This gives the appearance of a triangular spot of the desired color. Black and white colors can also be obtained if all the three dots are of same intensity.
The display mode may be either of graphics mode. The graphical elements are composed of picture elements called as pixel. The term resolution is used to describe the number of pixels per unit area of the screen. If there are few pixels per unit area, we say low resolution display and if many pixels per unit area we say high resolution display. The higher the resolution, the greater the detail can be displayed. The console mode display consists of 24 lines; each line consists of maximum 80 characters per line.
LCD (Liquid Crystal Display)
In LCDs a liquid crystalline material is sandwiched between two glass or plastic plates. The front plate is transparent and the back plate is reflective. There is a coating of thin film on the front plate. The coating is transparent and conductive. Its sections are in the shape of desired characters. An electrically conductive film or backplane is put on the back plate. A voltage is applied between a segment and the back plate to create electric field in the region under the segment. Commonly available LCDs are of two types:
-dynamic scattering
-field effect
In dynamic scattering type, the molecules are aligned in one direction under the segment where field is present. The crystalline molecules reflect more light in this condition and this produces etched glass looking light characters on a dark background. In field effect type molecules are polarized to absorb light where electric field is present. This produces dark characters on a silver gray background. LCDs do not emit their own light. Therefore, a light source is to be used. LCDs simply change the reflection of available light. Today, most LCDs used are of the types that produce dark images on a silver background. LCDs are lightweight and consume little power which makes them attractive for portable devices like laptop, PDA and mobile phone. The viewing angle of LCD screens is less.
No comments:
Post a Comment