Display devices

Some examples on display devices . Hope it will be usefull .

1. DISPLAY DEVICES DONE BY JERIN . M 2 nd BCA 13BCA 4219 DISPLAY DEVICES DONE BY JERIN . M BCA

2. A display device is an Output device for presentation of information for visual reception . The display systems are often referred to as Video Monitor or Video Display Unit (VDU) . Display devices are designed to project , show , exhibit or display softcopy information .

3. Some of the display devices are Cathode Ray Tube (CRT) Raster Scan Display Random Scan Display Liquid Crystal Display (LCD) Light Emitting Diode (LED)

4. Cathode Ray Tube (CRT) Electron Guns Electron Beams Focusing Coils Deflection Coils Anode Connection Shadow Mask Phosphor layer Close-up of the phosphor coated inner side of the screen

5. Diagrammatic representation of CRT

6. A cathode ray tube or CRT is a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface . It contains one or more electron guns , and a fluorescent screen used to view images . It also contains magnetic deflection coils and focusing system .

7. Electron Gun

8. Heat is supplied to the cathode by the filament . The free electrons are then accelerated toward the phosphor coating by a high positive voltage . A positively charged metal coating on the inside of the CRT envelope near the phosphor screen . Intensity of the electron beam is controlled by setting voltage level on the control grid . A smaller negative voltage on the control grid simply decrease the number of electrons passing through .

9. Accelerating Anode The accelerating anode is maintained at sufficient high relative potential to accelerate the beam to necessary velocity .

10. Focusing System

11. The focusing system is needed to force the electron beam to converge into a small spot as it strikes the phosphor . The electron beam will be focused properly only at the center of the screen . As the beam moves to the outer edges of the screen , displayed images become blurred .

12. Deflection Systems The deflection system contains a set of coils mounted at the neck of the tube . There are two sets of deflecting plates . One pair of plates is mounted horizontally to control the vertical deflection , and the other pair is mounted vertically to control horizontal deflection .

13. Spots of light Persistence : The time it takes the emitted light from the screen to decay to one-tenth of its original intensity . Resolution : The maximum number of points that can be displayed without overlap on a CRT . Aspect Ratio : This numbers gives the ratio of vertical points to horizontal points necessary to produce equal length lines in both directions on the screen .

14. Refresh CRT : One way to keep the phosphor glowing is to redraw the picture repeatedly by quickly directing the electron beam back over the same points . Phosphors : Phosphors has a property to emit light when electron comes and hits it . This property is known as phosphorescence .

15. Raster Scan Display In a raster scan system , the electron beam is swept across the screen , one row at a time from top to bottom . As the electron beam moves across each row , the beam intensity is turned on and off to create a pattern of illuminated spots . Picture definition is stored in a memory area called the refresh buffer or frame buffer . Refresh buffer or frame buffer : This memory area holds the set of intensity values for all the screen points .

16. Stored intensity values then retrieved from refresh buffer and “painted ” on the screen one row (scan line) at a time .

17. On a black – and - white system with one bit per pixel , the frame buffer is called bitmap . For system with multiple bits per pixel , the frame buffer is called pixmap . Refreshing on raster scan displays is carried out at the rate 60 to 80 frame per second .

18. Horizontal retrace : The return to the left of the screen , after refreshing each scan line .

19. Vertical retrace : At the end of each frame ( displayed in 1/80 th to 1/60 th of a second ) the electron beam returns to the top left corner of the screen to begin the next frame .

20. Random Scan Display Random scan monitors draw a picture one line at a time (Vector display , Stroke –writing or calligraphic displays) . Refresh rate depends on the number of lines to be displayed . Picture definition is now stored as a line-drawing commands an area of memory referred to as refresh display file (display list) .

21. The component lines of a picture can be drawn and refreshed .

22. Random scan displays are designed for line-drawing applications and can not display realistic shaded scenes.

23. Differences between Random and Raster Scan Display RASTER SCAN DISPLAY RANDOM SCAN DISPLAY Picture with better contrast Cannot produce contrast , memory doesn’t store intensity value of pixel Less resolution High resolution Capable of producing curves better Smooth line drawings Used in systems to display realistic images Cannot draw realistic shaded scenes Cost is less Cost is more Mainly used for point plotting Line drawing , known as vector display Ex . TV sets Ex . Pen Plotter

24. Liquid Crystal Displays (LCD) Used in small systems , such as calculators , laptop computers . Produce a picture by passing polarized light (from the surrounding or from an internal light source) through a liquid-crystal material that can be aligned to either block or transmit the light .

25. Two glass plates , each containing a light polarizer at right angles to the other plate , sandwich the liquid crystal materials . Rows of horizontal transparent conductor & columns of vertical conductors (put into glass plates) .

26. Polarized light passing through the material is twisted so that it will pass through the opposite polarizer . The light is then reflected back to the viewer . Liquid crystal : These compounds have a crystalline arrangement of molecules , yet they flow like a liquid .

27. On State Off State

28. Light Emitting Diode (LED) An LED display is a video display which uses light-emitting diodes . An LED panel is a small display , or a component of a larger display . They are typically used outdoors in store signs and billboards . LED panels are sometimes used as form of lighting , for the purpose of general illumination , task lighting , or even stage lighting rather than display .

29. All early devices emitted low-intensity red light , but modern LEDs are available across the visible , ultraviolet and infrared wavelengths , with very high brightness . LEDs are based on the semiconductor diode . When the diode is forward biased (switched on) , electrons are able to recombine with holes and energy is released in the form of light . This effect is called electroluminescence and the color of the light is determined by the energy gap of the semiconductor .

30. LEDs present many advantages over traditional light sources including lower energy consumption , longer lifetime , improved robustness , smaller size and faster switching . However , they are relatively expensive and require more precise current and heat management than traditional light sources . Applications of LEDs are diverse . They are used as low-energy indicators but also for replacements for traditional light sources in general lighting and automotive lighting . The compact size of LEDs has allowed new text and video displays and sensors to be developed , while their high switching rates are useful in communications technology .

31. LED Display

32. Thank You.....