Liquid Crystal Display
A liquid crystal display (LCD) is a thin, flat display device made up of any number of color or monochrome pixels arrayed in front of a light source or reflector. It is prized by engineers because it uses very small amounts of electric power. Each pixel consists of a column of liquid crystal molecules suspended between two transparent electrodes, and two polarizing filters, the axes of polarity of which are perpendicular to each other. Without the liquid crystals between them, light passing through one would be blocked by the other. The liquid crystal twists the polarization of light entering one filter to allow it to pass through the other. The molecules of the liquid crystal have electric charges on them. By applying small electrical charges to transparent electrodes over each pixel or subpixel, the molecules are twisted by electrostatic forces. This changes the twist of the light passing through the molecules, and allows varying degrees of light to pass (or not to pass) through the polarizing filters. Before applying an electrical charge, the liquid crystal molecules are in a relaxed state. When an electrical charge is applied to the electrodes, the molecules of the liquid crystal align themselves parallel to the electric field, thus limiting the rotation of entering light.
Top: When turned off, polarized light reflects off a mirror, passes through the liquid crystal layer, and leaves the display looking white.
Bottom: When turned on, polarized light is blocked by the rear polarizer. Without the reflection of light, affected areas of the liquid crystal layer look black.
Characteristics of Video Display Devices
• Persistence: Persistence is defined as the time it takes the emitted light from the screen to decay to one-tenth of its original intensity. It decides how long they continue to emit light after the electron beam is removed. Lower persistence phosphors require higher refreshing rates to maintain a picture on the screen without flicker. It is useful for displaying animations. On the other hand higher persistence phosphors are useful for displaying static and highly complex pictures.
• Resolution: Resolution indicates the maximum number of points that can be displayed without overlap on the CRT. It is defined as the number of points per centimeter that can be plotted horizontally and vertically.
• Aspect Ratio: It is the ratio of vertical points to horizontal points to produce equal length lines in both directions on the screen. An aspect ratio 4/5 means that a vertical line plotted with 4 points has the same length as a horizontal line plotted with 5 points.