DISPLAY SYSTEMS AND PHOTOSENSORS (PART 1) LCD TFT LED-OLED CCD CMOS LCD Display M.A.MOEENI SIMPLE TYPES OF LC PROPERTY OF LIQUID CRYSTALS (LC) -Three major characteristics of Liquid Crystal - The Thermal Nature Solid State (Crystal) Liquid Crystaline State Liquid State Low Temp High Temp Melting Point Clearing Point - The Optical Nature of a LC molecule NO change in polarization state Light Phase retardation will exist - The Electrical Nature of the LC molecules Electrodes No potential field AC potential THE HISTORY OF LIQUID CRYSTAL DISPLAY LCD Before the appearance of LCD, it is the Cathode Ray Tube (CRT) kingdom. The CRT monitors dominate almost all the display applications. The CRT, invented by Karl Ferdinand Braun, is also called Braun tube. Cathode rays exist in the form of streams of high speed electrons emitted from the heating of cathode inside a vacuum tube at its rear end. The released electrons form a beam within the tube due to the voltage difference applied across the two electrodes, and the direction of this beam is then altered either by a magnetic or electric field to trace over the inside surface of the phosphorescent screen (anode), covered by phosphorescent material. Light is emitted by that material at the instant that electrons hit it. - good image quality - no problem with response time - no problem with viewing angle But it is bulky and high power consumption, So Display engineers tried looking for alternative technologies like flat panel display. In 1990s, technology breakthrough brought the birth of active matrix LCD, along with the plasma display, both of which become the main stream of the flat display markets, replacing the CRT LCD DISPLAY (A) It has a mirror which makes it reflective (B) & (F) piece of glass with a polarizing film on the bottom side (C) & (E) a common electrode plane made of indium-tin oxide on top (D) layer of liquid crystal substance THREE COMMON TYPES OF LCD - TRANSMISSIVE TYPE LCD Light (Back Light) Eyes POLARIZER ON BOTH SIDES - REFLECTIVE TYPE LCD Incident Light POLARIZER ON THE FRONT SIDE - TRANSFLECTIVE TYPE Day Light POLARIZER ON THE FRONT SIDE REFLECTOR ON THE BACK SIDE LCD Night Light (Back Light) TRANSFLECTOR ON THE BACK SIDE An alternative way to achieve high-resolution LCD is to use the Liquid Crystal on Silicon (LCOS) devices. LCOS devices use only one glass substrate, and employ a silicon wafer for the back substrate. The pixels are then generally coated with a reflective aluminum layer, and then a polyimide alignment layer. This technology can also be used in personal viewer such as viewfinder in digital cameras and camcorders. LCD MODES Along with the development of LCD's driving infrastructure, different LCD modes were introduced to improve the image quality. -ASV -BINEM -Cholesteric -ECB -SSFLC -Guest-Host -IPS -LCOS -MVA -PDLC -Pi-Cell -PVA -STN -TN George Heilmeier in 1968 TWISTED NEMATIC (TN) MODE The TN mode is the "workhorse" for the LC display. It was first introduced by Schadt and Helfrich, and also by Fergason in 1971 TWISTED NEMATIC (TN) MODE The gray scale is achieved by applying intermediate voltages between 0 and the value at which light is completely blocked. A TYPICAL TN TYPE LCD CELL Polarizer (Axis 0 degree) Glass with electrodes NO power supply With AC Volts connected Polarizer (Axis 90 degrees) Cell Gap The separation between two glasses The smaller the cell gap, the faster response. Light Depending on how the LCD fluid is formulated. TWISTED NEMATIC (TN) MODE TWISTED NEMATIC (TN) MODE For a LCD, each pixel is divided into three subpixels, which have red, green and blue color filters. The exact color coordinates of the white point depend on the relative transmission and color purity of the red green and blue subpixels. ADVANCED SUPER VIEW (ASV) MODE The ASV mode was developed by Sharp. Because of the full circle rotation of the director, the viewing cone is very symmetric and viewing angle performance is excellent. POLYMER DISPERSED LIQUID CRYSTAL (PDLC) MODE -The PDLC display consists of droplets of liquid crystals inside a polymer network In the off state, the droplets are randomly aligned hence the light is scattered in a large angle towards the viewer. In the on state, light can be transmitted with a very high transmission. POLYMER DISPERSED LIQUID CRYSTAL (PDLC) MODE The working voltage and response time of the PDLC can be affected by: - the resistive and dielectric properties of LC inside the droplet and the polymer properties - size of the droplets - shape of the droplets - the viscosity of the of the droplets There are a few factors influencing the contrast ratio of the PDLC display: - the cell gap - the density of the droplets GUEST - HOST (GH) MODE In a Guest - Host system, the mixture is prepared by mixing LC and dichroic dyes. The dichroic dyes absorb the light whose E-field is along the long axis of the dye. When the LC molecules change their orientation, the dye will also change along with LC molecules, consequently, the absorption axis is changing, a light transmission can be modulated. There are three simple GH displays: - The Heilmeier type GH - The Double Layer type GH - The PDLC type GH.