DISCOVERY OF LIQUID CRYSTALS (LC) - Between 1850 and 1888, people from different fields like chemistry, biology observed strange behavior in some materials when the temperature is approaching the melting point. W. Heintz, a German biochemist, reported in 1850 that stearin melted from a solid to a cloudy liquid at 52 C, changed at 58 C to an opaque and at 62.5 C to a clear liquid In 1888, Friedrich Reinitzer, an Austrian botanist observed that there was two melting points for a certain material while he was making esters of cholesterol for studying cholesterol in plant. He also observed birefringence and iridescent colors between these two melting points. Lehmann and Reinitzer may thus with some justification be called the grandfathers of liquid crystal science
DISCOVERY OF LIQUID CRYSTALS (LC) -In 1922, Georges Friedel suggested a classification scheme to name different phases of liquid crystals called nematic, smectic and cholesteric, which are still used today - Carl Oseen in Sweden worked on elastic properties of liquid crystals and his results were used on the continuum theory by England's F.C. Frank. This theory became one of the fundamental theories in liquid crystals today. Alfred Saupe V. Freedericksz
DISCOVERY OF LIQUID CRYSTALS (LC) From 1945 to 1958, everything seemed slow down in the liquid crystal field. People thought they knew everything about liquid crystals and that nothing new could be expected in this area. No one could yet imagine how large a role liquid crystals would play in technical applications today In 1958 Glenn Brown, an American chemist, published an article in Chemical Reviews on the liquid crystal phase and subsequently sparked an international resurgence in liquid crystal research. Pierre-Gilles de Gennes, the laureate of Nobel Prize in Physics 1991, becomes the first and so far the only person receiving this prize in the field of liquid crystals.
- Three major characteristics of Liquid Crystal - The Thermal Nature High Temp Clearing PointMelting Point Liquid State Liquid Crystaline State Solid State (Crystal) - The Optical Nature of a LC molecule Light NO change in polarization state Phase retardation will exist Low Temp - The Electrical Nature of the LC molecules AC potential No potential field Electrodes PROPERTY OF LIQUID CRYSTALS (LC)
OPTICAL PROPERTIES OF LC LC is a birefringent medium. The electric and magnetic susceptibilities (the susceptibility of a material or substance describes its response to an applied field) are different along the director and perpendicular to the director. if the incoming light's polarization is at an angle other than 0 or 90 to the director, then a phase retardation will exist at the exit, and the light becomes elliptically polarized. These optical properties are used in the LC displays. Chiral nematic and smectic C phases are optically active, i.e. a linearly polarized light is rotated as it passes through such a medium.
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 Light (Back Light) - TRANSMISSIVE TYPE - REFLECTIVE TYPE POLARIZER ON THE FRONT SIDEREFLECTOR ON THE BACK SIDE Incident Light - TRANSFLECTIVE TYPE Day Light Night Light (Back Light) TRANSFLECTOR ON THE BACK SIDE POLARIZER ON BOTH SIDES POLARIZER ON THE FRONT SIDE LCD Eyes
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
TODAY’S LCD Duty Ratio Panel Size 1/1 Static 1/2 1/3 1/4 1/8 1/16 1/32 1/64 1/256 10 mm21001,000 10,000 100,000 Time pieces Hand Held Games Film Camera Calculator Data Bank Mobile Phone Digital Instruments PDA Digital Camera Office Equipment Notebook PanelsProjector Portable TV Active LCD Passive LCD STN TN 1M+ Pixels 100K Pixels 10K Pixels 1K Pixels 100 Pixels LCD TV & Monitors 1/128
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 With AC Volts connected NO power supply Polarizer (Axis 90 degrees) Light Cell Gap The separation between two glasses Depending on how the LCD fluid is formulated. Glass with electrodes Polarizer (Axis 0 degree) The smaller the cell gap, the faster response.
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.
HTN (HIGHLY TWISTED NEMATIC) & FSTN (FILM STN) TN 90 deg Twisted STN 180 deg or higher deg Twisted HTN 110 deg Twisted Narrow View Angle Wide View Angle BUT with Darker Color Background & Blue or dark blue patterns. Wider View Angle than TN but narrower than STN FSTN 240 deg or higher deg Twisted View angle same as 240 deg STN BUT in Grey Background Color & Black patterns. LCD Cell DSTN (Double STN Cells) 1st Cell with patterns Same as usual STN 2nd Cell without pattern But in reverse twisting Old way when NO Retardation film Polarizer
ELECTRICALLY CONTROLLED BIREFRINGENCE (ECB) MODE The ECB mode uses the applied voltage to change the tilt of the liquid crystal molecules, as a result, the birefringence is changed as a function of the tilt angle. In the TN mode, the transmission maxima will be reached when the condition is met: If we use applied voltage to change the tilt angle, we are able to shift the wavelength of the maxima.In general, ECB can be used to generate color to replace the color filter, by applying different voltages on each pixel corresponding the desired color the color obtained from ECB are not fully saturated because the spectrum curve are not sharp (narrow) enough, with some overlapping with neighbor colors. Therefore, it is not used often in practical applications for color generations.
POSITIVE MODE (PATTERN ON A CLEAR BACKGROUND) EitherTNHTNSTNFSTN Deg Twisted90110180-240240 Background Color Grey Yellow Green or Grey Grey Pattern ColorBlack Dark Blue or Blue Black Temp Range-40C to +85C-20C to +40C-20C to +70C View Angle60 deg80 deg120 deg110 deg Voltage2.5v min 5v typical 3v min 5v typical 3v min 5v typical 5v typical, (higher duty, higher volts) Negative Mode (Clear Pattern on a Color Background) TNHTNSTNFSTN Degree Twisted90 deg110 deg180-240deg240 deg Background Color Black (Seldom used)Black Pattern ColorClear (Seldom used)Clear
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.
1.Proceedings of the Liquid Crystal Seminar HK by E. Merck, Darmstadt, Germany. 2.Various articles in the SID International Symposium and Information Display by the Society for Information Display, Inc. USA 3.LCD Displays, the leading edge in flat panel displays, by Sharp Technical Library, Vol. 1, of Sharp Corporation, Osaka, Japan. The information in above are partly referring to the following documents