Liquid Crystal Displays (LCDs) consist of two pieces of glass with electrodes printed on the inside, within this glass sandwich is the liquid crystal (LC) organic fluid. When the LCD is in its “off” state i.e. no voltage applied, light passes through the LCD, when in its “on” state i.e. voltage is applied, the LC fluid twists in the direction of the electric field, this causes the light to be blocked creating dark areas on the LCD. By selectively applying voltage to the electrodes, a variety of patterns can be achieved. LCDs are passive displays, which means that they do not emit light; instead they use ambient light in the environment or backlight illumination from LED, EL or CCFL sources.
An LCD is basically a reflective part, it needs a light source to read it, this can be done by using the ambient light that is available or a backlight source i.e. a light source placed behind the LCD. There are several options available however the main three are as follows:-
EL backlights are very thin, light weight and provide a very even light. They are available in a variety of colours, with white or turquoise blue being the most popular for use with LCDs. While their power consumption is low, they require voltages of 80~100VAC. This is supplied by an inverter that converts a 5, 12 or 24VDC input to the AC output. El’s have a relatively low brightness 15~20cd/m² and a limited life of 10khrs~15khrs to half brightness.
LED is now the number one choice for backlighting LCDs, they are now available in numerous colours including both white and blue and offer hi-brightness and a long operating life of anywhere between 25khrs ~ 50khrs depending on colour. Being a solid state device they operate directly off 2~5VDC however a current limiting resistor is recommended for protection of the LEDs. The LEDs are mounted either in an array directly behind the LCD or along one edge within a diffuser assembly.
CCFL backlighting offers low power consumption and a very bright white light. The CCFL lamp(s) is generally mounted along the edge(s) of the LCD with a diffuser assembly, which distributes the light evenly over the rear of the display. CCFL lamps require 270~300VAC this is supplied by an inverter that converts 5, 12 or 24VDC input to AC.
LCDs are offered in three basic light transmission modes: reflective, transmissive and transflective (see following drawing)
In the reflective mode, available or ambient light is used to illuminate the display. This is achieved by combining a reflector with the rear polarizer. It is better suited to outdoor or well lit environments.
Transflective LCDs are a mixture of reflective and transmissive types; with the rear polarizer having partial reflectivity. They are combined with a backlight for use in all lighting conditions.
Transmissive LCDs have a transparent rear polarizer and do not reflect ambient light. They require the backlight to be on at all times to see the display. They work best in low level or dark conditions.
CSTN, colour super-twist nematic or passive LCD utilizes the same technology principles as Monochrome LCD (please see previous text) however instead of one dark pixel, with the use of colour filters each pixel in a CSTN display is broken down into a group of three dots, one red, one blue and one green generally arranged in a stripe formation. The LCD drive electronics applies a varying voltage depending on the colour required to all three coloured dots which untwist the liquid crystal allowing the exact amount of light through on each coloured dot to give the desired colour or shade. CSTN LCD can offer a maximum colour palette of 65k colours and are ideal for industrial applications requiring colour but not necessarily requiring extremely fast moving images.
Thin Film Transistor LCD basically describes the control elements that actively control the individual pixels, which has coined the term “Active Matrix LCD”. Each pixel is again broken down into three dots, red, green and blue which are produced by colour filters and are arranged in either a stripe or delta formation, each dot is driven by one transistor i.e. three transistors per one pixel. The liquid crystal in its “off” state no voltage applied will block all light coming through from the backlight, but when a voltage is applied across the liquid crystal elements they twist by up to 90° allowing a level of light through, the transistors control this degree of twist and hence the intensity of the red, green and blue elements of each pixel forming the image on the LCD. TFT LCD are ideal for all applications requiring fast switching colour images.
The basic structure of an OLED or OEL (Organic Electroluminescence) is a sandwich formed with a thin, transparent semi-conducting anode made from Indium Tin Oxide and a metal based cathode on both sides of an organic substrate. The organic material comprises of a hole transmission layer (HTL), an emitting layer (EL) and an electronics transmission layer (ETL). The emissive EL layer includes a polymeric substance that allows the deposition of the organic compounds in rows and columns on a flat carrier (usually glass) by using a simple printing method to create a matrix of pixels. When the correct voltage is applied holes injected in the anode and the electric charge from the cathode are drawn together and combine in the emissive layer, which causes the pixel to emit light. One of the major benefits of OLEDs over more traditional displays is that because they emit light they do not need a backlight which makes them ideal for battery powered applications.
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