DISPLAY SCREEN TYPES
LED - Liquid crystal displays are thin flat panels with a number of pixels in front of a back light. The pixe ls are filled with liquid crystals that are capable of showing colours or one colour only. The technology used is Electro-optical modulation. Liquid crystal displays are often used in battery-powered devices, because they use very little electricity. Fig.1 shows an example of LCD usage in a basic clock/radio.
                                                                                                                                                                        Fig.1 Clock/Radio.

TFT-LCD - Thin Film Transistor Liquid Crystal Displays use a technology which is used in LCD monitors and television displays. TFT technology gives one of the clearest pictures of any flat screen display, using less electricity than older screens. TFT displays are very fragile because they are made as thin and light as possible.

Thin Film Transistor Liquid Crystal Displays are made with a chemical technology known as chemical vapour deposition. With this technology very thin glass is coated with electrically conductive metal but remains transparent. Chemical vapour deposition technology is used to make computer and television display screens of the thinnest dimensions of any flat screen in production to date. Fig.2. shows an example of TFT-LCD technology.

                                                                                                                                                                     Fig.2 TFT-LCD Screen

OLED - Organic light-emitting diodes are an improvement of the older light-emitting diode (LED) technology. The difference between LED’s and OLED’s lies in the coating, the layer on an OLED that creates light is made of thin organic compounds. One of the uses of OLED technology is for thin display panels used in notebooks. OLEDs can be used to make displays that can bend due to the materials very thin structure. There are many uses for this comparatively new technology, which was designed in 2006 but proved to be inferior to the LED screens due to its shorter lifespan. Standard LED screen lifespan is 60,000 hours compared to the OLED screen lifespan of 5,000 hours. Experiments in 2007 created a type of OLED that worked for 198,000 hours. The organic compounds used in OLED’s are more susceptible to water damage. Fig.3. shows an example of OLED Panel technology.

                                                                                                                                                                        Fig.3 OLED Panel


Plasma Display Panel (PDP) - This is a type of flat panel display common to large TV displays (32 in/81 cm or larger). Many microscopic cells between two glass panels hold a mixture of noble gases, helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn). The gas in the cells is electrically charged and turned into plasma which emits ultraviolet light. This UV then excites phosphors to emit visible light. The plasma display system is ideal for the manufacture of large screens. It has become uneconomical to produce a screen which is less than 32˝ (diagonal dimension); PDP technology is therefore ideal for larger displays.

                                                                                                                                                      Fig.4. PDP (103˝ Samsung Plasma TV)
Touch Screen – This technology, as the name indicates, is a means by which the presence and location of a touch is detected within the display area. The touch on the display area may be a finger or a stylus.

One of the main advantages of touch screen technology is the ability to manipulate the objects on the touch screen without the need for keyboard or mouse. The items on the screen, icons or images, can be selected or moved around the screen and therefore arranged to the personal configuration of the user.

The touch screen has many applications, such as Personal Digital Assistant (PDA), which is commonly known as a palm top computer, satellite navigation devices, mobile telephones and video game machines. There are several technologies used in the construction of touch screen devices. The following descriptions are the most commonly used.
The touch screen panel is built up of several layers, two of which are thin metal layers which are electrically conductive and separated by a narrow gap. When the screen is touched the two metal plates touch at that point and the panel behaves like a pair of voltage dividers with connected outputs. This causes a change of electrical current which is sensed as a touch event and is processed by the touch screen controller.


                                                                                                                                               Fig.5. Resistive Touch Screen

This technology uses ultrasonic waves that pass over the touch screen panel. When the screen is touched, part of the wave is absorbed. The change in the ultrasonic wave is registered as a touch and the information sent to the controller. SAW screen panels can be damaged by outside elements and contaminants on the surface can interfere with the function.

                                                                                                                                                Fig.6. Surface Acoustic Wave Touch Screen


The capacitive touch screen is constructed from an insulator, usually glass, coated with a transparent conductive material such as indium tin oxide. By using the finger as a conductor the electrostatic field is distorted resulting in a capacitance charge. There are many different technologies available to determine the location of the touch. The information for example could be passed to a computer running a specific software application which will calculate its position.

This is the most basic of capacitance touch screen technology. Only one side if the insulator is coated with a conductive layer which has a voltage applied resulting in a uniform electrostatic field. When touched by the finger a dynamic capacitor is formed. The sensors controller determines the position of the touch by from the change in capacitance measured from the four corners of the screen. This system is moderately durable but has limited resolution; it is also prone to false signals from parasitic capacitive coupling. The unit needs to be calibrated during manufacture. The main use for this system is simple applications in industry and kiosks.

                                                                                                                                                                                                      Fig.7.  Surface Capacitance Touch Screen


This touch screen technology is the most commonly used in modern equipment, form point of sale equipment to Smartphone. The difference between basic capacitance sensing and projected capacitance is that the latter uses an etched conductive layer which forms an XY array. A single electrode is etched or two separate perpendicular layers are etched to form parallel forming a grid. Applying a voltage to the array produces a grid of capacitors. Bringing a finger or a stylus close to the screen changes the electrostatic field. The capacitance change can be very accurately determined at every point on the grid. This system has greater resolution and also permits multi-touch operation. Because direct contact with the screen is not needed, the conducting layer can be coated with a protective film or even toughened vandal proof glass.

                                                                                                                                                                                                      Fig.8. Projected Capacitance Touch Screen

Optical touch screen systems are constructed using light emitting diodes (LEDs) on two adjacent bezels of the display and Photosensors placed on the opposite two bezels to for an array. Placing an object, finger or stylus, on the screen interrupts the light intensity passing between LED – Photosensor pairs, causing a measurable light intensity drop which is measurable. This enables the touch position to be determined. There are two problems which prevent the widespread use of infrared touch screens. Firstly is the high cost of the technology compared to other touch screen technologies. Secondly is the performance of this type of technology in bright light. If the infrared touch screen is used in very light conditions, such as sunlight, the photosensors cannot detect the light from the LEDs and causes failure in the touch screen operation. The main feature of infrared touch screen technology, which gives it an advantage above most other touch screen technologies, is that it has a digital output signal. There is no need for the additional analog/digital converter to the circuit, there is less power consumption, more accuracy and has higher precision.
Fig.9. Infrared Touch Screen.
The strain gauge technology uses a screen which is sprung at the four corners and strain gauges are used to determine the deflexion when the screen is touched. This system can also measure deflexion on the Z-axis thus determining the force of a person’s touch. The technology is old, developed in the 1960s but recent innovation has made the system more commercially viable. The screens are typically used in public systems such as ticket machines due to a high resistance to vandalism.
                                                                                                                                                         Fig.10. Strain Gauge Touch Screen


This is a relatively modern touch screen technology. Two or more image sensors are placed on the periphery of the screen with an infrared backlight placed sensors field of view on the other side of the screen. The image sensors detect a touch, which shows up as a shadow, and triangulates its position. This technology is growing in popularity.


                                                                                                                                                                                                         Fig.11. Optical Imaging Touch Screen
This system was introduced in 2002 by 3M, using sensors to detect the mechanical stresses in the glass when touched. By using complex algorithms the information is interpreted and the location of the touch is determined.
Fig.12. Dispersive Signal Technology Touch Screen


This system was introduced in 2006 by the Elo division of Tyco International. The system uses more than two piezoelectric transducers located at positions of the screen and converts the mechanical touch on the glass screen into an electrical signal. The screen hardware uses algorithms to determine the position of the touch on the screen the same as 3Ms Dispersive Signal Technology but because of the number of sensors used it is more accurate, similar to the triangulation system used for GPS.
 Fig.14. Acoustic Pulse Recognition Touch Screen
This is the most up to date touch screen system; introduced by MIT Media Lab in December 2009. This system converts an LCD display into a camera which provides gesture control of the objects on the screen. Instead of an LCD, there is an array of pin holes in front of the sensors. Light passing through the pinholes strikes a block of sensors and produces a low resolution image. Each pin hole image is taken from a different position andwhen combined, produce an image with good depth information.
Fig.15. Coded LCD: Bidirectional Touch Screen.