MULTI TOUCH  Multi-touch refers to a touch system's ability to simultaneously detect and resolve a minimum of 3+ touch points. All 3 or more touches are detected and fully resolved resulting in a dramatically improved touch experience. Multi-touch is considered by many to become a widely-used interface mainly because of the speed, efficiency and intuitiveness of the technology.  Multi-touch technology is an advanced human-computer interaction technique that recognizes multiple touch points and also includes the hardware devices that implement it, which allow users to compute without conventional input devices.  Multi-touch consists of a touch screen (screen, table, wall, etc.) or touchpad, as well as a software that recognizes multiple simultaneous touch points, as opposed to the standard touchscreen which recognizes only one touch point at a time.  The most popular form are mobile devices (iPhone, iPod Touch), tables (Microsoft Surface) and walls. Both touchtables and touch walls project an image through acrylic or glass, and then back-light the image with LEDs

IMPLEMENTATION  When a finger or an object touches the surface, causing the light to scatter, the reflection is caught with sensors or cameras that send the data to software which dictates response to the touch, depending on the type of reflection measured.  Touch surfaces can also be made pressure-sensitive by the addition of a pressure- sensitive coating that flexes differently depending on how firmly it is pressed, altering the reflection.  Handheld technologies use a panel that carries an electrical charge. When a finger touches the screen, the touch disrupts the panel's electrical field. The disruption is registered and sent to the software, which then initiates a response to the gesture.  In the past few years, several companies have released products that use multi-touch. In an attempt to make the expensive technology more accessible, hobbyists have also published methods of constructing DIY touch screens.

HOW MULTI TOUCH WORKS  Tracking Fingers: The most advanced multi-touch screens respond to the motion and pressure of numerous fingers. In the Perceptive Pixel design projectors send images through an acrylic screen onto the surface facing the viewer. When fingers or other objects (such as a stylus) touch the surface, infrared light shone inside the acrylic sheet by LEDs scatters off the fingers and back to sensors. Software interprets the data as finger movements. Tapping the screen brings up command menus when desired. To create a signal, LEDs bounce light through the acrylic sheet. No light escapes. But if a finger is placed against the face light will scatter off it toward the sensors. Also, a pressure-sensitive coating flexes when pressed firmly or lightly, making the scattered fingertip signal appear slightly brighter or dimmer, which the computer interprets as more or less pressure.

 Touch Table : A projector inside Microsoft’s multi-touch table, called Surface, sends imagery up through the acrylic top. An LED shines near-infrared light up as well, which reflects off objects or fingers back to various infrared cameras; a computer monitors the reflections to track finger motions.  Apple has revealed some clues about how the touch screen works. A panel under the screen glass senses your touch using an electrical field. The panel then sends this reading to an LCD below it.  In other words, your finger changes the electrical charge, which in turn feeds the phone operating system and determines which pixels have changed and which activities have been triggered.  Every touch screen phone uses a similar method, but what makes the iPhone unique is how the iPhone OS responds so quickly to swipes, pinches, and finger presses -- so fast that there is a burgeoning market for high-quality iPhone games that some say rival even the mighty Nintendo DS and PlayStation Portable.

TECHNOLOGIES  From physical point of view there are several multi touch technologies, they are  Resistive touch screens are composed of two flexible sheets coated with a resistive material and separated by an air gap or microdots. When contact is made to the surface of the touch screen, the two sheets are pressed together. On these two sheets there are horizontal and vertical lines that when pushed together, register the precise location of the touch. Because the touch screen senses input from contact with nearly any object (finger, stylus/pen, palm) resistive touch screens are a type of "passive" technology.  Capacitive sensing is a technology based on capacitive coupling that is used in many different types of sensors, including those for detecting and measuring. Capacitive sensors are used in devices such as laptop track pads, MP3 players, computer monitors, cell phones and others. More and more design engineers are selecting capacitive sensors for their versatility, reliability and robustness, unique human-device interface and cost reduction over mechanical switches.  Projected Capacitive Touch (PCT) technology is a capacitive technology which permits more accurate and flexible operation, by etching the conductive layer. Due to the top layer of a PCT being glass, PCT is a more robust solution versus resistive touch technology. Depending on the implementation, an active or passive stylus can be used instead of or in addition to a finger.

 Surface acoustic wave (SAW) is an acoustic wave traveling along the surface of a material exhibiting elasticity, with an amplitude that typically decays exponentially with depth into the substrate.  Infrared touch screen uses an array of X-Y infrared LED and photodetectorpairs around the edges of the screen to detect a disruption in the pattern of LED beams. These LED beams cross each other in vertical and horizontal patterns. This helps the sensors pick up the exact location of the touch. A major benefit of such a system is that it can detect essentially any input including a finger, gloved finger, stylus or pen. It is generally used in outdoor applications.  Optical touch optical sensors track the movement of any object close to the surface by detecting the interruption of an infra-red light source. The light is emitted in a plane across the surface of the screen and can be either active (infra-red LED) or passive (special reflective surfaces).