A touch screen is a display screen that can sense when it is touched. Touching can either be done by a finger or some other passive device. A light pen is an example of a non-passive device and using this form of display would not be called a touch screen.
The idea of a touch screen is to replace or supplement buttons used to control the device. Being able to just touch an item is typically an intuitive user interface. There has always been an interest in touch screens on eBook devices. However, not everyone likes them and there are some problems. For example the implementation of some forms of touch screens degrade the display image but adding extra layers. A finger can also leave finger prints on the screen that degrades the display image. However, these limitations can be mitigated in some designs.
A touch screen uses similar technology as a touch pad on a laptop but provides absolute positioning on a screen with direct visual feedback.
 Types of screens
There are several types of touch screen technology. They fall into three classes.
- A stylus is required - These kinds of screens need a special pointing device. Technically these are no longer called a touch screen unless they also support finger access. An example is the Wacom design. This may be called Electromagnetic Handwriting Panel or is also known as a digitizer screen.
- A finger is required - These kinds of touch screen will not respond to a stylus but need a human finger to provide the sensing. An example is a capacitive touch screen. There are special stylus' that mimic your finger that will work on these devices.
- Either a finger or a pointer can be used. These screens typically depend on pressure and do not care where it comes from. An example is a resistive touch screen or an optical approach. Other technologies have dual hardware that can switch modes from a hardware pointer to a finger. An example is N-trig. Wacom also makes this technology using their special pointer, or finger.
 Resistive touch screen
A resistive touch screen is composed of several layers, the most important of which are two electrically conductive layers separated by a narrow gap. In one implementation each layer consists of a group of wires in parallel. On one layer the wires run horizontal while on the other they are vertical. When an object, such as a finger, presses down on a point on the panel's outer surface the two wires are pushed together at that point. The point of contact is then computed by the panel logic and sent to the controller as an X,Y coordinate of the screen position.
For transparent screens a better approach is to have each conductive layer made with a transparent conductive coating. The coating is connected to sensors and the full resistance if the layer is known. As the screen is "touched" the layers short together at that point which causes a voltage division that can be used to calculate the point on the screen that was touched. While these layers are transparent there is some loss of light reflection due to the layers themselves and will typically cause a discernible difference in the clarity of the display.
A very simple plastic stylus can be used for pin point accuracy on this type of screen.
 Capacitive touch screen
In this technology only one conductive layer is needed. The panel consists of an insulator, such as glass, coated with a transparent conductor. When the finger touches a point a capacitive contact is established between that point and and external ground supplied by the body of the person touching the screen. The point of contact due to this interference with the static electrical field set up on the layer can be measured using sensors located at the four corners of the screen to determine the location of the finger on the touch screen.
This measurement technique is not particularly accurate so more refined methods are often used. One method is to etch the conductor screen such that large number of capacitors are created in the form of a grid. Now, touching the screen will cause an interference to specific capacitors resulting in better definition. In addition multi-touch is supported in the configuration since there is now the ability to detect each touch independently from the others due to the independence of each capacitor in the grid. Detection can be made difficult if the screen is smudged with the oil from fingerprints. This form of touch screen is referred to as Projected Capacitive Touch (PCT) technology.
One advantage that the capacitive system has over the resistive system is that it transmits almost 90 percent of the light from the screen, whereas the resistive system only transmits about 75 percent. This can give the capacitive system a clearer image than the resistive system.
A capacitive touch screen does not support the stylus that you might use on a resistive touch screen however, there are special stylus' available for these screens.
IR, InfraRed touchscreens are available from Kobo and Amazon on some models. They work by having IR emitters on one edge of the bezel just above the screen and receivers on the opposite edge. This is repeated in the 90 degree direction. The invisible beams of light are intercepted (blocked) by your finger or perhaps a stylus like device and this detection is used to determine the location of the finger. The X Y coordinates can be determined by matching the touch in both axis. A touch of the actual screen is not required but permitted, and usually unavoidable. The advantage of this technique is that it will work with any interruption. You could, for example, be wearing gloves which a capacitance screen will not allow. Any object will work and no direct pressure on the screen is required which will minimize fingerprints. This also removes the need for an extra touch screen layer which might make the screen display sharper.
There are disadvantages as well but they may or may not be important depending on the use. Multi-touch may or may not be supported. The resolution may not be as high depending the the number of sensors involved. The screen cannot be flush with the bezel which can sometimes cast a shadow on the edge of the screen. Some may see this as an advantage, since it is less likely that your hand might touch the screen accidentally.
 ZForce IR
ZForce™ technology is from Neonode. This is a patented optical approach for both high resolution pen writing in combination with finger navigation. Touch, gestures, multi-touch, sweeps are all supported. zForce uses no overlay (like resistive and capacitive touch screens) on top of the E Ink display thus creating a 100% clear window free from reflexes and parallax effects and produces a true paper like experience. The power consumption has also be lowered with respect to capacitive technologies. This technology is used on the latest Sony devices.
zForce uses a small frame around the display with IR LEDs and photo-receptors on the opposite sides hidden behind a infrared-transparent bezel. A touch obstructs one or more of the beams which identifies the X and Y coordinates which also give area information. Interpolation combined with analog signal processing give multiple touch readings and high speed gesture feedback.
 Stylus Screens
These screens may or may not have finger touch capability but include Stylus specific hardware designs to facilitate stylus performance and accuracy.
The EMR® (Electro-Magnetic Resonance) Technology from Wacom uses a special stylus to touch the screen. They also have a capacitive screen technology that allows finger touching and EMR on the same screen. EMR touchscreen technology is also available from Hanvon. Features of this pen include: No battery, no setup or pairing required, Special high-friction pen tip (some units), Tilt detection, up to 2048 levels of pressure sensitivity.
This stylus is only available for iPad Pro tablets and the iPad 2018. It features pixel level resolution, angle (tilt) sensing, and pressure sensing with virtually no lag. The samples are 240 times per second. It also features palm rejection of your hand when the pen is in use. This is a powered device with 12 hours of battery life between charges. It is recharged by plugging into the iPad itself and will provide 30 minutes of writing with only a 15 second charge.
Logitech has announced a Crayon stylus at 1/2 the price for those who do not require a pressure sensitive stylus. It supports tilt sensing and palm rejection.
Another stylus solution is the Windows Bluetooth Pen with Palm block technology used on Surface 3 and Surface Pro 3. It can also work with the Surface Pro 4 and Surface Book. It uses low power Bluetooth 4.0 LE. It is powered with a AAAA battery and has 2 or 3 buttons. This is optional accessory on some models and works with a Capacitive touch screen. The pen supports 1024 pressure levels, has an eraser (button) on the other end. It can use different thickness points, 2H, H, HB, B and comes with HB.
This screen/pen technology is covered at N-trig.