A touchable hologram is a combination of an apparently three dimensional light projection, a sensor array, and some type of tactile feedback. It is impossible to actually touch and feel holograms, but the use of a sensor array and haptic technology can provide the illusion of touch. Sensors are used so that a computer will be able to adjust the behavior of a holographic projection when it is touched, and haptics are employed to provide an actual sensory input. A number of different haptic technologies can be used to create a touchable hologram, though ultrasound was chosen for some of the first designs.
Holograms are three dimensional light projections that are created from two dimensional recordings. This type of projection can be viewed from any angle just like a physical object even though it is composed of nothing but light. Since holograms are made of light and lack any physical substance, they cannot be touched or interacted with through any traditional means. In order to create a touchable hologram, at least two different techniques must be employed.
In order for a hologram to be felt, some type of haptic technology must be used. One way to create tactile feedback is to attach physical devices to a person's hands or body, though this can interfere with the illusion of touching a hologram. High powered air jets are another option, though ultrasonic devices can provide a similar effect. In either case, the haptic technology is used to impart a physical sensation to some part of a person's body in order to provide the illusion of touch.
The other main component necessary for creating a touchable hologram is some type of sensor apparatus. In order for the haptic technology to activate at the correct time, a computer must know where a person's hand or other body part is located. This can be accomplished through the use of a camera and specialized software, though reflective tape and other markers can make the process more accurate. Once the computer knows where a person's hand is located, it can activate the haptic technology at the correct time to create the illusion of touch.
These types of sensors can also allow a person to interact with a hologram. Since the computer responsible for generating and controlling the image and the haptic feedback knows where the person is located, it can respond accordingly. An example of this type of touchable hologram is a ball that is capable of bouncing off of someone's hand. The computer can track the location of the hand, project the hologram in the correct place, and activate the tactile feedback accordingly. If the hand is moved, the ball can be allowed to drop realistically.