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Also within FLATIR, an infrared sensor array is attached behind the display's LC-components. Unlike in MightyTrace, no active devices are used to trigger the sensors. The goal of FLATIR is to detect passive objects, in particular finger touches. In order to detect them, infrared light has to trigger the sensor at the position where the finger touches the surface of the screen. Using an interpolation, the exact finger position can be calculated from multiple sensor values. In order to achieve this, a second component is mounted in front of the display, which consists of a normal acrylic plate. Infrared light emitting diodes (LEDs) couple in infrared light into the acrylic plate from the side. Within the acrylic plate, the light is totally reflected and thus is guided inside the material. As soon as a finger touches the surface, the refraction index changes locally, since the coupling between acrylic and air is replaced by the copuling acrylic and finger. This cause an distortion in the total internal reflection and some infrared light is coupled out from the acrylic. This principle is known since the 1960s and is used for lightguides in advertisement or in background illumination for displays. The decoupled infred light triggers the sensors and thus allows an interpolation of the finger's position.
The following figure shows an cross-sectional view of FLATIR.



Coupling out infrared light from the acrylic plate depends on many different factors.



The following tables show some influencing factors. It is distinguished between the incopuling of the infrared light into the acrylic plate, and into the outcoupling by touching the plate with the fingers.






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