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frustrated total internal reflection (multitouch) (FTIR)

Frustrated Total Internal Reflection (FTIR) is a technique for touch-sensitive scanning devices such as those used in fingerprint scanners, touchscreens and multi-touch screens. FTIR is a technique based on light scattering and light reflections that can be used to accurately determine touch positions on the scanner surface.

The FTIR process is based on changing the angle of reflection of light. When light passes from one medium to another, the different refractive indices change the angle of reflection. This physical principle underlies the FTIR method, in which light is diffusely reflected in an acrylic disk.

Principle of Frustrated Total Internal Reflection (FTIR) without and with finger contact

Principle of Frustrated Total Internal Reflection (FTIR) without and with finger contact

In the FTIR technique, a plexiglass pane is irradiated from the edges with infrared light from many light-emitting diodes. The IR radiation spreads diffusely in the Plexiglas pane, but cannot escape from it because the critical angle conditions between the refractive indices of Plexiglas and air do not allow this. Only when the refractive indices at the surface of the Plexiglas change does the light experience a steeper angle of reflection and exit the Plexiglas. The change in refractive index at the Plexiglas surface occurs through contact, for example with a finger. The media transitionbetween finger and Plexiglas is different than between air and Plexiglas.

In order for the user to see where to touch the Plexiglas, the user interface and the application are projected onto the back. When touched, the infrared light emerges from the Plexiglas, and the position of the light emission is determined by a camera. One could also change the angle of reflection by bending an elastic Plexiglas pane by pressing on it with a finger.

There are several similar principles that work with light diffusion. Diffused Surface Illumination( DSI), for example, has a special acrylic plate as a diffuser, which provides uniform light distribution within the Plexiglas. In Diffused Illumination( DI), infrared emitters replace the externally mounted infrared LEDs and uniformly illuminate the Plexiglas pane from below.

Englisch: frustrated total internal reflection (multitouch) - FTIR
Updated at: 24.01.2013
#Words: 325
Links: fingerprint, light, scattering, scanner, process
Translations: DE

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