1. Daniel A. Taylor
University of Pitt-Bradford

2. Introduction
 Touch sensitivity is fairly common in
electronics today…
 …but only for a single point of contact at
any given time.
 Touch-sensitive devices that allow for
multiple points of contact have thus far
been expensive or difficult to produce.

3. Why multi-touch?
 Allows a user to interact with a system
with more than one finger at once
Touch Keyboards
Operations using both hands
 Also allows multiple users to interact on
the same touch-sensitive platform
Interactive walls
Interactive tabletops

4. Prior approaches to the problem
 Matrix of smaller sensors
Requires many connections, which severely
limits resolution
Visually translucent - no touch-screen
 Video cameras
Measuring of brightness
Identifying tracking markers in gel

5. A new approach…
FRUSTRATED TOTAL INTERNAL REFLECTION

6. Total Internal Reflection
 When light encounters a medium with a
lower index of refraction (e.g., going
from glass to air), its refraction depends
on the angle at which it hits the border.
 Beyond a certain critical angle, light is
not refracted, but instead reflects
entirely within the material.
This is the basis for fiber optics and other
optical wave guides.

7. Frustrated Total Internal Reflection
 If another material touches that within
which the light is reflecting, the reflection
is frustrated, causing the light to escape.
 This has been used in the past:
fingerprint imaging
early touch sensors (1970s!)
tactile sensors for robotic grips

8. A Schematic of FTIR
(Kasday, 1984)

9. Using FTIR for touch sensitivity
 A clear acrylic sheet is used as the
touch surface.
28 inches x 24 inches in prototype
 Edges of surface lit by infra-red LEDs to
produce total internal reflection.
 A video camera is mounted under the
surface and facing it.
 When the surface is touched, the light
escapes and registers on the camera.

10. Using FTIR for touch sensitivity
 Basic image-processing techniques are
performed on the camera output to
identify the points of contact.
 Computer-vision techniques are used to
interpret the motion of contact points as
discrete touches or strokes.
 Processing easily handled in real-time
by a 2 GHz Pentium IV processor

11. Advantages to this approach
 High capture rate and resolution
30 frames per second
640x480
 True zero-force touch sensitivity
 Inexpensive to construct
 Scalable to much larger (even wall-
sized!) surfaces
 Transparent: can be combined with rear-
projection display

12. How is projection achieved?

13. Disadvantages
 Requires significant space behind touch
surface for camera
 Gloves, certain types of styluses, and
even dry skin may not register
a function of refractive index
 Residues on surface (e.g., sweat) also
produce FTIR effect that may build up

14. Any Questions?