August 13, 2010

page 3 of 14

2.2 Camera-based Touchscreens

Very recent developments are camera-

based touchscreen setups. This technology

is growing in popularity, due to its scalability,

versatility and affordability, especially for

larger units. One typical setup is presented

in Fig. 2.

The system usually consists of two or more

IR line-scanning optical sensors, like used in

barcode or flat-bed scanners. Each one is

mounted in the upper left and right corner of

the screen bezel. The sensors monitor the

complete screen which is illuminated with

infrared light.

The infrared illumination of the screen area

is done by IREDs positioned in the upper left

and right corners, next to the line scanning

sensors, but optically isolated to avoid

crosstalk. Each of these IRED assemblies

illuminates the complete 90° angular range

of the screen.

The reflection of a stylus or object (e.g.

finger) triggers a rise in the signal of the

relevant

detector

cells.

By

special

computational algorithms (e.g. triangulation)

based on the readout of the two line

scanning sensors the exact coordinates and

even the size of the touching object or finger

tip can be calculated via software.

2.3 Camera-based with Light Guide

In a different arrangement, a light guide

based infrared lighting system is mounted at

the cameras opposite field of view, inside

the bezel (see Fig. 3). Practical realizations

of this backlighting system include high

power IREDs which couple light into both

ends of an edge emitting optical light guide

element. This light guide is mounted around

the screen and provides an IR light curtain.

In this case the touch of a stylus or object

shows up as a shadow generating a drop in

the relevant detector cells’ signal. Again,

special computational algorithms are needed

to do the calculation of the location resp.

size of the object.

Fig. 2: Reflection type principle of a camera-

based touchscreen. The influence of a stylus

in the light path on the cell’s signal of the line

scanning sensor is sketched in the graph.

Fig.

3:

Camera-based

touchscreen

realization with an edge emitting light guide.

The

light

guide

provides

a

diffuse

illumination of the screen.