August 13, 2010

page 2 of 14

complexity.

The third constraint, the ambient light

sensitivity, remains a very relevant design

challenge. There are several methods to

deal with this, both optically and electrically.

This will be discussed, among other issues,

in

‘General

Design

Considerations’

in

section four.

At the end a brief product selection guide

provides information for a rapid and

successful design-in.

2. Overview of IR Touchscreen

Principles

Generally speaking, IR touchscreens have

several desirable attributes that are not all

present in competing technologies. The

object used to generate the ‘touch’ can have

almost any shape and size and be made of

almost any material. This is in contrast to

most other touchscreen technologies where

some sort of stylus is required.

As IR touchscreens are a solid state

technology they have no moving mechanical

parts or anything placed on top of the

display to reduce the brightness. The latter

fact ensures crystal clear image quality and

robustness over time. This is especially

important as many device or display vendors

sell their products on the customers

perceived display quality. During the past

years several different technologies for IR

touchscreens have come up on the market.

The major ones will be explained in the

following sections.

2.1 IR Matrix-based Touchscreens

The

traditional

IR

matrix

touchscreen

technology is based on the interruption of a

light path in an invisible light grid in front of

the screen. A simplified schematic is

presented in Fig. 1.

In this concept an array of emitters (IREDs)

is employed and covered behind two

adjacent bezels of the screen frame and

creates the invisible optical grid. The

opposite bezels contain the respective

detector arrays (typically phototransistors or

-diodes). This arrangement shields the

active parts from environmental influences

and maintains the quality and brightness of

the image. Additionally it enables screen

retrofits,

and

is

in

fact

completely

independent of the screen for all practical

purposes.

If an obstacle (e.g. a stylus or finger tip)

appears inside the grid matrix it interrupts

the light beams and causes a reduction of

the

measured

photocurrent

in

the

corresponding detectors. Based on this

information the x- and y-coordinates can be

easily obtained.

The IR-matrix based principle is suitable to

recognize static operations as well as

motions. It is not really suitable for high

resolution motion detection, e.g. handwriting

recognition.

Fig. 1: Concept of an IR matrix-based

touchscreen. The influence of a stylus on

the

photocurrent

of

individual

detector

elements is sketched below.