SFH4645 datasheet(5/14 Pages) OSRAM

5 / 14 page

August 13, 2010

page 5 of 14

3. Application Specific Design

Guidelines

3.1 Matrix-based Design

Newly developed slim and cost effective

emitter resp. detector packages allow a

significant

reduction

bezel

height,

overcoming one of the main drawbacks of

this traditional concept.

The number of employed IREDs depends

mainly

screen

size

and

required

resolution. For simple applications their

spacing might be as wide as one IRED per

inch.

In most enhanced large screen systems an

IR controller sequentially pulses the IREDs.

This is important to avoid any simultaneous

crosstalk between different emitters.

If sequential operation is not feasible there

are some other measures necessary to

counterfight unintended crosstalk (although

intended optical crosstalk into neighboring

detectors is necessary to increase the

resolution beyond the IRED spacing).

The most important and best measure is the

proper mechanical design to achieve a good

optical shadowing. The combination with a

narrow-angled

detector

also

appropriate action to minimize ambient light

issues.

Suitable

emitter/detector

products

with

narrow half-angle and small package height

for matrix-based touchscreens can be found

in Tab. 2 at the end of this note. These slim

products enable a cost effective and

appealing design.

3.2 Camera-based with Direct Illumination

Depending on the optical design and

working

principle

camera-based

touchscreens, either diffuse wide-angle

IREDs for direct illumination or emitters for

coupling light into a light guide are

advisable.

To extract the signal from ambient IR-noise

the usual operation is in pulsed mode by

comparing

two

scans.

The

first,

the

reference scan (without IR illumination) is

compared with the signal scan (with IR

illumination). Based on the difference the

touch event can be extracted.

Suitable components for the former setup

are either pairs of SFH4050 or SFH4655,

which can illuminate the complete 90° field

of view. The slim package is an excellent fit

for a compact design.

3.3 Camera-based with Light Guide

Illumination

An efficient system requires a homogeneous

and diffuse illumination of the area above

the screen due to an. e.g. edge emitting light

guide. The selection of an IRED for coupling

into a light guide element depends on a

number of criteria. Most important is the

design of the light guide (fiber), especially

the distribution of the outcoupling elements

along the light guide. The spacing of these

elements is either uniform or gets narrower

with increasing distance from the IRED

coupling site. The latter variation is usually

designed

for

standard

wide-angle

components, whereas the first prefers

emitters with a more focused beam to

achieve the homogeneous and diffuse

outcoupling along the light guide. In general,

only

customized

solutions

provide

optimized illumination along the light guide.

However, for coupling light from the emitter

into waveguides there are some general

guidelines (see also the OSRAM application

note “Light Guides” for a more detailed

discussion). First of all, the air-gap between

the emitter and the light guide needs to be

minimized. Even better options include holes

in the acrylic glass for the emitter. To get a

good optical contact an index matching can

significantly reduce the Fresnel-losses (typ.

at least 2 x 4 % at the emitter – air-gap –

glass interfaces). A second issue concerns

the type of coupling, e.g. butt coupling

(perpendicular to the plain cut light guide

surface) or angled coupling. The first type

usually

employs

standard

wide-angle

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