August 13, 2010

page 6 of 14

devices, whereas the latter uses narrow-

angle IREDs, necessary to achieve total

internal reflection. It is worth to mention that

the

emitting

light

guides

radiation

characteristics depend on the design of the

outcoupling structures and the emitter’s

radiation characteristics in combination with

the coupling arrangement. To support

design activities OSRAM provides raytrace

models, available at the OSRAM website.

For narrow-angle applications, the e.g.

without lens family is an excellent choice for

standard wide-angle requirements. Both

feature slim packages and a flat-top to

minimize the air-gap between the fiber and

the IRED.

3.4 Projector-based with Diffuse

Illumination

In projector-based touchscreens with IR

illumination from the backside it is desirable

to achieve a diffuse and homogenous

illumination. Suitable high power products

are the DRAGON IRED series (SFH423X).

In

large

projector

applications

it

is

recommended to split the screen into

subscreens and use several IRED-arrays for

illumination

purpose.

Alternatively

high

power

IRED-based

modules

with

homogenous

and

diffuse

fields

are

Observation product family (SFH47XX).

3.5 Projector-based with FTIR

The

FTIR

principle

features

different

aspects. The mathematics behind is based

on Snell’s law: n

and the light path and n as the refractive

index of the material (see Fig. 6 for an

illustration). Using Snell’s law, the boundary

condition for total internal reflection at the

glass – air interface is around

ε

(assuming a refractive index of around n

1.49 for acrylic glass).

To achieve efficient FTIR a high number of

total internal reflections per unit length inside

the acrylic glass are desirable.

Direct coupling (also called butt coupling,

like depicted on the left side in Fig. 5),

employs IREDs with a wide half-angle to

achieve this target.

However, to increase the level of internal

reflections (increasing the light density to

achieve a more efficient FTIR) it might be

preferable to couple light into the acrylic

glass under angled conditions. This can be

achieved efficiently by either tilting the

standard wide-angle emitter by e.g. 45° or

by an inclined glass edge (see also Fig. 5,

coupling from the right side). A suitable

arrangement is e.g. cutting the glass edge

up to around

γ

g

≈ 35°. To ensure a

maximum

of

total

internal

reflections

simultaneously with a high power density,

components with a high radiant intensity and

narrow half-angle are recommended. Under

above conditions, emitters with a half-angle

of up to around 15° are preferable.

Compared to wide-angle emitters, narrow-

angle

IREDs

might

require

a

tighter

component spacing to avoid ‘dark’ spots

close to the coupling location.

To ensure a high coupling efficiency and

minimal Fresnel-losses (which typ. add up to

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

glass interfaces) a plain cut glass surface is

c

1

2

IRED

total

internal reflection

refraction

total internal reflection

ε

ε

ε

ε

ε

Fig. 6: Definition of Snell’s law and critical

angle

schematic also illustrates that under certain

conditions all light coupled into the light

guide is subject to total internal reflection.