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SFH3015FA_

Emitters and Detectors for Infrared (IR) Touchscreens

OSRAM GmbH 

Using visible LED sources (e.g. monitor

backlighting) in IR touchscreen solutions is

recommended, as these LEDs have no IR

content. In contrast most conventional (non-

LED based) light sources emit also in the IR

spectral range. E.g. sunlight and

incandescent bulbs contain components of

equal or even higher amplitudes in the

visible as well as IR wavelengths range1.

For applications where intense incandescent

or halogen illumination is expected, some

additional electrical-domain effort is advised

to enhance the signal-to-noise ratio.

There are several steps with increasing

complexity to counterfight the IR noise topic.

The implementation depends on the signal

compared to the IR noise level.

The simplest version is the operation of the

emitter and detector in a pulsed and

synchronized operation. An AC-coupling of

the detector signal might efficiently filter out

the present DC-components of the ambient

light. If the IR background noise becomes

more dominant, a more complex

implementation is necessary. This measure

compares two subsequent measurements.

The first, called the reference (without IR

illumination) is compared with the second,

the signal (with IR illumination). Based on

the difference signal the touch event can be

extracted.

The most demanding solution is the

inclusion of a lock-in amplifier type circuitry,

which demands a modulated emitter signal.

In this case the IRED is modulated with a

carrier frequency and the signal is detected

through a frequency synchronous receiver,

either in a homodyne or heterodyne

structure. Alternatively, digital signal

processing allows a direct detection by

employing computational algorithms (e.g.

Fourier-type). Such a system can be made

immune even to severe IR ambient light

(noise). The selection of the right modulation

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1 IR noise may contain many frequency components from

various light sources:

- DC (e.g. in sunlight, incandescent or halogen lamps)

- Mains frequency (50/60 Hz) and driver caused harmonics

(e.g. in incandescent and halogen lamps)

- Frequencies depending on the driver/power supply (e.g. up

to kHz range in fluorescent lamps)

frequency and the implementation of a

narrow band-/lowpass filter are key elements

for a high signal-to-noise ratio.

Similar considerations should be undertaken

for camera/line scan systems. In this case, it

is recommended as a first measure to insert

a narrow optical bandpass filter (matching

the IRED wavelength) in front of the camera.

This increases significantly the signal-to-

noise ratio.

4.4 Scalability Issues

Camera-based and projector-based

solutions have the advantage of easy

scalability. An increase in resolution is

commonly achieved by utilizing a sensor

with higher resolution. Additionally, the

optical power from the emitters might be

increased to keep the signal-to-noise ratio

stable.

In a more general sense, every free space

beam tends to broaden over distance. This

leads to a decrease of the irradiance over

distance.

The half-angle value is an appropriate

approximation concerning the broadening

and propagation of light in free space in the

so called far field regime2.

Mathematically the irradiance Ee(r) in the far

field drops with the basic relationship Ee(r) ~

1/r2. As a conclusion: A doubling in distance

r reduces the irradiance by a factor of four.

Note that the irradiance depends on the

distance and also on the angle (normal to

the surface of the emitter). The irradiance Ee

is related to the radiant intensity Ie (stated in

the data sheet of the optical source) by the

equation Ie = Ee·r2.

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2 The transition distance between the near- and far field

depends, among other issues, on the package type. For non-

lensed types the transition is usually within the first

centimeter. Devices with a domed lens and narrow angle

might have a transition distance of up to 25 cm. The main

difference between near- and far field lies in the shape and

distribution of the radiation characteristics. For proper

modelling OSRAM provides raytrace files if accurate

information is necessary. An important issue for practical

applications might be that the detector should be located

within the far field. By using non-lensed emitters this is

ensured for almost all applications.

August 13, 2010

page 8 of 14

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