Emitters and Detectors for Infrared (IR) Touchscreens
Application note
1. Introduction
Touchscreens as a popular user interface
are more and more common. Applications
span from public information systems to
customer self-service terminals. Thus, as a
logical step, more and more devices today
feature this kind of user interface, e.g. bank
automatic teller machines (ATMs), personal
digital assistants (PDAs), mobile phones and
PC displays. The widespread popularity is
actively supported by standard computer
based operating systems, like e.g.
Windows
®
7.
The rapid development of CMOS imaging
sensors and the development of high power
infrared (IR) emitters in slim packages have
led to a series of new optical touchscreen
technologies. Many of them contain
proprietary technology and solutions. Tab. 1
presents a general overview of different
technologies and their features.
This paper will give an overview on IR-based
touchscreen technologies with a special
focus on infrared emitting diodes (IREDs)
and photodetectors to be used in such
applications. It shall help touchscreen
designers to select suitable IR components
for their system and provide some general
optoelectronic guidelines.
Traditionally, IR touchscreens have faced
three criticisms: Size, cost, and ambient light
sensitivity. The first two concerns stem from
traditional matrix-based systems. However,
new technology and slim packages enable a
significant decrease in bezel height
combined with a decrease in cost. Camera-
based systems go even further by reducing
significantly the number of parts at the cost
of
added
computing
and
software
Surface
acoustic
wave
+
o
-
+
+
o
-
o
+
o
+
-
m
IR
matrix-
based
++
-
-
+
++
+
++
++
o
+
-
+
m/l
IR
camera-
based
++
++
++
+
++
+
++
++
++
+
o
++
m/l
IR
projector-
based
++
++
++
++
++
+
-
++
++
+
-
+
l
In-cell
optical
++
++
+
o
++
+
-
-
+
+
-
o
s
Feature
Clarity of image
quality
Resolution
Cost effective for
larger screens
Resistance to
vandalism
Stable calibration
Easy to manufacture
Retrofit possibility
Any object can
create a touch
Touch accuracy
Multitouch capability
Ambient light
insensitivity
Sealable, resistance
to dust
Main market
Resistive
-
+
-
-
-
+
++
o
o
-
+
+
s
Capacitive
o
+
-
-
+
o
o
-
+
+
+
+
m
Table 1: Summary of touchscreen technologies and their features.
(++: excellent, +: good, o: ok, -: does not perform well/does not have this function,
screen size: s: small (2” – 10”), m: medium (12” – 30”), l: large (>32”))
August 13, 2010
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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
Fig. 1: Concept of an IR matrix-based
touchscreen. The influence of a stylus on
the photocurrent of individual detector
elements is sketched below.
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.
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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.
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
Fig.
3:
Camera-based
touchscreen
realization with an edge emitting light guide.
The light guide provides a diffuse
illumination of the screen.
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.
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γ
ε
Fig. 4: Principle of projector-based
touchscreen realization. The depicted
system works with diffuse illumination (DI).
Fig. 5: Principle of a FTIR-based touch
detection system. Different IRED coupling
options are sketched.
waveguide usually by several IREDs located
on all sides of the screen. The light is
captured inside the waveguide by total
internal reflection.
If pressure or a touch is applied on the
polymer/acrylic surface due to a stylus or an
object (e.g. finger) light is coupled by FTIR
into the polymer (or into the finger if no
polymer is used), from where it is scattered
and remitted towards the IR sensitive
camera located in the rear part of the
screen. This technique is desired for
applications where IR emission through the
screen should be avoided, e.g. in touch
screens used in TV studios to avoid
interferences or saturation of TV camera
pictures by IR light.
It should be mentioned that FTIR combined
with camera sensors is also used in the
biometrics industry, most notably in
fingerprint scanning applications.
2.6 In-Cell Optical Sensing
2.4 Projector-based Touchscreens
Another group of systems are based on a
projector concept. Due to the setup their
main application is in large screens for
overview or presentation purposes. The
principle of such a technique is presented in
Fig. 4.
Usually the visible image is projected from
the backside onto a diffuse screen. One or
several IR sensitive cameras are mounted
behind the screen to monitor the reflected IR
image of the screen.
To illuminate the screen with IR radiation
there are various options. One makes use of
diffuse illumination (DI) from IR-sources
behind the screen. If a stylus or finger
touches the screen, a reflection occurs and
the IR camera detects the bright spot.
2.5 Projector-based with FTIR
Fig. 5 presents a similar version which works
on the principle of frustrated total internal
reflection (FTIR).
This setup uses the waveguide properties of
e.g. the acrylic glass as a part of the screen
to distribute the IR radiation. Usually a
pressure sensitive polymer layer is added on
top to display the projected image, as acrylic
glass is almost transparent to the visible
image. IR light is coupled into the acrylic
The in-cell optical sensing principle is an
integrated solution. Inside each pixel cell in
a LCD display there is typically a
phototransistor integrated. The principle
works without a designated light source. In a
bright environment the phototransistor sees
the shadow of the finger tip, whereas in a
dark or dim lit ambience the reflections of
the backlight generates the signal. The
absence of an active illumination is also the
drawback of this principle, especially a black
screen in dark environments.
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3. Application Specific Design
Guidelines
3.1 Matrix-based Design
Newly developed slim and cost effective
emitter resp. detector packages allow a
significant reduction of bezel height,
overcoming one of the main drawbacks of
this traditional concept.
The number of employed IREDs depends
mainly on 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
is
also
an
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
of
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 an
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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