SFH4740_ データシートの表示（PDF） - OSRAM GmbH
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SFH4740_ Datasheet PDF : 14 Pages
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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.
August 13, 2010
page 2 of 14
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