SFH4050_ 查看數據表（PDF） - OSRAM GmbH
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SFH4050_ Datasheet PDF : 14 Pages
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In some enhanced applications it might be
worth to consider an external lens in front of
the emitter to create a slim IR curtain only
above the screen. This might also be
advantageous concerning power saving
issues, as it allows reducing the IRED drive
current.
Exemplary Calculation
The following simple calculation illustrates
above relations in a matrix-type application.
The emitter is a SFH4650-V (90 mW/sr at
100 mA), the detector a phototransistor
SFH309FA-4. The detector is located axially
on the opposite screen side, inside the
bezel. Unfavorable ambient IR light
conditions incident on the detector are
assumed to be an equivalent irradiance of
25 µW/cm2 (e.g. shadowing the detector
behind the bezel and bright incandescent
light bulbs near the screen).
To achieve a signal-to-noise ratio (SNR) of
at least 6 at the detector, an Ee = 0.15
mW/cm2 is required (for a comfortable IPCE
≈ 0.45 mA). Using Ie = Ee·r2 yields for a 7-
inch screen (9.1 cm x 15.5 cm) a necessary
radiant intensity of 12 mW/sr (vertically
emitting) and 36 mW/sr (horizontally).
According to the data sheet a drive current
IF of 13 mA resp. 40 mA is required.
Increasing the screen size from 7-inch to 14-
inch and keeping the SNR stable results in
an increase in the required radiant intensity
by a factor of four (roughly quadrupling the
operating current – in this particular case it is
necessary to operate under pulsed
conditions).
In the above 7-inch screen example, the
crosstalk from a neighboring IRED (spaced
2 cm apart) is around 25 % of the signal for
the vertical grid (the angular arrangement
causes the radiation characteristics to drop
to 60 % resp. the detector’s sensitivity to 40
%). This value rises to over 80 % if the
screen size doubles. This demonstrates the
importance of sequentially pulsed operation
of larger matrix-based touchscreens. As an
alternative, a proper mechanical design
which shadows the detectors from
neighboring light, especially with increasing
screen sizes resp. resolution is advised.
More focused emitter / detector pairs are
recommended anyway to avoid x-y crosstalk
and reduce interference from ambient light.
4.5 Power Supply Considerations
If the electrical power supply is limited (e.g.
battery powered applications or USB
connected touchscreens with a max. 500
mA supply) it is recommended to operate
the IREDs in pulsed mode. Synchronization
with the detector is advised for better signal-
to-noise ratio and lower overall power
consumption. Please note that in pulsed
operation the IREDs can be operated at
higher drive currents resulting in higher
optical peak powers compared to continuous
operation. Please refer to the data sheet for
details.
5. Product Selection Guide
Tab. 2 - 7 present a short product selection
guide which highlights products and product
families of OSRAM which are suitable for IR
touchscreen applications. For many package
variations top- or side-emitting options are
available.
Please note that this guide provides just a
general overview. For more detailed
information and the latest products and
updates please visit www.osram-os.com.
6. Literature
[1] OSRAM-OS:
http://www.osram-os.com.
[2] G. Kaindl: Exploring multi-touch
interaction. VDM Verlag Dr. Müller, 2010.
[3] Interactive Displays Conference:
http://www.int-displays.com.
[4] LLFY-Network:
http://www.ledlightforyou.com
August 13, 2010
page 9 of 14
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