Texas Instruments | OPT3001/-Q1:Automatic control of Display backlight using an Ambient Light Sensor | Application notes | Texas Instruments OPT3001/-Q1:Automatic control of Display backlight using an Ambient Light Sensor Application notes

Texas Instruments OPT3001/-Q1:Automatic control of Display backlight using an Ambient Light Sensor Application notes
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Automatic control of Display backlight using an Ambient
Light Sensor with Human-eye response
When you think of displays, what do you think of?
Traditionally interpreted as a part of televisions and
computers, display technology is now everywherefrom thermostats, handheld terminals, automotive
infotainment systems, automotive clusters, to
electronic point of sale machines (EPOS) and more.
People like the display to be lit comfortably so it is
easy to see and not strain the eyes. In many of these
applications, ambient light sensors (ALS) are used to
set the display brightness based on the surrounding
light conditions. Depending on the environment a
product is designed to be used in, the problems
present in designing an ambient light sensor into these
products are trying to match human eye response,
reaching the correct IR rejection due to conditions
under dark glass, and maintaining long battery life.
Texas Instruments provides not only the OPT3001 to
solve these problems, but also the OPT3001-Q1 that
is an automotive qualified device to be used in
infotainment systems.
A problem with display backlights in portable
electronics is the need to provide a clear picture in all
ambient light conditions while simultaneously
optimizing the power consumption to preserve battery
life. The same problem occurs with display backlights
in stationary applications, such as thermostats, but the
environmental conditions are varied. Stationary
applications must provide a clear picture in a variety of
light conditions, like florescent light, incandescent light,
or no light at all. Ambient light sensors from TI are
being designed into products such as these to provide
information to the display about the ambient light level
surrounding the display. This light level can then be
used to automatically adjust the display brightness via
the backlight, providing just the right brightness level
for clear viewing, preserving power and avoiding end
user frustration that the display is too bright or too dim.
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Figure 1 shows a typical block diagram for a display
application.
Figure 1. Display Block Diagram
The end goal behind using ALS in any display
application is to be able to measure what a human
sees, so the display can be adjusted accordingly. Just
as the human eye adjusts under different light sources,
display brightness needs to change from one light
source to another. If these different light sources are
not accounted for it could result in user discomfort as
well as a non-optimum display power.
The photodiodes (PDs) in most light sensors are not
as responsive as the human eye due to the wide
spectrum of wavelengths the PD measures. The
human eye naturally has high IR rejection and the
OPT3001 matches the response of the human eye
and includes significant infrared rejection. The IR light
that is captured during a sunset for example is
interpreted as “high ambient” by a regular photodiode,
which leads to increased display brightness. The
OPT3001 will interpret the light source and provide
optimal display brightness. See Figure 2 for the typical
behavior.
Automatic control of Display backlight using an Ambient Light Sensor with
Human-eye response
Copyright © 2017, Texas Instruments Incorporated
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due to the wideband of the photodiode. The OPT3001
has a very high IR rejection and discards most of the
IR light reaching the actual sensor, measuring only
what is in the visible region. See Figure 4 for graphical
data. The Input Spectrum is the same graph seen in
Figure 3 of the different light sources. Green Curve:
Human Eye, Pink Curve: Florescent, Blue Curve:
Incandescent, Black Curve glass response.
Figure 2. Typical Photodiode Response
The OPT3001 response matches human eye
response. It detects the ambient light precisely and
enables optimum control of backlight, which leads to
extended battery and display life as well as an
improved user experience. Daylight is around 100k
lux, outdoor shadow is around 10k, and night is around
50 lux (200x less than shadow). To get the same
performance in each scenario, the backlight LED
current can be reduced by 200x during night. See
Figure 3 for OPT3001 Response data.
Figure 4. IR Rejection
The strong IR rejection also aids in maintaining high
accuracy when the design calls for mounting the
sensor under dark glass for aesthetic purposes. The
OPT3001 is designed for systems that create lightbased experiences for humans, and is an ideal
replacement for photodiodes, photoresistors, or other
ambient light sensors with less human eye matching
and IR rejection.
Figure 3. OPT3001 Response
To help achieve the reaction of the human eye the
OPT3001 also has an auto-gain setting feature that
adjusts automatically based on input light level. No
additional adjustments are needed, unlike those
required when using a discrete solution. The device
always stays in optimal range with good resolution and
tight accuracy between ranges. The relative accuracy
between gain ranges is 0.2%. This allows for optimum
display under all light levels.
The OPT3001 and OPT3001-Q1 devices are sensors
that measure the intensity of visible light and are
suitable for display applications. The spectral response
of the sensor tightly matches the response of the
human eye, includes significant infrared rejection. The
OPT3001 maintains performance regardless of light
source or application.
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Another problem that can occur with backlight display
is that light measurement changes under different light
inputs. Dark glass, for example, transmits IR. Sources
like incandescent light have high IR that reaches the
sensor due to glass transmissivity. If a photodiode
were to be used, there is a higher risk for inaccuracy
1.1
Related Documentation/Learn More
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OPT3001 Product Folder:
http://www.ti.com/product/OPT3001
OPT3001-Q1 Product Folder:
http://www.ti.com/product/OPT3001-Q1
OPT3001EVM:
http://www.ti.com/tool/opt3001evm
Trademarks
All trademarks are the property of their respective owners.
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Automatic control of Display backlight using an Ambient Light Sensor with
Human-eye response
Copyright © 2017, Texas Instruments Incorporated
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