BrightnessGate for the iPhone & Android Smartphones and HDTVs
Why Existing Brightness Controls and Light Sensors are
Effectively Useless
Wasting Lots of Energy and Battery Power,
Reducing Screen Readability, and Causing Eyestrain
Dr. Raymond M. Soneira
President,
DisplayMate Technologies Corporation
Copyright © 1990-2010 by DisplayMate Technologies Corporation. All
Rights Reserved.
This article, or any part thereof, may not be copied,
reproduced, mirrored, distributed or incorporated
into any other work without the prior written permission of
DisplayMate Technologies Corporation
This article is Part II of an article series on the iPhone 4 Retina
Display,
the Samsung Galaxy Super AMOLED Display, and the Motorola Droid.
Overview
Although consumers currently don’t pay
much attention to them, the Automatic Brightness control and Light Sensor on
smartphones and HDTVs has a major impact on displayed image quality, screen
viewability and readability, as well as preventing eye strain and headaches
when the screen is too bright or too dim for the current level of ambient
lighting, which varies considerably. But for many consumers, organizations and
even governments, it is their impact on power consumption that generates the
greatest concerns and emotions. Most smartphones and HDTVs run with the screen
considerably brighter than it should be, which wastes a lot of power in addition
to causing eyestrain. On smartphones the display can use as much as 50 percent
of the total phone power, which has a major impact on the phone’s running time
on battery – a big concern for all users. On HDTVs the display can use as much
as 75 percent of the total TV power, which can be over 200 watts. Since there
are 330 million TVs in the US, and they are on over 600 billion hours per year,
that adds up to a considerable amount of wasted energy, money, and oil – so
even federal and state governments get concerned… And there are other important
benefits to reducing display power – it lowers the internal heat and
temperature of both smartphones and HDTVs, which reduces component aging,
failure rates and repairs. But for all of its advantages Automatic Brightness
still needs to properly control the screen brightness over the very wide range
of ambient lighting, otherwise the consumer will disable it and just park the
brightness close to maximum. Unfortunately, that is what happens most of the
time – here’s why…
From the above discussion, you would
think that a properly functioning Automatic Brightness would be a big deal in
the design, engineering, and marketing of smartphones and HDTVs – since they
are extremely competitive, especially the battery run time on smartphones, plus
every manufacturer is claiming to be environmentally “Green.” As we will
demonstrate below with extensive lab measurements for smartphones (and HDTVs in
a later article) this is instead a sham of incredibly poor design and engineering
– with Automatic Brightness in a great majority of smartphones and HDTVs being
effectively useless. And because they don’t work properly consumers simply turn
them off altogether, making matters even worse for power use, screen
readability and viewing comfort. This deserves to be called BrightnessGate – a
scandal with several causes…
You’re wondering how this could
possibly be the case – it’s because Automatic Brightness is being treated by
manufacturers as just a marketing feature for brochures and spec sheets (and
cursory Energy Star operating cost estimates for HDTVs). As a result it isn’t
given the engineering attention and expertise that it needs in order to
function properly and effectively, so that it is actually useful, and then
actually used by consumers – this latter point is the only one that matters!
It’s the Rodney Dangerfield function of smartphones and HDTVs because it “gets
no respect.” This article will show you why that needs to change…There are many
problems that need to be corrected – some are in the Automatic Brightness
control software and its user interface, which can be fixed with downloadable
updates, and some have to do with the light sensor, which will have to wait for
the next generation of smartphones and HDTVs. We’ll also outline how a fully
functional Automatic Brightness should work, including an interactive user
interface that adapts automatically to the user’s personal screen brightness
preferences. That is the key to its future success and to gaining the respect
and appreciation of consumers, manufacturers and even governments.
How Automatic Brightness
Works – In Principle
Both smartphones and HDTVs have a light
sensor located in the bezel right next to the screen that measures the ambient
light together with control software that appropriately raises or lowers the
screen brightness based on the measured light level. If you are watching in the
dark the screen should be appropriately dim. When the ambient light level is
higher the screen needs to be made appropriately brighter for two reasons:
because of glare from ambient light reflected off the screen, which washes out
the image, and because the eye’s light sensitivity decreases substantially as
the ambient light level increases. Unfortunately, none of the above currently works
properly in smartphones and HDTVs for the following reasons:
The Light Sensor
In a smartphone the light sensor is
facing your head and is measuring the brightness of your face instead of the
ambient light level that is behind and to either side of the phone, which is
what actually sets your eye’s light sensitivity and what should be determining
the brightness level of the screen. Similarly, in HDTVs the light sensor is
measuring the light level behind the TV viewers instead of the light that is
behind and on either side of the TV, which again determines the eye’s light
sensitivity level. The existing front facing light sensors are good for
measuring and correcting the image for glare from screen reflections (by
modifying the display transfer functions), but not for setting the screen
brightness. To do that in both smartphones and HDTVs a rear and side facing
ambient light sensor with a different angular profile than the current
Illuminance sensors is needed for future hardware designs. Note that a front sensor
for glare is not as important since screen reflectance can be very low – around
5 percent for many smartphones and HDTVs – see Part I of this
article.
Automatically Adjusting
the Brightness
The screen brightness
needs to be set carefully and systematically based on the data from the light
sensor. Here the smartphones and HDTVs fail again with poor and even bizarre
behavior that we document below. Another sign of careless engineering – all
three of the smartphones that we tested have operational bugs or errors with
their Automatic Brightness. One essential feature missing from both smartphones
and HDTVs is allowing users to interactively adjust the display for their own
visual preferences on how the screen brightness should vary as the ambient
light changes – and it should be accomplished automatically as we’ll outline
below. Some people and applications prefer a brighter or dimmer screen, and
some people are willing to put up with a dimmer screen that may not be as easy
or comfortable to read – in return for longer battery running times. So it’s
important to implement a properly functioning Automatic Brightness that
automatically adapts to the user’s own brightness preferences – otherwise it
will be disabled by the user.
Results Highlights
Here are the main results from our
extensive labs tests and viewing tests on three smartphones that we performed
to evaluate the Automatic Brightness Controls and Light Sensors under a wide
range of ambient lighting conditions.
Determining the Optimum Screen Brightness
The first step in evaluating Automatic
Brightness is to determine how the screen brightness should change with ambient
light level for optimum viewing. To demonstrate the proper relationship I read
an article from the New York Times on the iPhone 4 under a wide range of
ambient lighting conditions. I turned Auto-Brightness Off and then manually
adjusted the screen brightness for my own optimum viewing comfort – not too
dim, not too bright, just right – for each of 7 different ambient light levels,
from total darkness up through moderate outdoor lighting levels. After each
reading I measured the Ambient Light Brightness (Illuminance in lux) and the
screen’s Brightness (white Luminance in cd/m2). The results appear
as the black data points in Figure 1 along with a solid black trend line. At about 1,000 lux (which is at the low end of
outdoor lighting levels) I reached the maximum screen brightness for the iPhone
4, which is 541 cd/m2 – it is the brightest mobile display I have
ever measured, but above 1,000 lux the iPhone 4 can’t provide as much screen
brightness as I would like to have. The screen is still readable well beyond
10,000 lux (which is full daylight that is not in direct sunlight) but it gets
increasingly hard to comfortably make out the contents of the screen at the
higher ambient light levels.
The optimum screen brightness values
will vary due to personal preferences, and also with screen size and viewing
distance, but the proportional linear increase with ambient brightness
indicated by the solid black line in Figure 1 should be similar for everyone.
The dashed black lines in Figure 1 also show a wide range of alternative
brightness relations – the dashed lines labeled Dim and Very Dim are for
aggressive power savings at high ambient lighting or for people with more
sensitive eyes, and the Bright relation is for people or applications that need
particularly high screen brightness with ambient light. We’ll explain how to
automatically implement all of this functionality below. Now let’s look at the
Apple iPhone 4 and two Android phones (Samsung Galaxy S and HTC Desire) to see
how they perform…
Figure 1.
The measured Screen
Brightness for various measured Ambient Brightness levels. The manually
determined optimum brightness settings are the black data points with their
trend line. The values for five different Auto-Brightness slider settings of
the iPhone 4 are labeled Auto Minimum to Maximum. Circles are the data points.
The dashed lines show a wide range of alternative brightness relations. The
graph is linear from 0 to 2,000 lux and then jumps in steps to 10,000 and
100,000 lux. The labels from Pitch Black to Direct Sunlight roughly identify
the lux levels associated with them. The maximum Luminance of the iPhone 4 is
541 cd/m2.
iPhone 4 Auto-Brightness
Next, I turned Auto-Brightness On and
then measured the screen brightness (white Luminance cd/m2) that the
iPhone 4 produces under a wide range of ambient light levels, from 0 lux (Pitch
Black) up through 100,000 lux (Direct Sunlight). When Auto-Brightness is turned
On the Brightness slider adjusts the Auto behavior to allow consumers (in
principle) to set their own individual screen brightness preferences for
ambient light. To evaluate this, I measured 5 different settings of the slider:
Maximum, ¾, ½ (center), ¼ and Minimum. The results are plotted as the colored
lines in Figure 1 – the circles are the measured data values. None of the Auto
Brightness settings even remotely approaches the desired behavior discussed
above. It certainly looks as if no one at Apple ever bothered to set or check
Auto-Brightness for useful performance, which is why there are lots of user
comments questioning how it works on the web… This is BrightnessGate for the iPhone…
The iPhone 4 comes from the factory
with the Brightness slider set to ½ (center) and with Auto-Brightness turned
On. At 2,000 lux, where just about everyone will want the display operating at
maximum brightness, Auto-Brightness sets it to only 60 percent of maximum, so
Auto-Brightness is throwing away 40 percent of the precious brightness needed
for screen visibility. And at 10,000 lux, which is full daylight, the screen
brightness is still below 90 percent of maximum. The ¾ setting is much too
bright and power wasteful for all indoor viewing and yet it still throws away
20 percent of the screen brightness at 2,000 lux for outdoor viewing. The
Maximum setting is useless because it varies the screen brightness (and power)
by less than 10 percent and the ¼ and Minimum settings are far too dim to be
useful for humans.
The iPhone 4
Auto-Brightness performs in a bizarre fashion where it typically makes the
screen too bright at lower indoor ambient light levels (which is important for
saving battery power) and too dim at higher outdoor levels (which is important
for screen readability) – it’s always wrong, usable but very inefficient and
wasteful. But BrightnessGate for the iPhone gets even worse…
iPhone 4 Auto-Brightness
Bug
One behavior of the iPhone 4
Auto-Brightness that is a serious operational error or bug is that it locks
onto the brightest ambient light sensor value that it has measured at any point
starting from the time it was turned on, and then continues to use that highest
value indefinitely to set the screen brightness until the display turns off –
either by cycling through sleep mode or full power off. This means that the
screen brightness is frequently set too high, which wastes power and can cause
eye strain if you move to lower ambient light levels. Auto-Brightness should
always follow the current ambient light level (with appropriate time averaging
and filtering). Apple should correct this with a software update. To easily
verify this behavior with your own iPhone turn On Auto-Brightness under
Settings and set the Brightness slider near the middle of its range. Go to a
very dark location. Click the sleep/wake button on the top of the phone to turn
the display off. Then wake it up with the sleep/wake button or the Home button.
Note the screen brightness in the dark. Now take the phone to a very bright
outdoor location (such as in direct sunlight) then go back (with the display
on) to your original dark location and monitor the screen brightness. The
display will remain at very high brightness indefinitely until the iPhone
enters sleep mode again (or runs out of battery). What’s even more shocking is
that BrightnessGate
is even worse on Android phones…
Figure 2.
The measured Screen
Brightness for various measured Ambient Brightness levels for the Samsung
Galaxy S and HTC Desire. The Manual Optimum relation and other elements are the
same as in Figure 1.
Android Automatic
Brightness
There are currently a large number
smartphones running Google’s Android OS, and all of the models that we have
looked at appear to work in the same way. There is a slider for manual
adjustment of screen brightness, but when Automatic Brightness is enabled the
slider disappears and there aren’t any user settings or preference adjustments
(unlike the iPhone 4) – you get whatever screen brightness settings Android and
the smartphone manufacturers have pre-programmed into them. Unfortunately,
those Automatic Brightness settings are incredibly primitive and crude – on the
Samsung Galaxy S and HTC Desire that we lab tested Automatic Brightness
produces only four fixed screen brightness levels when the ambient lighting
changes from pitch black all the way up to direct sunlight, with each
manufacturer setting their own breakpoints as shown in Figure 2. For this
reason alone, Auto Brightness is effectively useless for Android. But BrightnessGate on
Android gets even worse…
Android Automatic Brightness Bugs
Both of the Android phones we lab
tested have their own Auto Brightness operational errors or bugs. On the
Samsung Galaxy S two of the four Android Automatic Brightness levels are set
ridiculously high: 7,000 and 30,000 lux – they are about a factor of 10 too
high to be useful. The Galaxy S screen brightness remains at an incredibly low
170 cd/m2 up until near Full Daylight, only about 50 percent of the
screen brightness that it can deliver, and it waits up until almost Direct
Sunlight to move up to it’s maximum screen brightness of 305 cd/m2.
Since there are no available settings or adjustments it’s better to leave the
Automatic Brightness permanently off until this gets fixed with a software
update. The HTC Desire has a somewhat better choice of brightness level
breakpoints than the Galaxy S, but it has a bug similar to the iPhone – once
the light sensor detects a light level over 100 lux it won’t allow the screen
below Android brightness Level 2 until the display is cycled off by going into
sleep mode using the power button or Screen timeout.
Conclusion for the Current
Auto Brightness
Automatic Brightness on existing
smartphones is close to functionally useless because the manufacturers have not
made the effort required to develop, evaluate and test the software and
hardware so that they work properly and effectively. All of the models we
tested also have serious operational errors and bugs indicating how little an
effort has been made to make them work (or rather not work) properly. It’s
clear that most manufacturers are using ad hoc implementations instead of
methodical science and engineering, which is shameful and shocking… As a result
most smartphones are operating without Auto Brightness because consumers
disable them when they don’t work properly, which means the screen brightness
is seldom set correctly for the wide range of ambient lighting conditions that
most smartphones experience. It also means that the display is very likely set
by the consumer to a perpetual high screen brightness. As a result the battery
runs down much sooner than if the brightness and power were actively and
intelligently managed automatically, as they should be. We outline how to do
that next…
How Automatic Brightness
Should Work
There is one more thing… to make this
work smartphones and TVs need a convenient brightness control to tweak and
train the Automatic Brightness. Every TV and smartphone in the solar system has
a convenient Volume Control but in most cases you have to go down a couple of menu
levels to get to a cumbersome Brightness Control. My suggestion for all
smartphones: temporarily shift the Volume buttons to Brightness buttons by
pressing both the + and – buttons at the same time – which will activate a
temporary Brightness Shift. It’s fast, convenient and easy, and then have them
automatically time out and shift back to Volume Controls when you’re done
adjusting the brightness. This same suggestion applies to TV remote controls –
use a shift button to temporarily convert the Volume Control buttons into
Brightness Control buttons. Every display needs a convenient external
Brightness Control – not buried under several levels of menus. In all cases
it’s best to implement it using the existing Volume Control together with an
appropriate shift button.
The above is guaranteed to work nicely
and conveniently for all consumers, solve BrightnessGate, maximize viewing
comfort, screen readability, energy efficiency and battery run time all
together. For HDTVs it will lower your electric bill and even make a dent in
oil imports… I hope the manufacturers are listening…
Special Thanks to Jay Catral and Konica Minolta Sensing
for their instruments and technical support. To measure the Ambient Light Brightness (Illuminance in lux) we used a Konica
Minolta T-10 Illuminance Meter and for screen Brightness (Luminance in cd/m2)
we used a Konica
Minolta CS-200 ChromaMeter.
About the
Author
Dr. Raymond Soneira is President
of DisplayMate Technologies Corporation of Amherst, New Hampshire, which
produces video calibration, evaluation, and diagnostic products for consumers,
technicians, and manufacturers. See www.displaymate.com.
He is a research scientist with a career that spans physics, computer science,
and television system design. Dr. Soneira obtained his Ph.D. in Theoretical
Physics from Princeton University, spent 5 years as a Long-Term Member of the
world famous Institute for Advanced Study in Princeton, another 5 years as a
Principal Investigator in the Computer Systems Research Laboratory at AT&T
Bell Laboratories, and has also designed, tested, and installed color
television broadcast equipment for the CBS Television Network Engineering and
Development Department. He has authored over 35 research articles in scientific
journals in physics and computer science, including Scientific American. If you
have any comments or questions about the article, you can contact him at dtso.info@displaymate.com.
About
DisplayMate Technologies
DisplayMate Technologies specializes in
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technology see the Summary description of our Adaptive Variable Metric
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Article
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Copyright © 1990-2010 by DisplayMate Technologies Corporation. All
Rights Reserved.
This article, or any part thereof, may not be copied,
reproduced, mirrored, distributed or incorporated
into any other work without the prior written permission of
DisplayMate Technologies Corporation
