Your phone display's brightness consumes up to half of its power. And chances are it's being horribly mismanaged, thanks to bad engineering and a backwards user interface. Here, Dr. Raymond Soneira of DisplayMate explains Brightnessgate for iPhone 4 and Android.
Although consumers currently don't pay much attention to them, the Automatic Brightness control and Light Sensor on smartphones 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.
As we will demonstrate below with extensive lab measurements, brightness controls are a sham of incredibly poor design and engineering—Automatic Brightness in a great majority of smartphones 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.
How Automatic Brightness Works—In Principle
Smarphones 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 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. 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 smartphones 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. 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 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 smartphones 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.
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
We've already shown that Automatic Brightness is important and is currently functionally useless on smartphones for many reasons. We've also discussed some of the changes needed for the Ambient Light Sensors, but by far the most important factor is getting the user interface for screen brightness to work properly so that consumers can use it to adjust the screen brightness based on their own visual preferences, in a natural fashion that automatically implements and tweaks the screen brightness they would like to see for different ambient lighting conditions. That will maximize viewing comfort, screen readability, energy efficiency and battery run time if it's done correctly.
Right now the user interface for brightness controls is completely backwards—the Light Sensor measures the ambient light and the smartphone or HDTV adjusts the screen brightness based on some ad hoc and mysterious algorithm based on an earlier user setting of a brightness control. The solution is very simple – do it in the opposite way – the consumer initially adjusts the screen brightness manually to whatever they want for the current ambient lighting. The Ambient Light Sensor then measures this light level. The value is recorded and used to interpolate the screen brightness whenever the ambient lighting changes. In principle, only two such user settings are needed to train the Automatic Brightness for a linear interpolation as shown in Figure 1. Whenever the user doesn't find the current screen brightness to their liking, they manually tweak the brightness and the new value and ambient light level are used to update the Auto Brightness calibration.
There is one more thing. To make this work smartphones need a convenient brightness control to tweak and train the Automatic Brightness. Every 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. 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. 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.
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 firstname.lastname@example.org.