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Where can I found information about the power consumption of an iPhone 4 display? Which technology is used? Does it save battery lifetime when the screen displays black or dark pixels?

Additionally, there is a brightness control in the settings. Does this setting really affect battery lifetime?

I'm asking about the hardware to learn the details so I can develop iOS applications and be sensitive to power consumption based on the settings I apply in our applications.

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migrated from superuser.com Jan 24 '12 at 15:04

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not sure where you can get the specs for it. But yest. Black pixels are non initialzed pixels, and need to be pure black rgb(0,0,0). This however does not save much power as the adjacent pixels need to be set on the same line, so it has to refresh the whole line. You will notice power saving when the brightness is down at 20%.. extends battery up to 40%. But setting 100% black, and turning backligh off will imitate screen lock –  ppumkin Jan 24 '12 at 12:03
    
LCD displays need continuous refreshing. Depending on the technology it may use slightly more energy making black, or slightly more making white (normally-transmissive or not.) –  Thomas O Jan 24 '12 at 12:27
    
usenix.org/event/atc10/tech/full_papers/Carroll.pdf That article gives you a good breakdown of the components of a smartphone and their respective power consumptions. While they didn't specially test an iPhone, it will follow a similar pattern. –  cksum Jan 24 '12 at 16:22
    
Correct me if I'm wrong, but "black = nothing = power saved" was relevant for CRTs, but has not been relevant since the invention of the LCD screen and beyond. –  Jason Salaz Jan 24 '12 at 18:45

3 Answers 3

LCDs (which the iPhones use) use virtually the same amount of power no matter what color is being displayed.

So, no, a black screen will make no practical difference whatsoever. (Although, as someone else mentioned, brightness of the screen definitely will. However, note that brightness is NOT the same as color being displayed).

AMOLED screens DO use less power when displaying black, because when they display black they actually turn that pixel off, and so it uses almost no power. LCD does not behave like this. All LCD does is block the light the pixel emits. Some would even say that it wastes energy in this way.

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Yes - brightness is the major thing you can change to save power if the screen is prevented from dimming. Having the screen off entirely significantly extends the runtime of the battery across the iOS platform.


I haven't found a good reference to determine if the natural state of the IPS transistors that control whether the pixel is open (colored) or closed (blocking or black) but the amount of power needed to activate all of the transistors and let all the backlight through is certainly dwarfed by the amount of power needed to run the backlight itself.

Since the backlight is dimmable, it could be more of a factor when the backlight is set at the lowest light level, but I would expect the actual panel itself to still be negligible compared to the backlight (and more importantly negligible compared to the CPU / GPU and radio power usage)


If you take a look at the official specs you can see for the iPhone 4S:

  • 6 hours: 3G Internet - screen on all the time
  • 8 hours: 3G Calling - screen off most of the time
  • 9 hours: WiFi Internet - screen on all the time
  • 10 hours: Video Playback - screen on all the time
  • 40 hours: Audio Playback - screen off most of the time

It would be interesting to see how long the audio playback test would last if you had a simple app to keep the screen on the entire time...

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Setting a black background will probably not make a big difference on battery life on an iPhone (see some tests with large monitors: http://techlogg.com/2010/05/black-vs-white-screen-power-consumption-24-more-monitors-tested/17).

Reducing the brightness probably will be much more important. That is because the LCD screen is led-backlit all the time and leds are eating up most of the energy.

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