Saving Power with Your Smartphone Display

When I was at my local Verizon today looking at phones I couldn’t help but notice that many of the choices for wallpaper were not optimal for saving power, and were chosen for reasons that had more to do with the graphics designer’s idea of what looked good.

Most people now understand that for an OLED display it only consumes power when and where the screen is lit, so a good screen saver would be one that is mostly black.

When it comes to an RGB stripe LCD the best way to save power is to turn the display off when the panel is not in use since the backlight is otherwise turned on.  However, for images that do not have lots of white brightness, Content Adaptive Backlight Control (CABC) in today’s RGB stripe displays will also turn down the backlight and increase the transmission of the LCD.

How about for a PenTile RGBW LCD?  In these displays there is a clear (white) subpixel, which together with improved aperture ratio increases light transmission to save power.  In addition there is an algorithm which we call Dynamic Backlight Control (DBLC) which continually analyzes image data for highly saturated color with high luminance.  The reason for this is that all RGBW displays must pay attention to an effect known as Simultaneous Contrast Error.  Simply stated, if you put a bright and highly saturated color on the screen at the same time as a bright white, it is necessary to supply more power for the display so that the eyes and brain will perceive the correct shade of saturated color.  This is especially true for yellow.   For PenTile displays, to combat this effect, the backlight brightness is turned up to increase the luminance of the colored subpixels while the white level is turned down tomaintain the desired perception of color to make a perfect looking yellow.  In other words the worst case power consumption for a PenTile display is when you are displaying more than a minimal amount of highly saturated colors, regardless of whether it is red, green, blue or a combination of these colors.  This is true even if the image is largely dark.  The best power consumption, other than a totally black screen, would be to use unsaturated colors or simply black and white.  The worst case power consumption for OLED  is the best case power consumption for a PenTile RGBW display.

Saturated Color Wallpaper

Unsaturated Color Wallpaper

 

 

 

 

 

 

 

So if you have one of the new Motorola smartphones with a PenTile RGBW LCD display, you might want to replace the  wallpaper featuring saturated colors with one that is either black and white, or colors of less than 50% saturation.  The power benefits should soon be apparent.

10 comments on “Saving Power with Your Smartphone Display

  1. I think something is wrong in this sentence:

    “Most people now understand that for an OLED display it only consumes power when the screen is totally black, so a good screen saver would be one that is mostly black.”

    Shouldn’t it say, “that an OLED display does not consume power when the screen is totally black”?

    Also, follow up question to the post. So a full black background on the Bionic pentile display would be the most power efficient?

    • Thanks for the catch. You are absolutely correct and I will fix that error.

      As for the Bionic that is a PenTile RGBW LCD. For an LCD the backlight is on almost all of the time. The one exception is that we detect a black screen and turn off the backlight as part of our global dimming. Therefore there is almost zero power consumed for a black screen. The next best situation is to maintain color saturation below 50%. In that case the backlight power is reduced and the LCD is most transmissive leading to power savings of about 50% relative to RGB stripe displays. As color saturation increases to high level we incrementally increase the brightness of the LEDs in the backlight to preserve the quality of color, avoiding what is known as simultaneous contrast error, a situation where the eyes and brain perceive saturated colors as darker than they really are due to the presence of brilliant white.

      Therefore, other than a totally black image, a screen with black and white. or colors of less than 50% color saturation will guarantee you the best battery life.

  2. Almost right, but not quite. What I commented on before was how to save power with PenTile RGBW LCDs such as the Motorola Atrix, Photon, Droid 3, or Bionic. In that case unsaturated colors such as black and white, or even photos or movies (yes they too are almost always unsaturated) save power. That is not the way to save power with PenTile OLED such as the Galaxy Nexus or the Droid RAZR. For the PenTile OLED display the best display is the one with the most black or darker images and the least bright or fully white images. As you can see there is a big difference between the way that PenTile RGBW LCDs work and how PenTile OLEDs work. Perhaps it is time for a new blog on why this is the case.

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