Bright smartphone displays drain the battery much faster than dark ones. This is just as little in doubt as the relationship between the pressure on the accelerator pedal and the fuel consumption. But even the colors displayed by a display affect the energy requirement.
The principle itself sounds easy to understand, at least in the case of the OLED display: the colors of a pixel are generated using tiny subpixels that specialize in certain colors and whose power requirements vary. If a pixel should glow red, only the red subpixel shines in it; with a white-looking pixel, on the other hand, all subpixels must be switched on, which you only see in high magnification. In a black pixel, all subpixels are simply dark and then need almost no power. The very varying construction of the subpixel matrix determines which of the tiny LEDs must be lit.
Most smartphones today have an OLED display, but some instead have a liquid crystal display, or LCD for short. With them an always active and therefore power-hungry backlight radiates liquid crystals, which are modulated according to the intended color scheme and brightness. Even if the LCD is to display a black pixel, the backlight is active. This suggests that the power consumption of LCDs does not depend on the color to be displayed. Our measurements show that this is not so easy.
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