Why black pixels do not switch off an LCD
An LCD creates its picture by controlling how much light passes through each pixel. Behind the pixels, a backlight shines across the screen; liquid crystals act like adjustable blinds, letting more or less of that light through. A black pixel blocks nearly all the light, but it does not turn off the backlight behind it.
That shared light source is why a dark interface usually has little effect on an LCD’s power use. The backlight is still illuminating the display, whether the pixels are showing a white page or a black one. A dark image can change how much light escapes through the panel, but it does not eliminate the main source of illumination.
Why the same setting can help an OLED
An OLED screen makes light at the pixel level. Each pixel produces its own light, so a black pixel can be switched off rather than covering a backlight. A dark interface can therefore reduce the number of lit pixels—and the energy they use.
The practical difference is the screen’s construction, not the color black itself. On an LCD, dark mode mainly changes the image the pixels form; on an OLED, it can also reduce how much of the display is emitting light. The same black screen can save power on one kind of display and do little on another.
