AMOLED, true black and calm devices
How AMOLED pixels work, why a true black interface saves power on OLED screens, when it does not (bright content, PWM flicker, burn-in) and how dark UI design can make a device calmer.
On an AMOLED screen, true black is not a colour. It is a pixel that has been switched off. That single fact explains why dark interfaces can save battery on OLED phones, why they look so deep, and why the savings are smaller than people often claim. This post goes through how the pixels work, where true black helps, where it does not, and why we care about it for calm devices like our Humanly Phone concept.
How an LCD makes black
Start with the older technology, because the contrast is the whole point. A liquid crystal display has a light source at the back, the backlight, which is usually a sheet of white LEDs behind a diffuser. In front of that sits a layer of liquid crystals between two polarising filters, then a colour filter with red, green and blue subpixels.
The liquid crystals act as shutters. Apply a voltage and they twist the light so it passes through the front polariser. Change the voltage and they block it. To show black, an LCD keeps the backlight on and closes the shutters as far as it can.
Two consequences follow. First, the shutters are never perfect, so some light leaks through and LCD black is really a dark grey. You see it in a dark room as a faint glow. Second, the backlight draws roughly the same power whatever is on screen. A black image and a white image cost about the same. Mini-LED and local dimming improve this by splitting the backlight into zones, but each zone still covers many thousands of pixels.
How OLED and AMOLED work
An OLED panel has no backlight. Each subpixel is a tiny organic light-emitting diode that produces its own light when current flows through it. More current, more light. No current, no light.
AMOLED stands for active-matrix OLED. The "active matrix" is a thin-film transistor layer, the backplane, that sits behind the organic layer and controls every subpixel individually. It holds each pixel's state between refreshes, which is what makes large, high-resolution OLED screens possible. Almost every OLED phone, watch and laptop screen you can buy is an AMOLED panel, whatever the marketing name.
So on an AMOLED screen, black means the pixel is off. It gives no light, so contrast in a dark room is effectively infinite. And it draws close to no power. The backplane still needs a little, and the display driver runs regardless, but an unlit pixel costs next to nothing.
Why true black saves power, and by how much
Because each pixel pays for its own light, an OLED screen's power draw depends on what it shows. Roughly, it scales with how many pixels are lit and how brightly. Different colours cost different amounts too. Blue subpixels are the least efficient, and white needs all three subpixels at full output, so white is the most expensive colour you can show.
That is why dark mode saves battery on OLED devices and does almost nothing on LCD ones. The size of the saving is where people tend to exaggerate. A 2021 study from Purdue University measured dark mode on several OLED phones and found large display power savings at full brightness, on the order of 40 percent, but only single-digit percentage savings at the 30 to 50 percent brightness most people actually use indoors. The display is also only one part of a phone's power budget. Radios, the processor and background work often matter more.
The other lesson is that "dark" and "true black" are not the same. A dark grey background like #121212 still lights every pixel, just dimly. Pure #000000 turns them off. On an interface that is mostly background, that difference shows up in the numbers.
When it does not help, or hurts
True black is not free of trade-offs, and anyone designing for OLED should know them.
Bright content wipes out the saving. A web page with a white background, a photo, a map or a video lights most of the screen whatever your theme is. On a white page, an OLED screen can draw more power than a comparable LCD. The benefit of dark UI only shows up if the content is dark too.
PWM flicker. Many OLED panels dim by switching pixels on and off very quickly, called pulse-width modulation, instead of lowering the current. At low brightness, the off periods get longer. Some panels do this at a few hundred hertz, and some people notice it as eye strain or headaches, even when they cannot see the flicker. Newer phones increasingly use much higher PWM frequencies or DC-like dimming at low brightness, which helps, but it varies a lot by model. Ironically, a dim dark theme is exactly where PWM tends to be at its worst.
Burn-in. Organic materials wear with use, and they do not all wear at the same rate. Blue ages fastest. A static element shown bright for thousands of hours, such as a status bar, a navigation bar or a logo, can leave a faint permanent ghost. Manufacturers fight this with pixel shifting and compensation, and it is much less of a problem than it was a decade ago, but a design that puts the same bright shape in the same place all day is asking for it.
Black smear. A pixel that is fully off takes slightly longer to switch on than one that is already dimly lit. When dark content scrolls or moves fast, some panels show a brief purple or grey trail. Some designers use a very dark grey instead of pure black for moving surfaces to avoid it.
Readability. Bright white text on pure black is very high contrast, and some readers, particularly people with astigmatism, see it halo or bloom. Off-white text works better than pure white, and body text is more comfortable slightly dimmer than headings.
Dark UI for calm devices
We care about true black less for the battery and more for what it does to attention. A screen that is mostly off is quiet to look at. When only the things that matter are lit, the lit things carry meaning. That is the idea behind the Humanly Phone concept: a black interface with off-white text and one orange accent for the one thing that needs you right now.
If you are designing for that kind of device, here is what we would suggest:
- Use pure black for the background, but not for everything. Very dark grey is fine for surfaces that move, such as sheets and lists, where smear would show.
- Make text off-white, around 85 to 90 percent brightness, not full white. It reads better and costs less.
- Keep lit areas small. No full-width bright banners, no large coloured buttons. Outlines and text do the same job with far fewer lit pixels.
- Move static elements. A clock or indicator that shifts a few pixels over time is kinder to the panel.
- Use one accent colour and use it rarely, so it means something when it appears. Warm accents lean on the red and green subpixels, which are more efficient than blue, a small bonus.
- Test at low brightness on real hardware, where PWM and near-black gradation problems show up.
What this means for the phone concept
The Humanly Phone is a concept, not a product, and it is not for sale. Any battery figures we have given for it are estimates on paper. But the display logic is the most solid part of it. A home screen with fewer than a hundred lit characters on true black, no wallpaper and no animated content would spend far less on the display than a typical phone, because most of the panel would simply be off.
The weak point is everything that is not the home screen. The camera viewfinder, maps and photos are bright by nature, and no theme changes that. So the real design rule is less about colour and more about time: keep the screen dark by default, and keep bright moments short. We will look at the other side of this, e-ink versus AMOLED for a calm device, when we return to the phone's hardware later in the year.