E-ink vs AMOLED: choosing a screen for focus
How e-ink and AMOLED screens work, what that means for power, refresh, ghosting, colour and eye comfort, and which one we would pick for a device built around focus.
When we sketched the Humanly Phone concept in January, the hardest decision was the screen. E-ink vs AMOLED is the central argument in any device meant to help you focus, and we promised to come back to it properly. This is that post: how the two technologies work, what each one is good and bad at, and which claims about them hold up.
A quick reminder that the phone is a concept, not a product, and nothing here is for sale. We are a software studio. We read spec sheets and teardowns like everyone else, and where the evidence is thin we say so.
How e-ink works
E-ink is the common name for electrophoretic displays, and E Ink is the company that makes most of them. The screen is a thin layer of tiny capsules, or cells in some newer designs, each filled with a clear fluid and two kinds of pigment particle: white ones and black ones, with opposite electric charges.
Under the capsules is a grid of pixel electrodes. Above them is a transparent electrode. Apply a voltage one way and the white particles move to the top, so the pixel looks white. Reverse it and the black particles rise instead. Intermediate positions give shades of grey. What you see is pigment, lit by the room, in the same way you see ink on paper.
The key property is that the particles stay where they are when the voltage is removed. The display is bistable: it holds an image with no power at all. A reader can sit on your desk for a week showing a book cover and use nothing to keep it there.
How AMOLED works
An AMOLED screen is built from organic light-emitting diodes. Each subpixel, usually red, green and blue, is a thin stack of organic material between two electrodes, and it glows when current flows through it. "Active matrix" means each subpixel has its own transistors on a thin-film backplane to switch it on and hold its brightness.
On top of the emissive layer sit a thin-film encapsulation to keep out moisture and oxygen, a circular polariser to stop room light bouncing off the metal electrodes, a touch layer and the cover glass. The whole panel is well under a millimetre thick, which is why flexible and folding phones use it.
Because every subpixel makes its own light, a black pixel is simply off. That gives AMOLED its perfect blacks and very high contrast, and it is why a mostly black interface on an AMOLED screen uses less power than a mostly white one.
Power, refresh and ghosting
The two technologies spend power in opposite ways.
E-ink spends power on change. Holding a page costs nothing. Turning a page costs a burst of energy to move the particles. So e-ink is extraordinarily efficient for content that changes rarely, such as a page of text or a map you look at for a minute, and much less so for anything animated. The screen is rarely the biggest drain in an e-ink device. The processor, radios and front light usually use more.
AMOLED spends power on light. Power depends on how many pixels are lit and how bright they are, all the time. A dark interface at modest brightness is cheap. A white web page in sunlight is expensive. Modern panels can drop their refresh rate very low when the image is static, which helps, but an AMOLED screen that is on is always using meaningful power.
Refresh speed is where e-ink pays for its efficiency. Moving pigment through fluid takes time, so a full-quality update takes a noticeable fraction of a second. Fast modes exist and make scrolling and typing usable, but they trade away contrast and leave faint traces of the previous image. That trace is called ghosting. To clear it, the controller periodically does a full refresh, which is the familiar black-and-white flash. Controllers have become much smarter about when and where to do this, and ghosting is less of a problem than it was ten years ago, but it has not gone away.
AMOLED refreshes in milliseconds. Video, maps that pan with your thumb and a camera viewfinder all work without thinking about it. This is the main reason we kept AMOLED for the phone concept: calls, directions and the camera all want a fast screen.
Colour and light
Colour e-ink exists, and it has improved a lot. The most common approach puts a colour filter array over a black-and-white panel, which is simple and fast but gives muted colours and slightly lower resolution in colour areas. Other designs use several coloured pigments in each cell and produce richer colour, but they refresh much more slowly. Both are good for book covers, charts and comics. Neither looks like a phone screen, and nobody claims they do.
The other difference is where the light comes from. E-ink is reflective. In daylight it looks better the brighter it gets, and in direct sun it is excellent. In the dark it needs a front light: LEDs along the edge push light into a thin light guide on top of the screen, which spreads it down onto the pigment. You are still looking at reflected light, but it is light from a lamp built into the device, and many readers let you shift it towards warm tones.
AMOLED is emissive. It is great in a dim room and good indoors. In direct sunlight it has to push very high brightness to compete, which costs battery and still does not look as calm as paper.
What we can say about eye comfort
This is the part where marketing gets ahead of the evidence, so we will be careful.
Many people find e-ink more comfortable to read on for long periods, and some studies of reading on e-ink versus backlit screens support that for long sessions. It is plausible that a reflective surface with no flicker, lit by the same light as the rest of the room, is simply closer to paper. But the research is mixed and a lot of it is small. We would not claim e-ink prevents eye strain.
Blue light is a common claim, and it is weaker than it sounds. Screen light is far too dim to damage your eyes, and major ophthalmology bodies do not recommend blue-light blocking for eye health. There is better evidence that bright light late in the evening can shift sleep timing, but that is about brightness and timing more than the type of screen. A dim, warm front light at night is probably better than a bright phone. A bright front light is probably no better than a dim phone.
Flicker is a real, specific issue with some AMOLED screens. Many panels dim by switching pixels on and off rapidly (pulse-width modulation), and at low brightness some people notice headaches or eye strain from it. Some phones now use higher PWM frequencies or dimming modes designed to reduce this. E-ink does not flicker while static, which is one point in its favour that does not depend on anyone's marketing.
Most eye strain from screens comes from how we use them: long sessions, not blinking enough, holding the device too close, not looking away. No panel fixes that.
Which screen for which job
After all of that, the choice comes down to the job the device has to do.
| Long reading | E-ink. Low glare, no flicker, battery for weeks. |
|---|---|
| Notes and sketching | E-ink, if you accept a little lag. |
| Calls and messages | Either works. Messages barely care about refresh. |
| Maps and directions | AMOLED. Panning and zooming need speed. |
| Camera | AMOLED. A viewfinder has to be live. |
| Outdoors in sun | E-ink looks better. AMOLED works at high power. |
| Dark room | AMOLED with a black interface, or e-ink with a warm, low front light. |
There is also a quieter argument. An e-ink screen is slow in a way that discourages the behaviour focus devices are trying to reduce. Endless scrolling feels bad on it, and video barely works. That friction is part of the appeal. An AMOLED screen can be made calm with a strict black interface and very little on it, which is what we drew for the phone, but the calm comes from the software. Remove the software discipline and it is a normal phone screen again.
That is why we landed where we did. A phone has to handle a camera and turn-by-turn directions, so it gets AMOLED and a lot of self-restraint in the interface. A device whose main job is reading and writing is a different story. We are working on a concept for exactly that, a reading slate, and we will publish it later in the year. We expect it to have an e-ink screen, and most of the interesting design problems start with that choice.