Why does "flashing" prevent ghosting on E-Ink displays?

Question

Anybody who owns an E-Ink device (like a Kindle) will be familiar with the phenomenon of "flashing" -- basically, when turning a page, the device will first flip all of the pixels to black, then draw a "negative" of the page, and then invert the whole thing.

The Wikipedia page for "Electronic Paper" gives a brief description of the problem, and attributes it to the need to prevent the "ghosting" of the previous image onto the new one. This is corroborated by my own evidence: if I use the KDK to write an application that doesn't flash the screen, the ghosting is evident.

My question is, why does ghosting happen, and why does flashing prevent it? I have a rough understanding of how E-Ink works (thanks to the aforementioned Wiki article), but nothing there explains to me why ghosting occurs, or why reversing the charge a few times alleviates the problem.

Answer 1

A pixel is comprised of tiny balls full of black ink suspended in white fluid, and how black the pixel looks depends on what percentage of the balls are near the top of the fluid. For a black pixel they're ideally all at the top and for a white pixel at the bottom. If only some of them are at the top, or many of them are floating halfway down, etc., the pixel may appear to be some shade of grey. You might think of the floating balls as subpixels.

The balls get to the top or bottom by having an appropriate charge applied to each cell. However, each cell might be influenced by its neighbors as well as the applied charge. To the extent the balls are attracted to charge on a neighboring cell (horizontally) rather than its own cell (vertically) they won't wind up in the intended place. If a cell is changing from black to white and all its neighbors are also, it will transition more completely than if some neighbors are staying black or are going the other direction. This is where ghosting comes from.

The solution is to drive the entire screen white-black-white (or similar) so that no cell has a problem from neighboring cells, and then apply the desired screen image. Every screen write starts with a screen that has been wiped clean so there is no afterimage of the previous screen.

Answer 2

While EInk has patented a black particle in white fluid display, the shipping article is a dual particle system consisting of white particles of one charge and black particles of opposite charge.

These are electrophoretic displays - which is just a fancy way of saying "moving particles through a fluid with an electric field". The particles themselves come pre-charged and the applied voltages creates an electric field to drag the particle around in the display. The particles are prevented from sticking to each other through a process of steric stabilization. The particles are meant to keep their locations in the fluid through the control of viscosity in the fluid.

The particles and fluid are encapsulated in small transparent flexible spheres (they call the black and white spheres in fluid the "internal phase") which are applied in a uniform layer across a TFT panel. The micro-encapsulation is to prevent lateral migration of particles from lateral electric fields caused by neighbouring pixels being at different levels.

The grey scale is determined by the state of white vs. black particle mixture. Because they have opposite charge one can easily see that full voltage one way will pull all the black particle to the top whilst full voltage reversed will pull all the white particles to the top. An intermediate state is a mixture of the two.

Where the problem arises is that there are many possible voltage settings that could potentially produce the same gray-state. The reason is actually quite simple, if for example you have a grey state that is only slightly darker than the whitest white, that means that you only need a few dark particles near the top. Where the rest of the black particles are does not determine the darkness but they will effect the electrical charge state in the cell. You could have all the black particles at the back of the display or all in a layer just under a bunch of white particles.

What this really means is that there is hysteresis in the system and the appropriate voltage to apply to a pixel to get a certain grey-scale will depend very much on it's history. If you have two scenarios 1: you have 5 scenes in a row where you have a pixel being white and then need to drive to black on the 6th frame or 2: if you have 6 scenes in which the pixel is at the same black level. Those two scenarios require different voltages on the pixel when you transition from the 5th to the 6th frame.

The controller that drives these displays tracks the voltage history of each pixel over time, but eventually it runs out of room to be able to hit the right gray-scale in the next frame. What then happens is a display reset in which the pixels are flashed to white then black and then re-written. This starts the tracking of the optical trajectory all over again.

Typically the reset pulse happens every 5 - 8 screen refreshes.

So no, the applied voltage does not inject charge in the system, the charges are already present, they are moved around by the applied voltage. No, the reset pulse is not to correct the adjacent pixel corruption. That is solved by micro-encapulation. This is a two-particle system, not a system of black particles in White ink.

Here is a cross section from a patent USPTO 6987603 B2:

122 = spacer ball to maintain separation of front panel from TFT

104 = the flexible micro-encapsulation - in it's crushed down state in a display

110 = a white/black particle

108 = a black/white particle

118 = TFT electrode

114 = the common (aka Vcom) ITO electrode

Answer 3

The flashing evens out the charge. Without it you have residual charge from the previous page.

By filling the whole page with one charge, then reversing that charge, you are cleaning up that residual charge.