E-Paper Technology Explained
Unlike traditional displays that use liquid crystals to form an image, E-Paper displays use pigmented ink which is a bit like the toner ink used in printers. In a similar fashion, the ink is “printed” on the screen using electric charge, which gives a paper-like appearance - hence the name.
How the technology works
An E-Paper screen is made up of tiny microcapsules. Each microcapsule contains positively charged white particles, and negatively charged black particles, suspended in a clear fluid. When a negative or positive electric field is applied, corresponding particles with opposite charge move to the top or bottom of the microcapsule. By forming different patterns, this makes images visible to the viewer.
Due to the technology used in E-Paper, a field of electric charges needs to be applied on the whole screen in order to form an image. Unlike LCDs that constantly need to refresh, with E-Paper a full screen refresh is only applied once, and the image is retained even after the power is turned off. The only disadvantage is it takes a bit longer for an image to appear on screen, and usually needs to cover the whole screen size. The reason for this delay is because the image needs to be refreshed several times to ensure no residual pixels remain, a process which takes 1-4 seconds for mono, and 10-22 seconds for colour E-Paper.
Fortunately, there is a technique for partially refreshing an E-Paper screen, which requires less time and only affects the area of the screen where the image appears. So the question you may ask is, why do we need a full refresh, when the display can be partially refreshed?
Full refresh vs partial refresh
Regardless which method is used, an electric field needs to be applied to the whole screen. Unlike full refresh, the partial refresh technique is more sophisticated as it makes a comparison between the current image on the screen, and the one that needs to be displayed next. Once a partial refresh is initiated, the only area of the screen that will be updated whilst the whole screen refreshes, is the area that is different between the two images.
Considering the need to pull the ink particles to the top or bottom of the screen, a partial refresh is usually “weaker”, as it is faster and covers a small area of the screen. Therefore, a full refresh is still required from time to time, usually after 10 partial refreshes, to stabilise the ink particles and retain a clear image. It should be noted that not all E-Paper displays support partial refresh, especially those with three colours, due to the structure and controller IC capabilities.