e-Readers use a special display technology on flexible substrates (usually either paper or plastic), that are modified electronically in order to more closely mimic the appearance of printed ink on paper. It requires no energy to leave a text or image displayed on the device’s screen.
Unlike conventional display techniques that require a backlight or the emission of photons, the electronic paper is purely reflective and uses the ambient light in the same manner as conventional paper. An e-ink screen must be able to display text and images indefinitely without consuming any energy, either for display or for possible data processing systems, and must allow for changes in what it displays. Most electronic paper consumes energy only when the displayed content is changed, as when you turn a page in an e-book. The pixels of such a system must have several different stable states, so as to keep intact the content displayed in the absence of power supply.
Electronic paper was developed to overcome some limitations of conventional computer screens. For example, the backlight of some screens may cause eye strain or interrupt sleep cycles when used for long periods of time, whereas electronic paper reflects light just like a sheet of conventional paper would, so these problems are not an issue. It is very easy to read on electronic paper, regardless of the angle at which it is viewed. In addition, electronic paper is lightweight, durable, and very flexible compared to other types of displays (but less flexible than paper). Rochie De Mireasa has written about other technology gadgets on her blog
More technically speaking, e-readers work by constantly arranging and rearranging a series of positively charged white particles and negatively charged black particles. When applying a negative electric field, the white particles are placed on one end of the capsule and black on the other. Placing millions of these capsules on a surface and ordering them by electric fields, results in any arrangement of particles to form words or black and white pictures. A filter array may also be added to allow access to colors. The system can have 4 electric fields to each microcapsule allowing to have 4 levels of gray: 100% white; 75% white and 25% black; 50% white, 50% black; 25% white, 75% black and 100% black.
This system is bistable, a single polarization pulse is sufficient to determine whether the pixel is “on” or not. Moreover, the very high contrast display makes it unnecessary to use direct or indirect lighting, all this results in a significant energy savings. Finally, the support may be semi-flexible, which is a major improvement when compared to other more conventional display methods.
The first electronic paper was developed in the 1970s by Nick Sheridon at the Palo Alto Research Center of Xerox. My friend Colin has written about this on his technology blog. The first commercial electronic paper, called Gyricon, consisted of polyethylene spheres with a diameter of between 20 and 100 micrometers. Each sphere was divided into two parts: a black half-sphere that was negatively charged, and a white hemisphere that was positively charged, forming an electrical field that can be controlled through two electrodes. The spheres are held captive inside of an oil bubble which allows them their freedom to rotate, but not escape the designated area. All of this exists within a transparent silicone sheet. The polarity of the voltage applied to the electrodes of each dipole determines which of the two sides is pointing upwards. You can control each one independently of the others.
In the 1990s, another type of electronic paper was invented in the laboratories of MIT by Joseph Jacobson, co-founder of E Ink (E Ink was acquired in December 2009 by the Taiwanese manufacturer of LCD screens PrimeView International (PVI ). This new paper used clear pockets (microcapsules 40 micrometers in diameter) filled with white particles electrically charged, immersed in colored liquid paraffin. Electronic circuits allowed him to control the position of the white particles to the top of the capsule (white pixel) or the bottom of the capsule (the color pixel of the oil). This technology is strongly reminiscent of the displays based electrophoresis, but the use of microcapsules instead of glass has achieved such displays on flexible plastic sheets.
There may be mentioned, for example, the use of titanium dioxide particles (white) negatively charged and dipped in black dye. The microcapsules were maintained in a layer of transparent liquid polymer sandwiched between two grids of indium-tin oxide electrodes, a transparent conductive material. Each pixel is located at the intersection of two-row electrodes (one for each layer). The sheet was protected by a transparent plastic sheet, the total thickness of which was 80 micrometers (more than twice that of conventional paper).
The electrode array is then connected to an electronic circuit complete with the display manager, which handles the switching of pixels between their white and black states. In a more recent version of this technology, the use of a single electrode layer proved sufficient to control pixels. Further research on electronic paper focused on the use of organic field effect transistors (OFETs) embedded in a substrate flexible7,8, even to trials to install organic transistors directly on traditional paper.
Bistable liquid crystal was another technology developed during this same general time frame. This technology is based on a unique principle called “surface anchoring breaking.” With this technology, the liquid crystal has two stable states, “uniform” status (commonly referred to as”U” or “uniform”) and “turned” state (“T” or “twisted”), each selected simply by applying an electrical signal. Once either state has been selected (white or black), it is retained indefinitely, all without consuming a drop of energy. An electrical impulse along the molecules of the surface changes them by breaking the weak anchoring. The shape of the falling edge of the pulse defines the organization of molecules in a state “U” or “T”. This technology is currently being explored and researched by IT companies around the world.
The leading manufacturer of this unique bistable LCD electronic paper was the French company Nemoptic, which unfortunately went bankrupt in 2010 (it was actually the Seiko Japanese company that manufactured, sub-contracted, and manufactured most of the screens developed by Nemoptic while they were still operating).
Well, that’s enough with the science/history lesson. All my posts won’t be this technical, I promise! The bottom line is that ereaders are a great technology that uses almost no power, is very versatile, and can revolutionize the way we read and consume information.