Electronic paper, e-paper and electronic ink are a range of display technology which are designed to mimic the appearance of ordinary ink on paper.[1] Unlike conventional backlit flat panel displays which emit light, electronic paper displays reflect light like ordinary paper. Many of the technologies can hold static text and images indefinitely without using electricity, while allowing images to be changed later. Flexible electronic paper uses plastic substrates and plastic electronics for the display backplane.
Electronic paper is often considered to be more comfortable to read than conventional displays.[2] This is due to the stable image, which has no need to be refreshed constantly, a wider viewing angle, and that it reflects ambient light rather than emitting its own light. An ideal e-paper display can be read in direct sunlight without the image appearing to fade. The contrast ratio in available displays as of 2008 might be described as similar to that of newspaper, though newly-developed displays are slightly better.[3] There is ongoing competition among manufacturers to provide full-color ability.
Applications of electronic visual displays include electronic pricing labels in retail shops, and digital signage,[4] time tables at bus stations,[5] electronic billboards,[6] mobile phone displays, and e-readers able to display digital versions of books and e-paper magazines. Electronic paper should not be confused with digital paper, which is a pad to create handwritten digital documents with a digital pen.
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Electronic paper was first developed in the 1970s by Nick Sheridon at Xerox's Palo Alto Research Center. The first electronic paper, called Gyricon, consisted of polyethylene spheres between 75 and 106 micrometres across. Each sphere is a janus particle composed of negatively charged black plastic on one side and positively charged white plastic on the other (each bead is thus a dipole).[7] The spheres are embedded in a transparent silicone sheet, with each sphere suspended in a bubble of oil so that they can rotate freely. The polarity of the voltage applied to each pair of electrodes then determines whether the white or black side is face-up, thus giving the pixel a white or black appearance.[8] At the FPD 2008 exhibition, Japanese company Soken has demonstrated a wall with electronic wall-paper using this technology.[9]
In the simplest implementation of an electrophoretic display, titanium dioxide (titania) particles approximately one micrometer in diameter are dispersed in a hydrocarbon oil. A dark-colored dye is also added to the oil, along with surfactants and charging agents that cause the particles to take on an electric charge. This mixture is placed between two parallel, conductive plates separated by a gap of 10 to 100 micrometres. When a voltage is applied across the two plates, the particles will migrate electrophoretically to the plate bearing the opposite charge from that on the particles. When the particles are located at the front (viewing) side of the display, it appears white, because light is scattered back to the viewer by the high-index titania particles. When the particles are located at the rear side of the display, it appears dark, because the incident light is absorbed by the colored dye. If the rear electrode is divided into a number of small picture elements (pixels), then an image can be formed by applying the appropriate voltage to each region of the display to create a pattern of reflecting and absorbing regions.
Electrophoretic displays are considered prime examples of the electronic paper category, because of their paper-like appearance and low power consumption.
Examples of commercial electrophoretic displays include the high-resolution active matrix displays used in the Amazon Kindle, Barnes & Noble Nook, Sony Librie, Sony Reader, and iRex iLiad e-readers. These displays are constructed from an electrophoretic imaging film manufactured by E Ink Corporation.
The EPD technology has been developed also by Sipix and Bridgestone/Delta. SiPix Imaging Inc. is now part of AU Optronics Corp, one of the three largest LCD-panel manufacturers in the world. Other than E-Ink's 0.04mm-diameter micro-capsule structure, Sipix's is 0.15mm-diameter microcup.[10] On the other side, Bridgestone Corp.'s Advanced Materials Division has been cooperating with Delta Optoelectronics Inc. in developing the Quick Response Liquid Powder Display (QR-LPD) technology.[11][12] The Motorola MOTOFONE F3 was the first mobile phone to use the technology, in an effort to help eliminate glare from direct sunlight during outdoor use.[13]
Electrophoretic displays can be manufactured using the Electronics on Plastic by Laser Release (EPLaR) process developed by Philips Research to enable existing AM-LCD manufacturing plants to create flexible plastic displays.
An electrophoretic display forms visible images by rearranging charged pigment particles using an applied electric field
In the 1990s another type of electronic paper was invented by Joseph Jacobson, who later co-founded the E Ink Corporation which formed a partnership with Philips Components two years later to develop and market the technology. In 2005, Philips sold the electronic paper business as well as its related patents to Prime View International. This used tiny microcapsules filled with electrically charged white particles suspended in a colored oil.[14] In early versions, the underlying circuitry controlled whether the white particles were at the top of the capsule (so it looked white to the viewer) or at the bottom of the capsule (so the viewer saw the color of the oil). This was essentially a reintroduction of the well-known electrophoretic display technology, but the use of microcapsules allowed the display to be used on flexible plastic sheets instead of glass.
One early version of electronic paper consists of a sheet of very small transparent capsules, each about 40 micrometres across. Each capsule contains an oily solution containing black dye (the electronic ink), with numerous white titanium dioxide particles suspended within. The particles are slightly negatively charged, and each one is naturally white.[8]
The microcapsules are held in a layer of liquid polymer, sandwiched between two arrays of electrodes, the upper of which is made transparent. The two arrays are aligned so that the sheet is divided into pixels, which each pixel corresponding to a pair of electrodes situated either side of the sheet. The sheet is laminated with transparent plastic for protection, resulting in an overall thickness of 80 micrometres, or twice that of ordinary paper.
The network of electrodes is connected to display circuitry, which turns the electronic ink 'on' and 'off' at specific pixels by applying a voltage to specific pairs of electrodes. Applying a negative charge to the surface electrode repels the particles to the bottom of local capsules, forcing the black dye to the surface and giving the pixel a black appearance. Reversing the voltage has the opposite effect - the particles are forced from the surface, giving the pixel a white appearance. A more recent incarnation of this concept requires only one layer of electrodes beneath the microcapsules.[15][16]
Electro-wetting display (EWD) is based on controlling the shape of a confined water/oil interface by an applied voltage. With no voltage applied, the (coloured) oil forms a flat film between the water and a hydrophobic (water-repellent) insulating coating of an electrode, resulting in a coloured pixel.
When a voltage is applied between the electrode and the water, the interfacial tension between the water and the coating changes. As a result the stacked state is no longer stable, causing the water to move the oil aside.
This results in a partly transparent pixel, or, if a reflective white surface is used under the switchable element, a white pixel. Because of the small size of the pixel, the user only experiences the average reflection, which means that a high-brightness, high-contrast switchable element is obtained, which forms the basis of the reflective display.
Displays based on electro-wetting have several attractive features. The switching between white and coloured reflection is fast enough to display video content.[17]
It is a low-power and low-voltage technology, and displays based on the effect can be made flat and thin. The reflectivity and contrast are better than or equal to those of other reflective display types and are approaching those of paper.
In addition, the technology offers a unique path toward high-brightness full-colour displays, leading to displays that are four times brighter than reflective LCDs and twice as bright as other emerging technologies.[18]
Instead of using red, green and blue (RGB) filters or alternating segments of the three primary colours, which effectively result in only one third of the display reflecting light in the desired colour, electro-wetting allows for a system in which one sub-pixel is able to switch two different colours independently.
This results in the availability of two thirds of the display area to reflect light in any desired colour. This is achieved by building up a pixel with a stack of two independently controllable coloured oil films plus a colour filter.
The colours used are cyan, magenta and yellow, which is a so-called subtractive system, comparable to the principle used in inkjet printing for example. Compared to LCD another factor two in brightness is gained because no polarisers are required.[19]
Examples of commercial electrowetting displays include Liquavista,[20] ITRI,[21] PVI and ADT.[22][23]
Electrofluidic displays are a variation of an electrowetting display. Electrofluidic displays place an aqueous pigment dispersion inside a tiny reservoir. The reservoir comprises <5-10% of the viewable pixel area and therefore the pigment is substantially hidden from view.[24] Voltage is used to electromechanically pull the pigment out of the reservoir and spread it as a film directly behind the viewing substrate. As a result, the display takes on color and brightness similar to that of conventional pigments printed on paper. When voltage is removed liquid surface tension causes the pigment dispersion to rapidly recoil into the reservoir. As reported in the May 2009 Issue of Nature Photonics, the technology can potentially provide >85% white state reflectance for electronic paper.
The core technology was invented at the Novel Devices Laboratory at the University of Cincinnati. The technology is currently being commercialized by Gamma Dynamics.
Other research efforts into e-paper have involved using organic transistors embedded into flexible substrates,[28][29] including attempts to build them into conventional paper.[30] Simple color e-paper[31] consists of a thin colored optical filter added to the monochrome technology described above. The array of pixels is divided into triads, typically consisting of the standard cyan, magenta and yellow, in the same way as CRT monitors (although using subtractive primary colors as opposed to additive primary colors). The display is then controlled like any other electronic color display.
Examples of electrochromic displays include Acreo,[32] Ajjer,[33] Aveso[34] and Ntera.[35]
Electronic paper technologies have a very low refresh rate compared to other low-power display technologies, such as LCD. This prevents producers from implementing sophisticated interactive applications (using fast moving menus, mouse pointers or scrolling) like those which are possible on mobile devices. An example of this limit is that a document cannot be smoothly zoomed without either extreme blurring during the transition or a very slow zoom.
Another limit is that a shadow of an image may be visible after refreshing parts of the screen. Such shadows are termed "ghost images", and the effect is termed "ghosting". This effect is reminiscent of screen burn-in but, unlike it, is solved after the screen is refreshed several times. Turning every pixel white, then black, then white, helps normalize the contrast of the pixels. This is why several devices with this technology "flash" the entire screen white and black when loading a new image.
Electronic paper is still a topic in the R&D community and remains under development for manufacturability, marketability, and reliability considerations.
Several companies are simultaneously developing electronic paper and ink. While the technologies used by each company provide many of the same features, each has its own distinct technological advantages. All electronic paper technologies face the following general challenges:
Electronic ink can be applied to flexible or rigid materials. For flexible displays, the base requires a thin, flexible material tough enough to withstand considerable wear, such as extremely thin plastic. The method of how the inks are encapsulated and then applied to the substrate is what distinguishes each company from others. These processes are complex and are carefully guarded industry secrets. Nevertheless, making electronic paper promises to be less complex and costly than making traditional LCDs.
There are many approaches to electronic paper, with many companies developing technology in this area. Other technologies being applied to electronic paper include modifications of liquid crystal displays, electrochromic displays, and the electronic equivalent of an Etch A Sketch at Kyushu University. Advantages of electronic paper includes low power usage (power is only drawn when the display is updated), flexibility and better readability than most displays. Electronic ink can be printed on any surface, including walls, billboards, product labels and T-shirts. The ink's flexibility would also make it possible to develop rollable displays for electronic devices.
Other proposed applications include cloths, digital photo frames, information boards and simply keyboards (useful for less represented languages or special non alphabetical applications like video editing or games).
