When Amazon dropped the first Kindle on the world in 2007, it single-handedly created the e-reader market. Even though devices like the Kindle and the Kobo can’t compete witha high-end tabletin terms of 24-bit color depth or 60Hz refresh rate, they blow those devices out of the water when it comes to battery life by taking a different approach to their displays.
A tale of two screens
The screens that you’re used to on your tablets and phones are called emissive displays. Most phone screens use liquid crystal display (LCD) or organic light-emitting diode (OLED) technology. In LCD screens, an array of LEDs is used as a backlight. That light is filtered by a layer of millions of tiny red, green, and blue LCD subpixels to create an image. OLED displays use thin layers of organic materials that emit light when electricity is applied to them. Thin strips of red-, green-, and blue-light emitting OLEDs are arranged on a screen to create subpixels.
From a consumer perspective, the primary difference between LCD and OLED display technology is that OLEDs have better contrast between dark and bright colors due to the lack of a backlight. But, because both technologies rely on producing light to create an image, the displays can be indiscernible in bright light, especially direct sunlight.
Electronic readers use a different kind of display technology called e-paper. Instead of emitting light via LEDs, they reflect the ambient light around them. So, instead of being harder to see in intense light, images become easier to see. Reflective display technology can be achieved in several ways, but the e-reader on your coffee table is almost certainly powered by an E Ink electrophoretic display.
E Ink is the name of the company that produces the displays used in modern e-readers; e-ink is a generic term for the pigments used in electrophoretic displays.
One E Ink to rule them all
At the heart of E Ink’s e-paper displays are millions of microcapsules filled with electrically charged white and black ink particles suspended in a transparent oil. The negatively charged white particles act as the white on a page, while the positively charged black particles act as the ink. A top and bottom layer of electrodes sandwiches the ink-filled capsules and controls where the ink is displayed via an electric field. The process of using an electric field to control the movement of particles in a fluid is known as electrophoresis.
The nature of electrophoresis also gives rise to the significantly longer battery life e-readers have compared to devices that use LCD screens. Whereas LCDs require continuous power input (most of it powering the LED backlights), electrophoretic displays only use power to refresh the display. That means as you read your e-book, the e-reader is in sleep mode between page turns, and if it runs out of power, it holds its last image indefinitely.
To achieve color display, commercial e-readers use the same trick that color camera sensors use to detect color: a color filter array (CFA). A CFA is an overlay with a pattern of red, green, blue, and clear color filters, only allowing specific wavelengths of light to pass through. Beneath the CFA, the e-paper is limited to displaying either black or white pigment, but by activating white pigment beneath red and blue portions of the CFA, the e-paper can be made to appear purple.
CFAs aren’t without problems. One of them is that they reduce the overall resolution of the display. Between four and nine black-and-white pixels are needed to produce one color pixel. Another problem is a loss of brightness since light is lost passing through the CFA, reflecting off of the e-pigments, and passing through the CFA again.
The newer Kaleido displays mitigate these problems in a few ways but don’t eliminate them. The first trick E Ink uses is to place the CFA closer to the ink layer of the screen. Instead of printing on a glass substrate as it did for the Triton displays, the CFA is printed on the plastic covering the electrodes. Perhaps most importantly, the Kaleido displays have a front light designed to reflect the most light possible back to the reader.
Despite these improvements in design, CFA-based e-paper displays are darker than screens without CFAs. The latest color displays from E Ink eliminate the CFA and use an e-ink with four pigments (cyan, magenta, yellow, and white) instead of two. These pigments are controlled electrophoretically, but E Ink uses particles of different sizes and charges and replaces the spherical capsules used in its traditional paper with trapezoidal cups, which allow for finer control over the vertical positions of the pigments in the cups. Tablets with this technology, which E Ink calls Gallery 3, are just now popping up in consumer devices.
The wide world of e-paper
There’s no question that E Ink and its displays rule the e-reader marketplace, but alternatives to its ubiquitous electrophoretic screens exist. Some of these reflective display technologies were sold in e-readers at one time, some never made it to market, and at least one is available for purchase right now.
Interferometric modulation
Qualcomm’s Mirasol display came closest to finding a niche in the marketplace. Mirasol operated on the principle of interferometric modulation to produce full-color bistable images. Essentially, as light reaches the display, some of it is reflected from the surface, and some is reflected from subsurface mirrors. The array of mirrors is positioned so that the light waves reflecting off it interfere with the light waves reflected from the surface. Depending on the position of the mirrors, the interference can create a visible color (red, green, or blue) or modulate it out of the visible spectrum (black). This optical principle is also responsible for iridescence.
Only a handful of devices using the Mirasol technology made it to market. None of them were widely successful despite Mirasol having a more vibrant color than what E Ink offered at the time and a faster refresh rate. Qualcomm officially discontinued Mirasol production in 2013.
Electrophoresis
In the early days of e-readers, Bridgestone (yes, the tire company) developed a technology called liquid powder display, which it marketed as Aerobee. Instead of suspending the pigment in a fluid as E Ink did, Aerobee used air, making the response time of its display faster than contemporary E Ink displays. Bridgestone targeted the business market rather than the consumer market with its Aerobee displays but abandoned e-paper technology entirely in 2012 without releasing a product.
Reinkstone and Topjoy released e-readers based on a technology calleddisplay electronic slurry(DES). It suspends its pigments in plasma instead of oil or air and separates them with “cofferdams” instead of capsules or cups. According to its distributor, Good Display, DES screens can operate under a wider range of temperatures, with higher resolution, sharper contrast, and lower power draw. Despite this potential, DES has yet to find market success, with theReinkstone R1 coming in at $700to mixed reviews. The Topjoy Butterfly, which wasfully funded on Kickstarter back in 2021, still has funders who have not received their devices (and the ones who have aren’t pleased with what they received).
A cofferdam is an enclosure built in water to allow dry working conditions on sea and river beds.
Reflective Liquid Crystal Display
Of all the reflective display technologies that have challenged E Ink over the years, only reflective LCD (RLCD) is available in consumer devices. The science behind RLCDs is similar to emissive LCDs in that an LCD layer is responsible for creating the red, green, and blue subpixels on the screen, but instead of an LED backlight illuminating those subpixels, the layer behind the LCD reflects the ambient light.
You might remember this type of display from the One Laptop per Child initiative or the Pixel Qi displays that found their way into a few products. Recently, RLCD has been used in smartwatches like the Amazfit Bip and the Garmin Fenix. Modern RLCD technology is often dubbed “transflective” because it allows for the transmission of LED light through the reflective layer of the display, making it visible in low-light scenarios.
The latest transflective displays also incorporate memory-in-pixel (MIP) technology. The idea behind MIP devices is each subpixel has a one-bit memory circuit, meaning that it doesn’t have to be updated every refresh if what it’s displaying doesn’t change, making MIP-compatible transflective displays bistable and therefore power efficient. However, the largest MIP-based RLCD screens only measure around four inches, so they’re too small for an e-reader or a tablet.
You can buy the RLCD-equipped e-reader,the Eyemoo EPaper S1, for $500. It runs on Android and can display the full 16.7 million colors of the RGB color space. You can still find the Hisense Q5, a monochrome RLCD tablet, for as low as $400 on eBay.
There can be only one
There are a few reasons that E Ink’s electrophoretic displays have dominated the consumer market. First off is Amazon. It had the capital resources to buy hundreds of thousands of screens for its Kindle devices, meaning that E Ink was flush with cash to develop its displays. Amazon also had the digital infrastructure in place to make iteasy to get books, making it easy to overlook that it existed in a closed ecosystem. It also didn’t hurt that Oprah named it one of her favorite things in 2008.
For any other technology to find a niche, it will have to survive long enough to work out the kinks in the first generation of displays. The manufacturers making e-readers will also have to create reliable devices with open software ecosystems. The only reason Kindle gets away with being so locked down is because of the massive Amazon library. Finally, it must offer something that E Ink doesn’t, whether that’s brighter colors or lower prices. Consumers need a reason to try something new. Until that happens,the best e-readers on the marketwill continue to be made with E Ink.
