ePaper displays are very different from other technologies such as LCD, TFT, and (O)LED. They have their advantages but also their limitations. Due to their operating principle, ePaper displays offer viewers a contrast ratio similar to that of paper and excellent, flicker-free readability from any viewing angle (180°), even in direct sunlight. Further, power consumption is extremely low compared to other technologies, as the displays only need power when the displayed content is refreshed. This means the display remains totally visible even when the current and voltage are zero. Depending on the actual size of a display, only a few microwatts of power are required to refresh the content.
ePaper displays are, therefore, ideal for battery-powered and wearable (IoT) applications. Current applications include supermarket shelf labels and price tags featuring product information. The evolution of ePaper from monochrome to color display is giving this technology a whole new boost.
New application areas thanks to full-color display
N color ePaper displays from Holitech are based on all-in-one IC technology and are able to provide a color image with seven basic colors even without the use of a color filter. The displays comprise micro-capsules. Each of these capsules contains four color particles (magenta, yellow, cyan, and white) dissolved in a transparent liquid. The color pigments are charged differently: yellow, cyan, and white negative, magenta positive. By applying a specific voltage, each micro-capsule displays a corresponding combination of the color particles and thus a specific hue (Fig. 1).
The individual pixels are controlled by means of matrix displays. If different voltages are applied, the corresponding color capsules move to the surface, where they become visible to the viewers (Fig. 2). Combining the seven basic colors (red, green, blue, cyan, magenta, yellow, white) enables the display of colored images. The first available N color ePaper display is 7.3" tall and has a resolution of 800 × 480.
Extensive application areas despite physical limits
ePaper displays have two limiting factors that are decisive for their application areas. First off, the operating temperature for ePaper is limited. For a black/white ePaper, it ranges from 0 °C to 50 °C, and for the N color version, from 15 °C to 35 °C. This is due to the physical properties of the micro-capsules. Outside this temperature range, ePapers do not work properly or might even be damaged. Outdoor applications are, therefore, only possible to a limited extent; heating is highly recommendable at temperatures below –20 °C. Moreover, direct exposure to UV light has a negative impact. The sun’s rays dry out the ink capsules and cause irreparable damage. However, UV blocking films offer a quick-fix solution.
The second limitation is the relatively sluggish content refresh rate as a result of particle rearrangement. The full refresh rate for complete renewal of image information is in theory 28 seconds, but in practice it is significantly less. N color technology is, therefore, well suited for displaying information that does not have to be refreshed regularly.
Retail, logistics, and industry are already benefiting from this cutting-edge display option. Typical applications include electronic shelf labels (ESL) and price tags that are controlled from a central location via a bus system, for example in supermarkets or warehouses where labels need adjusting from time to time due to changing stock or products. Smart home and IoT applications are also an obvious choice for ePaper displays. For example, they can be used to indicate the occupancy of meeting rooms or rooms in hospitals, or to display passenger information and schedules in railroad stations and on public transport.
The full-color display will also open up new application areas for ePaper displays, for example as menus in restaurants. They are usually made of high-quality paper, and even minor changes result in a lot of reprinting. When using N color displays, it is easy to update and adjust prices, menu details, and ingredient information electronically. Further, the dishes and drinks available can be shown both pictorially and in written format on the display. Needless to say, the new full-color ePaper displays will inspire designers to come up with whole new ways of using them.
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