Revisiting electronic ink
Prying Eyes: Electronic ink has changed in the seven years since we last took a look at it.
By Robert Cravotta, Technical Editor -- EDN, 8/7/2008
 Electronic ink has moved into the e-book market. Although the electronic-ink film is flexible, current e-books are implementing a rigid display with a glass cover for physical protection and as many as seven layers of coating for ultraviolet protection of the display. |
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 Electronic ink supports small and low-power displays, such as for USB flash disks and wrist watches. |
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 The electronic-ink film consists of millions of microcapsules arranged in a honeycomblike structure. When you magnify the structure, you can see the microcapsules and how the pigment fragments do not have to align on a one-to-one basis with the microcapsules to form images. The addressing backplane rather than the size of the microcapsules limits the dot-per-inch resolution. |
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The pigment fragments within each microcapsule move independently based on the polarity of the charge on the addressing electrodes that sandwich the electronic-ink film. Higher voltages, such as 15V on the electrodes, result in faster movement of the pigment fragments for refresh rates of 250 msec. Lower voltages would require slower refresh rates to allow the pigment fragments to fully shuttle through the microcapsule fluid. When the addressing electrodes are powered off, the pigment fragments maintain their position within the microcapsule for up to a year without consuming any more power in the system. |
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When
EDN looked at electronic ink seven years ago, there were two visible commercial approaches to delivering display information.
E Ink’s approach has made the larger visible move from the lab to product within the last three years as a display technology; however, the underlying approach has changed somewhat. The approach remains fundamentally the same except that the millions of microcapsules, with a 50-micron diameter arranged in a honeycomb structure, now hold a clear fluid instead of a colored oil, and it employs a two-pigment particle method instead of a single-pigment particle method suspended in the fluid to display information.
In 2001, possible products for electronic ink primarily included signage, and, although signage remains an important opportunity, electronic ink has found its way into e-books. One reason that electronic ink suits use in e-books is that it shares the same viewing characteristics as paper. The two types of pigment particles in electronic ink are carbon for black and titanium dioxide for white, similar to those pigments for paper. Another significant reason electronic ink suits use for e-books is that it consumes power only when changing the display contents; it consumes no energy to retain an image for as much as a year. This passive capacity for retaining a display image makes electronic ink ideal for displays with slow or infrequent content-update rates.
Electronic ink is available as a film that a display integrates. The two pigment fragments are oppositely charged so that, when you apply a positive or a negative field to the electronic-ink film, one set of the pigment fragments moves to the front and visible side of the microcapsule, and the other moves to the back of the microcapsule. Reversing the field’s charge switches the position of the pigment fragments and causes each area of the display to change from white to black and vice versa. The refresh rate depends on the viscosity of the clear fluid and the strength of the addressing field near the ink film. The refresh rate for production electronic ink is 250 msec, whereas, in a lab using a 15V field, the rate can be as fast as 50 msec. In theory, you can refresh the display in the lab by applying a 3.3V field for a longer time, resulting in a slower refresh rate. Using a lower voltage may not necessarily mean a lower overall power consumption because the addressing field must remain active longer to allow the pigments to shuttle between the front and back of the microcapsule.
The contrast of contemporary electronic ink is similar to the ink on a newspaper. The addressing backplane limits the number of available dots per inch with electronic ink because both types of pigment fragments can occupy different parts of the same microcapsule for submicrocapsule resolution. Production resolutions are 167 and 200 dpi in 6- and 5-in. displays, respectively. Researchers have demonstrated display resolutions as high as 397 dpi in the lab with an Epson addressing backplane.
One promising area for electronic ink is in small displays, such as a storage indicator embedded on a USB flash disk or the face of a watch. Current displays are monochrome; using active filters would make color displays available. There is ongoing research in the lab for electronic ink that includes using a three-pigment method.
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