ePaper

The development of electric paper (ePaper) has proved to be a surprisingly difficult challenge. Electric paper should be paper-like: reflective and easy to read. You should be able to write on it, and magically, your writing would be transformed into the ascii characters you'd written. Electric paper should have pleasing tactile characteristics - pliable, bendable, foldable - it should feel like a sheet of paper. Given these requirements, it's not surprising that so little progress has been made.

This talk describes some of the earliest work to create a paper-like display. The majority of the work was done at Xerox PARC. We focused on the electronic and material properties of the display rather than its tactile characteristics. Three interesting, patentable ideas were developed.

One of the most difficult challenges we confronted was the need to make a near perfect, defect-free surface of picture elements (pixels). Any approach to creating a paper like display built using transistor technology must provide some solution to the near certainty of fabrication defects. A single defect in a bus line will result in a line defect: a row or column of pixels that are inoperative. Line defect artifacts are among a class of visual impairments that are particularly noticeable to the human eye.

The human retina is composed of a complex matrix of light-sensing cells and neuronal interconnection cells. They are not simply sensors that relay intensity information to processing centers in the brain. They are also cross-linked to provide low level processing of the image projected onto the retina. Retinal sensor cells are linked to their neighbors via inhibiting synaptic connections. That is, they are linked in a winner take all configuration, in which the sensor receiving the most light inhibits its neighbors thus standing out by pushing his neighbor's output lower. This innate retinal wiring makes us exceptionally well enabled to detect lines and edges. The transition between regions in Mach Bands provides a good illustration of this perceptual phenomenon.

Our ePaper used an ink paradigm to create images. It used a special, electric ink composed of microscopic, chemically treated particles of ordinary white house paint (titanium oxide) suspended in a dark blue solution of liquid Freon. The chemically treated paint particles (the paper component) acquire an ionic charge that enables them to be pulled to or away from the viewing side of the display. Because of the ionic nature of this approach, the displays are stable without sustaining power.

The displays are based on an innovative active matrix circuit substrate: an integrated circuit at each pixel. Each pixel is composed of four redundant circuits that are spatially dispersed to resist common fabrication defects. The electrostatic field within each defined pixel is time multiplexed. This enables each pixel to exhibit gray-scale uniformly across each pixel or to provide localized blending to support anti-aliasing.