E-paper in color – how does it work?

We have already talked about e-paper many times. This is a topic that becomes more and more interesting with the passage of time and the rapid development of this technology. We have presented the more interesting EPDs (Electronic Paper Display) but also the history of the beginnings and evolution of such solutions, as well as their exemplary applications in selected environments. We have mentioned color variants of e-paper almost everywhere – now it’s time to take a closer look at them.

To describe how the color EPDs work, it is necessary to first look at the basic representatives of this technology – their black and white counterparts. Today, most e-paper displays are based on the prototype created by Joseph Jacobson, the later founder of E Ink. Its modified variants are the basis of modern e-paper solutions. Their operation is based on the phenomenon of electrophoresis, i.e., the movement of dispersed particles relative to a fluid under the influence of a spatially uniform electric field.

Electronic ink is made of microcapsules the size of human hair. Each contains positively charged white particles and negatively charged black particles that are suspended in a clear liquid. The capsules are also placed in liquid to form “ink”, which covers the electronic circuit. When a voltage is applied in each cell, the pigment molecules move up or down, creating the desired image.

An additional color

It was only a matter of time before grayscale-only solutions were no longer sufficient. The need for EPD modules using a wider range of colors began to emerge in both industrial applications and consumer electronics. The E Ink company – a world leader in e-paper technology at that time, holding that title to this very day – began to react quickly by introducing innovations to the market.

The first color solutions in the E Ink offer were Spectra displays, in which, apart from black and white pigments, a third one was introduced – a choice of yellow or red. Examples of such solutions in Unisystem’s offer are the USE0154-BWYH-01 (yellow) and USE0154-BWRH-01 (red). With the next generations of Spectra EPDs, it was possible to use both of the colors mentioned above at the same time.

From one extra pigment through 4096 colors…

In 2010, E Ink introduced the Triton display that was able to present content using grayscale and 4096 colors. The possibility of displaying such a large number of such in the case of e-paper was possible by using an RGB filter in the module itself. The reflected light takes on a red, green, or blue color, which, when combined, can create the above-mentioned color palette. However, the degree of saturation and contrast is limited with this technology. Both the first Triton and its next-generation – Triton 2 were used in e-book readers, but they did not gain a wide fan base.

The Kaleido display, the successor of the Triton, was presented in 2019. The basis of this solution was again a black and white e-paper display with advanced color filters applied. This time though, they were made of plastic instead of glass. In 2021, E Ink introduced an improved model – Kaleido Plus (also known as Kaleido 2 or New Kaleido). These types of EPDs are perfect for color e-readers –  not only for books, magazines, or comics but also, e.g., reports with tables and diagrams.

…to 32 000 different colors.

The two last models presented by E Ink turned out to be a breakthrough in the developing of this technology. The Advanced Color ePaper (ACeP) design was based on the use of four pigments (cyan, magenta, yellow and white) in each capsule. The particles are not identical; they differ, e.g., in size, polarity, and charge – changes in voltage cause the appropriate colors to shift to the display surface. Thanks to such a solution, the color filter on the screen’s surface were no longer needed. ACeP can display up to 32,000 colors – this is currently the most perfect solution developed so far, reproducing the full range of colors on e-paper.