Mastering e-Paper displays (Part 1)

Introduction

E-paper displays (EPDs), also known as e-ink screens, were for a long time reserved for very specific applications, such as e-readers.

However, technological advances in recent years have greatly expanded their possibilities, allowing EPDs to now be found in digital signage, electronic shelf labels, IoT devices, information panels, and many other uses.

In addition, the arrival of new display models and controllers has brought this technology into the maker world, making it much more accessible for experimentation and personal projects.

This tutorial, divided into two sections— fundamentals and applications — is designed as a practical, step-by-step guide to help you navigate the fascinating world of EPDs. In the first section, divided into four parts, we’ll explore how they work and what’s behind their unique behavior; in the second section, we’ll apply this knowledge through three practical projects focused on real-world use cases.

The Magic of EPDs

Take a look at the image below: an e-paper display (EPD) showing color graphics. At first glance, it doesn’t seem all that special, does it?

But look again closely. Do you notice anything unusual? The display is completely disconnected! The flexible cable on the right isn’t attached to any control board or power source. So how is it still displaying an image?

Here’s another amazing fact: I used this display a year ago, and since then it’s been stored away in a box, completely unpowered. How is it possible that a display can keep its content intact after a whole year without any energy at all?

No other display technology can achieve such a feat. It’s precisely this remarkable ability to maintain an image with zero power consumption that gives e-paper its truly unique and almost magical quality.

Features and Applications

This ability to retain an image without power isn’t the only standout feature of EPDs:

• They don’t emit light; instead, they reflect ambient light.
• They offer excellent readability, displaying sharp images even in brightly lit environments.
• Since the image doesn’t need constant refreshing and there’s no backlight, they cause significantly less eye strain than other types of displays.
• They’re environmentally friendly, thanks to their extremely low energy consumption.

This unique combination of features has not only made EPDs the go-to technology for e-readers, but has also driven their adoption in a wide variety of other applications.

The Science Behind the Magic

E-paper displays have almost magical qualities, but they operate based on a scientific principle known as the electrophoretic effect.

The electrophoretic effect refers to the movement of electrically charged particles within a fluid under an electric field. It was first observed in the early 19th century, and around 1970 the first attempts were made to apply this principle to electronic displays, although the technology was not yet mature enough for commercial products.

A major breakthrough came in 1995, when a group of researchers at MIT developed a method to encapsulate charged particles in microcapsules, giving rise to electronic ink (E-Ink) technology and modern EPD displays. Shortly after, these MIT researchers founded the company E-Ink, which later became the global leader and main supplier of e-paper screens.

Writing with Electronic Ink

Let’s take a closer look at how electronic ink is used in an EPD. Inside their structure, EPD screens contain millions of tiny spherical microcapsules, each filled with electrically charged pigments of different colors, suspended in a liquid.

The image below is a zoomed-in view of an e-reader screen, where you can see this microcapsule structure.

In the case of a monochrome display, each microcapsule contains black and white pigments that have been given opposite electrical charges during manufacturing. For this explanation, let’s say the black pigments are negatively charged (–) and the white pigments are positively charged (+).

The microcapsules are embedded in a film coated with a transparent electrode on the top side and another electrode on the bottom, which is formed by a matrix of thin-film transistors (TFTs), each of which can act as a switch.

When a voltage is applied between these electrodes, the electrophoretic effect causes the pigments to move inside the capsule according to their charge:

• If the top electrode is negative relative to the bottom, the white pigments move to the top and become visible, so you see a white dot from the outside.
• If the top electrode is positive relative to the bottom, the black pigments rise to the surface and the dot appears black.

To create complex images on the display, a driver chip is added to this structure; it’s responsible for activating the transistors in the TFT matrix. The driver applies a sequence of voltage pulses between the two electrodes to move the pigments to their desired positions.

The role of this driver is crucial in EPDs, so we’ll come back to it in more detail later on.

Bistability

Once the pigments have moved to their final position, there’s no need to keep applying voltage: the pigments stay in place, and the image remains on the screen without consuming any power. This property of EPDs is called bistability, meaning that each of its two optical states is stable and doesn’t require energy to be maintained.

Beyond Black and White

Monochrome EPDs, limited to just black and white, have trouble displaying images and fine details clearly. Adding grayscale levels improves legibility by smoothing edges and allowing for simple shading, but in many cases, that’s still not enough.

Color EPDs can highlight information, establish visual hierarchy, and grab attention more effectively — all without losing the key benefits of e-paper, like low power consumption and excellent visibility.

Let’s look at the different technologies that make this possible.

More Grayscale Levels

To achieve shades between black and white, a clever trick is used: instead of making the pigments move completely to the surface or the bottom (for pure white or black), they’re moved to an intermediate position, creating a visual mix of pigments that the eye perceives as a shade of gray.

This positioning is achieved by varying the duration of the voltage pulse applied to the electrodes: if the pulse is wide, the pigments move further; if it’s narrow, the movement is smaller.

This method doesn’t allow for infinite modulation, so the number of grayscale levels is limited. Most commonly, you’ll find displays with 4, 8, or 16 different shades.

Adding Colors

In recent years, significant resources have been dedicated to developing color EPDs that retain the key benefits of this technology. As a result, two main approaches now exist for adding color to e-paper, each producing different results: the use of color filters and the multipigment technique.

Color Filter

This method aims to mimic the way other display technologies, like LED screens, create colored pixels.

Each pixel consists of three capsules with pigments that can display white, black, or different shades of gray. On top of these capsules, a layer with a matrix of red, green, and blue (RGB) color filters is placed. By adjusting how much light each capsule reflects, you get a specific level of each primary color.

By combining these primary colors at different levels, the human eye perceives a wide range of tones, allowing the display to reproduce thousands of different colors.

Multipigment

The multipigment technique can be seen as an evolution of basic monochrome technology. Instead of using just two types of pigments, additional colors are added, such as yellow or red.

Each pigment has its own electrical charge and mass, which gives it a different level of mobility. This allows for selective control by applying the right voltage and pulse sequence, making it possible for only the red particles or only the yellow ones to rise to the surface and become the visible color.

In this design, spherical capsules are replaced by trapezoidal cups, which improve the stability and control of the pigments inside each cell.

This technology is widely used to manufacture electronic shelf labels and signage displays that use three or four colors.

Advanced Color ePaper

A promising variation of the multipigment technique is Advanced Color ePaper (ACeP). In this approach, the same trapezoidal microcups are used, but they’re filled with four different pigments: white, cyan, magenta, and yellow (CMY).

By applying a complex sequence of electrical pulses, each pigment can be positioned at different depths within the microcup. As a result, they combine at the surface to produce a wide color gamut that can reach up to 60, 000 colors.

The Dark Side of EPDs

At this point, I have some bad news: EPDs aren’t perfect and they do have a dark side (or two).

It’s absolutely true that they’re unbeatable when it comes to power consumption and offer excellent contrast even in bright environments. However, there are a couple of drawbacks that are inherent to how they work: they’re slow, and they support only a limited number of colors.

So what are EPDs actually good for?

These displays are ideal for showing information that doesn’t need to be updated frequently, in conditions where excellent visibility is needed even under bright lighting, and where ultra-low power consumption is a must.

They’re not suitable for displaying video or information that changes rapidly, although, as we’ll see later, there are tricks to refresh small sections of the screen quickly when needed.

In the second part of this tutorial, we will explore the anatomy of an EPD and what driver boards are