EPD vs ChLCD: The Battle of E-Paper Technologies

November 11, 2023
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The most popular e-paper technologies on the market are electrophoretic display (EPD) and cholesteric liquid crystal (ChLCD). Both reflect ambient light to display images, do not require a backlight, and protect eyes by reducing blue light. The bistable nature means both displays only use power when updating content, and require zero energy to maintain static images. So EPD and ChLCD are the leading technologies for energy-saving and sunlight-readable display applications.

We share the structural differences and advantages of the two technologies so you can choose the best e-paper technology for your display applications.

What is an electrophoretic display?

Electrophoretic display (EPD) is an e-paper technology that uses charged ink microcapsules to display colors. It is the dominant technology for black-and-white ebook readers. EPD screens contain black and white ink microcapsules with different states of charge. When a black pixel is required, the screen changes the pixel charge to make black microcapsules float up, and white microcapsules sink, forming a black pixel. If a white pixel is required, the pixel charge reverses to make white microcapsules float up, and black microcapsules sink, appearing white.

Diagram of color E-paper formation principle
EPD e-paper uses charged ink capsules to create pixels.

An EPD color e-paper technology requires multiple color microcapsules (such as cyan, magenta, yellow) or color filters to display images. The precision required to control multiple color capsules by an electric field to create different colors and grayscale is the challenge faced by electrophoretic color displays.

What is a cholesteric liquid crystal?

Cholesteric liquid crystal (ChLCD) is a color e-paper technology made by stacking three liquid crystal layers. The three layers reflect red, green, and blue colors respectively to form color images. The display uses different electric fields to control the rotation of liquid crystal particles, which remains bistable even after the electric field is off:

  • The planar state has the liquid crystal particles lined up uniformly to reflect specific wavelengths of light, to form unique colors.
  • The focal conic state lines up the liquid crystal particles in a way to let light pass through, displaying the colors underneath.
ChLCD wavelength segment visible to the human eye
Altering the ChLCD particles on different layers to fully reflect light (white), partial reflection (gray), or no reflection (black).

By changing three ChLCD layers, we can display more than 16.78 million vibrant colors. It is the key color e-paper technology to replace light-emitting LCD and LED displays.

Common features of the two e-paper technologies

Both EPD and ChLCD have the following advantages:

  • Bistable feature that only consumes energy during screen refreshes, and uses no power when displaying a static image.
  • No backlight means a huge reduction in blue light harming the eyes.
  • Screens are visible under direct sunlight.

Advantages of EPD e-paper

EPD e-paper's ink microcapsule design has the following advantages:

  • Ink capsules have great contrast, meaning blacks are darker, while whites are whiter. This is suitable for displaying texts.
  • Ink capsules have a wider viewing angle. It appears paper-like even when viewed from a small angle.

Advantages of ChLCD e-paper

The ChLCD e-paper has the following advantages:

  • Three liquid crystal layers (RGB) can mix the three colors to achieve over 16 million colors, similar to LCD and LED. This is suitable for picture books and digital advertising applications where color presentation is critical.
  • ChLCD does not increase capsule movement time when updating the screen, as opposed to EPD, which experiences an increase in capsule movement time with color addition. Therefore, ChLCD has a faster refresh rate.
  • While most ambient light is reflected off ChLCD, about 10-30% of the light is absorbed by the bottom layer. So an embedded solar module at the bottom of ChLCD can generate renewable energy to create a zero-emission Infinity Display.
Infinity Display trichromatic light wavelength principle
Use solar cells to replace the backsheet of the ChLCD to generate electricity from ambient light.

Color differences between EPD and ChLCD

The more varied ink capsules in an EPD display, the more complex it is to control them. So the latest color EPD can only display 4,096 colors.

ChLCD's liquid crystal layers can mix RGB layers to create more than 16 million colors. So cholesteric liquid crystal's better color presentation is more suitable for commercial and retail displays than EPD.

Left) ChLCD screen has more than 16 million colors, right) EPD screen has 4,096 colors
Left) ChLCD screen has more than 16 million colors, right) EPD screen has 4,096 colors.

Speed differences between EPD and ChLCD

Apart from color performance, another key metric for color e-paper is refresh speed. The more color ink microcapsules the EPD needs to control, the more complex the control method and the longer the update time. The 7-color EPD screen below requires around 60 seconds to refresh an image.

A ChLCD e-paper can drive three layers of RGB liquid crystals at the same time, and it only takes 1-2 seconds to update a full-color image.

Based on the pros and cons of the two e-paper technologies, EPD is more suitable for black-and-white ebook readers and shop labels that need fewer colors. While ChLCD is better suited for color ebook readers and vivid digital signage applications.

If you would like to know more about color e-paper applications, please get in touch with us.