Reflective Digital Displays

Our daily experience is typically with displays that emit light, our phones, televisions, monitors, tablets and watches all use technologies such as LCD (with LED backlights) or OLED or direct view LED. However reflective displays are also commonplace, calculator displays, shelf-edge labels, and of course Kindle e-readers. But there is significantly more potential for reflective displays, especially for outdoor signs where their significant benefit of being sunlight readable and low power provide considerable opportunity.

My first imaginings of the future of displays were along the lines of living room walls and cereal boxes being turned into displays, the stuff of science fiction. Nearly 50 years later we are still not at that stage, not commercially in any case. Well before knowing the terminology or the implications concerning power, ambient light, etc, I favored the idea of e-paper as the future of display technology. Today, after many years in the display industry, I am more pragmatic and favor whatever best meets the needs of the application and is actually available, but reflective displays are a key part of the display technology mix.

There are a number of reflective display technologies that have been proven and commercialized to various extents:

  • LCD
  • e-paper (charged particle)
  • Electro-wetting
  • Electrochromic
  • Other: Electromechanical (updated Arp 12, 2021)

There are some very significant benefits to these reflective display technologies for certain applications, for example outdoor signage where they offer:

  • Excellent visibility in sunlight. This compares with emissive displays that are competing with the ambient light.
  • Very low power. The reflective display technologies typically only use power to change the image or require a very small amount of power to maintain their image. The clear benefit of this is that they can be installed in locations where they are dependent on solar power.

LCD

LCD is best known as an emissive display technology for its use in televisions and mobile devices, it normally has a backlight and is part of an emissive display assembly. However LCD doesn’t emit light, the backlight to the display assembly provides the light, and these days that is usually made using LED, the LCD acts as a shutter allowing light to pass or not. LCD can work very effectively as a reflective display but efforts to make large or color reflective LCD displays have not been particularly successful (so far). There is a hybrid known as transflective, this can operate as both reflective and backlit.

Kent Displays have commercialized a use for reflective LCD technology with their Boogie Boards, note however that the Boogie Board is not an electronic display, see https://kentdisplays.com/.

Otherwise reflective LCD is used extensively in shelf edge labels, calculators and watches.

ePaper – Charged Particle

A display technology that uses particles that physically move to a visible surface.

E Ink, a market leader for reflective displays with a history dating back to 1997, saw their first major commercialization breakthrough being e-readers however they are now a major provider to the electronic shelf edge label market and have a growing presence in the outdoor digital sign market. They explain their technology here: https://www.eink.com/electronic-ink.html. As detailed on the E Ink website (https://www.eink.com/application.html) the number of applications has grown significantly, including larger higher resolution signage displays. A significant advantage with these e-paper displays is that they are both sunlight readable and bistable, ie they hold their image without power, and only require very low power to change the image making them suitable for solar or wind powered installations. We are using this with our products: https://www.volantidisplays.com/epaper/sign-displays.html.

E Ink reflective 32inch
E Ink reflective 32″ display – photographed in full sunlight

Another company with a charged particle display technology for reflective displays is Clearink. An explanation of their technology is here. https://www.clearinkdisplays.com/technology. They have apparently seen a market opportunity with e-readers, shelf labels and wearables.

Electrochromic

Electrochromic displays are based on the property of a material to change from transparent to opaque when a voltage is applied. They make effective reflective displays though they require a small amount of power to hold their On state. As far as I am aware they are not bistable like some of the other technologies, they need a very small amount of power to maintain their state. They also promote the benefit of being cost-effective to mass-produce.

RDot, https://rdotdisplays.com/, and Ynvisible, https://www.ynvisible.com/, are examples of electrochromic display solutions that have been commercialized. RDot has a good explanation of their technology at: https://rdotdisplays.com/displays.

Electro-Wetting

Electro-wetting involves moving a liquid so that it is visible or not. Here is a video from 2013 that explains and demonstrates the technology being used to make a high resolution display: https://www.youtube.com/watch?v=Bf1GjCaYzYg (Samsung sold this business to Amazon in 2013). However, it seems the direction might be for outdoor display use.

eTulipa, https://etulipa.com/, are bringing electro-wetting display technology to the market and have a number of real world installations viewable on their website.

Liquivista, one of the pioneers that I recall for electro-wetting. As noted in Wikipedia, the company was started as a spin-off from Philips, then bought by Samsung, then Amazon then closed in 2018. A company called LookGadgets acquired the web domain and have written up a good explanation of the electro-wetting technology, see: https://liquavista.com/

Other

The only other reflective display technologies that come to mind are electro-mechanical displays. These use mechanical shutters and some people may recall the click clatter of the train station timetable displays.

Solchroma, electroactive polymer actuators. The details of this technology, that uses liquid inks can be seen here: https://solchroma.com/. The actuators move the ink to the display surface where it becomes visible.

Image from the Solchroma website

Flipdot https://flipdots.com/en/home/: They manufacture both character and matrix displays: https://flipdots.com/en/products-services/small-7-segment-displays/, they also have flipdot panels: https://flipdots.com/en/products-services/flip-dot-boards-xy5/.

Links – further reading