US-based Solchroma develops a novel reflective inks and actuators based display technology, targeting the outdoor signage market (at least as a first step).

Solchroma full-color system schematic

The company introduced its interesting technology at SID Displayweek 2021. The company's CEO, Dr. Roger Diebold was kind enough to explain the company's technology and business to E-Ink-Info.

Hello Roger, thank you for your time. Can you explain Solchroma's ePaper technology?

Solchroma creates reflective digital displays to change how we interact with the built environment and world around us. Solchroma’s technology is based on hydraulically forcing of colored liquid ink in and out of view. A backplane of actuators acts as an array of electrically-controlled pumps that push on nearby reservoirs of ink. Ink reservoirs are shielded from view by a white reflector, while simultaneously allowing passage of ink from the reservoirs behind the reflector to three sealed, viewable chambers stacked one atop the next on the viewable side.



By controlling how much ink is present in each viewable chamber, the pixel color can be changed. Behind the reflector, each pixel has three separate ink reservoirs containing cyan, magenta, and yellow inks, with individual actuators for each color ink. To create white, all ink is withdrawn below the reflector (revealing the white reflector), and to create black, all three inks are layered above the reflector to create composite black. Greyscale is accomplished by progressively introducing or removing more or less of each ink. None of the inks physically mix during operation.

One of the main advantages seems to be the tandem architecture and the large fill factor. How will your displays compare to other ePaper displays on the market, In terms of color, response time, density?

Color saturation and contrast are Solchroma’s strong suits, which leads to an ability to access large color gamut volumes; we have measured color gamut volumes comparable to SWOP (magazine color quality). Because our electrical control apparatus is located out of view, we have an unusual ability to refractive index match in the optical stack; high refractive index transparent conductors such as indium tin oxide are not required in view.

Solchorma full-color pixel array photo

Full color 16.6 mm pixel array where a bezel reducing optical element is employed (left). On the right - the primary color states possible in the full color system (RGBW CMYK)

Most display technologies, including reflective displays, use indium tin oxide or similar transparent electrodes in the optical stack, which often cause large and multiple Fresnel reflective light losses that limit the light coupling efficiency of the system. In combination with Solchroma’s vertical color filter stacking to make effective use of the pixel area, our display is uniquely positioned for vivid, full color generation more so than existing reflective display approaches.

There are not a lot of data available on emerging reflective display performance, but published data on E-ink’s ACeP system indicate SWOP color gamuts are not readily accessible. Speed is not a performance goal for our target applications in the near term. Our response times are designed to be sub-second, but not necessarily video refresh rate as outdoor applications don’t always need video; in fact, video is banned in many instances for billboards or other signage applications. That said, in general, smaller pixels are expected to transition faster.

Our pixel pitches will be on the larger end of the spectrum for a typical display, comparable to pitches found in outdoor LED signs. We have built 10mm –25mm pitch prototypes, but the pitch for each application is driven by the expected viewing distance. For example, as a billboard in the US, a 16mm pixel pitch is appropriate for a significant portion of the market. That said, there is no fundamental reason we cannot move to single mm pitch or smaller and retain the unique benefits of our architecture.

Most surfaces we interact with on a daily basis are both reflective and static – we strive to provide a means to convert those reflective surfaces into dynamic ones that retain their aesthetic. To further this goal, we aim to miniaturize over time and expand into new markets that desire high color quality, reflective digital displays.

Can you estimate the power consumption of a Solchroma display? Maybe in comparison to a comparable E Ink display?

Power consumption is highly dependent on how the system is used. That said, our system is bistable, meaning that power is only consumed during image transitions. We expect to consume power on the same order of magnitude as a commercial E-ink display, but the exact value would depend on the configuration of our system.

How are Solchroma displays produced? We'll be happy to learn more about the production process, and whether it can be expanded in volume and display size easily?

The details of Solchroma’s manufacturing process are proprietary, but involve the use of conventional macro-level fabrication. We do not need a semiconductor fab for production, which allows us more freedom in selecting supply chain options. Similar to large area LED display construction, we aim to build modular panels that can be tiled seamlessly to create larger signs.

Our target for billboards, for example, is a 400mm x 400mm panel that will fit within existing sign scaffolding; this form factor may be appropriate for other large-area applications in On-Premise signage or Architecture.

Our materials set is intended to leverage commodities already available at scale and with proven service lifetimes in outdoor scenarios.

We understand your first product will be a monochrome display, targeting application such as fuel prices and traffic safety. Can you tell us the reasoning behind that?

Our first product line will be 2-color for use in fuel pricers and traffic safety applications. There are two reasons for going in this direction first: (1) An acute need exists for the low-light pollution and power saving attributes of Solchroma displays for these applications, in part illustrated by zoning code regulations in a significant number of municipalities in the US and communities globally, and (2) it is simpler, faster, and cheaper to launch a 2-color product, more quickly achieving our regulatory and outdoor performance proof points.

While our full color system is similar in construction to 2-color, and will enable larger market access, we see the 2-color product as a near-term option that checks a lot of boxes for us.

Do you see front-lighting as a mandatory addition to your displays?

There are certainly creative ways to light reflective displays, internally as well as externally, but for large area applications such as signage, most customers already have front lights on their static signs.

We would take advantage of such existing lighting infrastructure and repurpose those front lights for a Solchroma digital upgrade. Therefore, we strongly recommend an appropriate front light for nighttime visibility. However, we are aware that some architectural or other users may choose to omit front lights to retain the natural aesthetic that comes with our technology.

Most of the built environment is made from reflective materials that are made with pigments or dyes. Solchroma allows users to retain that aesthetic in the way that suits their needs best.

Will Solchroma be able to scale-down its display technology to target applications such as e-readers or tablets?

There is no fundamental reason why the key aspects of Solchroma’s display cannot be miniaturized for use as an e-reader, tablet, or other high resolution display. Smaller fluid volumes indicate that transition speeds would increase and enable video refresh rates. Miniaturization would enable a massive expansion into different aspects of the built environment, which goes along with our vision to be a display company that changes the way we look at and interact with our physical world.

During SID you discussed the effect of light pollution, and how communities are starting to ban or restrict harsh LED signage displays. We'll be happy to hear your views on light pollution.

Solchroma is creating a means to bring the surfaces in our everyday environments to life – doing so without emitting light. The incumbent technology for large area outdoor displays, LEDs, are the most successful technology in that setting because they are bright enough to compete with the sun. That brightness is a double edged sword, however, as it causes light pollution.

The International Dark Sky Association defines light pollution as “the inappropriate or excessive use of artificial light”. Light pollution is a persistent problem for many communities across the world as it reduces quality of life, potentially causing adverse health effects that are still being fully understood.

Dark Skies communities have organized against local changes that would create light pollution, and have instituted recommendations on how it can be limited, in some cases acting as a force to drive legislation and regulation.

Furthermore, light pollution is poised to become an ever-increasing problem with the broader trend of urbanization, increasingly blurring the line between commercial and residential areas. Community push back can be seen in a number of ways, particularly against LED based digital signs in the US because of their high brightness and nighttime contrast. Cities like Los Angeles and Denver have instituted significant restrictions or outright bans on LED-based digital signs in recent years.

However, because of the compelling benefits digital signage holds for sign operators and advertisers, LED digital sign installation continues to grow; in some cases, the community can financially benefit from digital sign installation. This natural tension between the downsides of light pollution and financial benefits to stakeholders does not have to be so oppositional with reflective digital displays like Solchroma’s.

Solchroma provides energy-efficient digital capability with a printed look, comparable to a static sign, offering a compelling alternative to communities looking for something more than LED.

Thank you Roger, for the interesting interview. I hope to hear more about Solchroma's displays and progress in the future!

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