Emissive Displays

The display sector has been historically slow in moving, but the past few years have seen the creation of several new and exciting display technologies. These include electrofluidic displays, Microsoft Research’s telescopic pixel, and Unipixel’s Time Multiplexed Optical Shutter (TMOS), all of which can be classified in the emissive display category.

Unveiled by the University of Cincinnati in April 2009, electrofluidic displays comprise two sheets of plastic: one sheet with pixels that each includes a hole filled with pigment and surrounded by air or oil, topped with another sheet containing a transparent electrode. When a voltage is applied, the pigment is drawn from the hole and revealed. The surrounding air or oil prevents it from mixing with pigments from surrounding pixels. While the creators promise superior brightness, color saturation, and video speed, this technology is still in its early stages and is years away from commercialization.  

Microsoft Research has positioned its telescopic pixels display technology as a superior alternative to LCD, with faster response time and the ability to form a brighter display. Telescopic pixels release 36% of the incident light, compared to 6% with LCD. This addresses another deficiency of LCD by providing enhanced viewability in outdoor lighting environments. Like electrofluidic displays, telescopic pixels have not yet been tested in the commercial market.

The last of the emerging technologies in the emissive category is Unipixel’s TMOS display technology. Interestingly, TMOS was originally developed as a display solution in avionics applications. The display’s light source is essentially a total-internal-reflection light box driven by color-field-sequential LEDs. The pixel elements are flexible optical shutters switched to frustrate internal reflection and emit light from the display screen. Unipixel claims that its TMOS display technology provides a brighter, higher-contrast, more colorful image that is readable in sunlight, while also significantly reducing power.

While electrofluidic displays, telescopic pixels, and TMOS are all exciting and innovative developments, their ability to address the widening power gap is questionable. As emissive displays, all three technologies consume significant power to emit light. While they mark an incremental improvement over LCD technology, they face the same challenges as their predecessor—high power consumption and significantly decreased readability in outdoor lighting environments.

Reflective Technology: Displays Of The Future?

So if emissive displays aren’t the answer to the mobile “energy crisis,” then what is? The second class of display technologies that has seen significant innovation in recent years is the reflective display category. Bistable reflective technologies are the most power savvy of the emerging display technologies. They also have proven their commercial viability through incorporation in today’s mobile devices. This category includes E Ink’s e-paper display and Qualcomm’s mirasol display.

The E Ink display is perhaps best known for its incorporation in Amazon’s Kindle and more generally for its role in defining the e-book market. It comprises a plastic film that has been laminated to a layer of circuitry, which forms a pattern of pixels controlled by a display driver. The display is designed to mimic ordinary ink on paper for maximum readability. It can hold a stable image without constantly needing to refresh, and it reflects ambient light rather than emitting light.

These attributes contribute to enhanced energy efficiency and visibility in direct sunlight—two of the primary deficiencies of the LCD. While black and white E Ink displays are well suited to e-reader applications, color e-paper displays have yet to be featured in devices. Current generations of e-paper displays are more suitable for relatively static displays that change less frequently and require little to no color. Furthermore, electronic paper has a relatively slow response time, rendering it unsuitable for video and similar functions.

Qualcomm MEMS Technologies’ Interferometric Modulation (“IMOD”) technology, however, makes a natural leap from bichrome to color and supports video-rate functions. The company’s engineers developed the mirasol display by studying and mimicking nature’s processes and structures. The display uses a combination of mirrors and thin-film layers common in LCD and semiconductor-like fabrication technologies to create a full spectrum of pure colors by reflecting light so specific wavelengths interfere with each other to select the emitted colors (Fig. 2).

This is the same phenomenon that makes a butterfly’s wings shimmer. When ambient light hits the display structure, it is reflected both off the top of the thin-film stack and off the reflective mirror membrane. Depending on the height of the optical cavity between the mirror membrane and the thin-film layers, light of certain wavelengths reflecting off the membrane will be out of phase with the light reflecting off the thin-film structure, and other wavelengths will be in phase. Based on the phase difference, some wavelengths will constructively interfere, while others will destructively interfere.

The mirasol display technology brings two essential value propositions to the mobile experience: longer battery life and direct-sunlight viewability. One of the key advantages is its bistable nature, which allows for near-zero power usage in situations where the display image is unchanged. This means that mirasol displays benefit from considerable power savings, especially compared to displays that continually refresh, such as LCDs.

The power currently used for battery-draining backlights can be reallocated to support functions and applications, improving response and refresh rate as well as lengthening battery life. Furthermore, the color remains bright, crisp, and saturated across a broad range of lighting conditions, including direct sunlight. While a relatively new technology, mirasol displays have already been selected for multiple products and will scale to greater sizes and enhanced color quality in the near future.

The Future

Bistable reflective technologies are the most power savvy of the emerging display technologies. They not only enhance the consumer experience, they also provide value throughout the mobile value chain. Designers can develop sleeker devices using smaller (and less expensive) batteries. Product managers don’t need to skimp on device features. And, carriers could see ART rise as consumers increase usage time on their handsets. These emerging display technologies are still maturing, but the core value proposition has the elements of a disruptive technology, making this a space to be watched.

References

1. Rosaleen Ortiz, “Ohio Engineers ‘Ink’ New E-Paper,” IEEE Spectrum, April 28, 2009,  www.spectrum.ieee.org/semiconductors/materials/ohio-engineers-ink-new-electronic-paper-technology
2. Prachi Patel, “A New Competitor to LCD,” Technology Review, July 21, 2008, www.technologyreview.com/computing/21104/
3. Unipixel, “Technology,” www.unipixel.com/tech.htm

4. E Ink, “Technology: How It Works,” www.eink.com/technology/howitworks.html