What can a butterfly teach about technology? Quite a lot, actually. The physics behind the iridescence seen in butterfly wings, peacock feathers, and the inside of a sea shell, has inspired an innovative display technology based on the same principles.

The phenomenon of iridescence is caused by multiple, intermingling reflections of light from a multi-layered, semi-transparent surface. The light reflections become modulated by interference and phase-shifting. As with other wave phenomena, the phase-shifting resulting from wave forms interfering with each other will amplify some frequencies and diminish others — constructive and destructive interference. With light, that means some colors will become brighter, while others will dim.

The effect permits the perception of colors where no color inherently exists. The iridescent soap bubble is actually transparent. The iridescent peacock feather is actually brown from the standpoint of pigment. The nature of a specific iridescent effect being observed depends upon the microstructures in the surfaces and how they reflect the light.

Enlarge this picture Fig. 1. Basic structure of an iMoD element subpixel.

In an iridescent soap bubble, the interference of light will be dynamic and random, exhibiting a dancing array of hues. Now imagine having complete control of the process, controlling the surface structure of the bubble to determine just how light gets reflected at each and every point.

That is the concept behind the Interferometric Modulator Display, iMoD for short, which was invented by Mark Miles, an MIT-trained scientist who co-founded the company Iridigm to commercialize the idea. Iridigm was later acquired by Qualcomm, San Diego, where research and development on the iMoD display continues.

The potential for iMoD technology to make low-power, reflective displays that excel in sunlight makes it well-suited for portable electronic devices. Qualcomm says the first targeted application is a secondary display for a mobile phone. MP3 players, GPS devices, digital cameras, laptop computers and displays for automobile dashboards are also being explored, but the technology could also be used for outdoor signage and televisions, ATMs, and other outdoor applications, because iMoD actually harnesses the power of sunlight, while other displays must fight it.

Qualcomm says it has a strategic relationship with Taiwan-based Prime View International, a manufacturer of small-format and medium-format displays, but won’t give a specific timeframe for when iMoD displays will hit the market. North American operations, Prime View Display, are based in Irvine, Calif.

How it works

Enlarge this picture Photo-illustration of a mobile phone showing what a first-generation, bichrome iMoD display might look like.

An iMod element is a MEMS device, measuring anywhere from 10 microns to 100 microns. The element is comprised of two conductive plates. (See Fig. 1.) The upper plate is a stationary thin-film stack on a glass substrate. The lower plate is a suspended, movable, reflective membrane. A given pixel in an iMod display contains a number of iMod elements, each acting as a sub-pixel

In the element, the optical gap between the partial reflector in the thin film upper plate and the lower reflective membrane exhibits both constructive and destructive interference. The wavelength at which constructive interference occurs determines the color of light that will be reflected by that particular element. Different gap heights produce different colors. (See left side of Fig. 1.)

The application of voltage between the thin-film stack and reflective membrane creates an electrostatic field. When the field exceeds a certain threshold, the field causes the gap to collapse. (See right side of Fig. 1.) In the collapsed state, the element appears black, because destructive interference occurs across the entire visible spectrum.

Reducing the applied voltage below another threshold releases the reflective membrane, which then travels back to its original, open position, where the element reflects its specified color.

The competition between mechanical forces and electrostatic forces creates a condition of hysteresis in the element that imparts a bistable behavior; the element maintains an open or closed state with only a nominal bias voltage. Short pulse voltages are used to change it from one state to another. This bistability allows an iMoD display to be addressed in a power-saving, passive matrix form, as opposed to the more common active-matrix display that must be constantly refreshed.

Benefits

Enlarge this picture Fig. 2. Structure of an iMoD pixel showing how the principle of interference is used to produce color.

The bistable iMoD display operates at near-zero power when the display is static, providing an energy-efficiency advantage in portable, battery-powered applications, but there are other benefits, as well. The most important one is readability. Qualcomm says that initial iMoD displays typically demonstrate reflectivity in excess of 45 percent and contrast ratios better than 8:1, and future generations will better those numbers. The displays also deliver a wide, symmetrical viewing angle similar to an emissive display.

One advantage of a purely reflective display is that it automatically adjusts its brightness to ambient light levels. The obvious disadvantage is that it needs help in low light situations. For these occasions, the displays can be outfitted with low-power supplemental illumination in the form of a front light.

As the ability to display video becomes increasingly important for consumer applications, the iMoD display affords an advantage there also, because of its high switching speed. Qualcomm says that an iMoD element can switch states in roughly 10 microseconds, which would make it 1,000 times faster than many traditional displays. This translates into an improved, sharper image with no motion blur.

Operating environment is another consideration. Unlike LCDs, where liquid-crystal viscosity is affected by temperature extremes, Qualcomm says that the iMoD’s design allows it to operate in temperatures ranging from –30 DegC to 70 DegC, and the iMoD display is impervious to UV exposure.

From a design perspective, the absence of a backlight in the equation permits a thinner form factor than a backlit LCD. Integration of an iMoD into an existing product design should not pose much of a problem either, because the iMoD is compatible with industry-standard hardware, software, and interfaces.

The cost of an iMoD unit has not yet been fixed, but given that the components can be manufactured on a subset of existing flat-panel display fabrication lines, it is expected that manufacturing costs would ramp quickly downward as volume increases. And while the new technology will initially target small-format applications, the iMoD displays will eventually be able to scale up to the same size range as LCD-TFT units, given that the technology can leverage existing LCD-TFT infrastructure.

For more information email: egavin@qualcomm.com