Each pixel is made up of rows of two-layered reflective surfaces separated by air, and acts as a tiny interference chamber. When ambient light hits each of these subpixels, some will reflect off the top surface, and some will pass through and be reflected off the bottom surface. The two waves constructively interfere with each other, combining to create light whose color is determined by the distance between the two surfaces–on the order of hundreds of nanometers. Within each pixel are three of these chambers, each a different depth, to create red, green, and blue. To turn the pixel off, a small voltage is applied to collapse the bottom reflector against the top. Once a subpixel is either on or off, it will stay there until power is applied again.
These Mirasol pixels can switch between on and off in 10 millionths of a second. “Because it’s mechanical, it’s very fast,” says Galley. Touch-screen Mirasol prototypes at the conference showed video almost as color-saturated and rich as an LCD, and the response rate felt faster than an E-Ink screen.
The company showed other prototype displays incorporating a front light, which had a crisper picture. Front lights are more energy-efficient than the backlights, but are tricky to engineer. Galley says the first product will likely incorporate a touch screen, and the front light will be added to future generations of products. Monochrome Mirasol displays are already in a few products in Asia with very small screens, including a Bluetooth headset and a cell phone.
“The battle in reflective displays is, who can get reasonable color and motion,” says Semenza. “That’s where Qualcomm has leapfrogged other companies.” The interference-based displays may prove to have some cost advantages as well. Most displays must be built on a thin-film transistor backplane, which is one of the most expensive components; the MEMS system in the Mirasol doesn’t require a transistor array. However, the real cost of the displays won’t be known until they’re manufactured in large quantities.
Qualcomm is currently manufacturing the devices at sizes of about a meter squared at a plant in Taiwan, but the company would not comment on their current manufacturing capacity. Galley says Qualcomm has addressed problems with earlier prototypes, but would not go into detail. In addition to the e-reader prototypes, company representatives are showing small static displays that demonstrate further improvements in brightness. Qualcomm is partnering with another manufacturer to develop e-readers using Mirasol displays.