Electronic Design

Unusual OLED Characteristics Create A Large-Screen Illusion

By combining ICs, microdisplays, and optics, eMagin Corp. of Hopewell Junction, N.Y., has created a virtual image equivalent to a real image on a computer monitor or large-screen TV. Using this technology, the company has unveiled an SXGA (1280-by-1024-pixel), active-matrix organic light-emitting-diode (OLED) display.

OLEDs are emissive devices. While LCDs require a separate light source, OLEDs create light. OLED devices also use less power. Additionally, they can produce very bright pictures with sharp color. More importantly to near-eye applications, OLEDs provide full Lambertian light output. In other words, they emit light in all directions (see the figure).

"The only other technologies that do that are the CRT and the inorganic electroluminescent displays," says Gary Jones, eMagin's CEO. "If you look at an LCD slightly off axis, the light fades off. And anything that is directional like a reflective display exhibits an even narrower angle before the light falls off."

With non-Lambertian displays, the image interference pattern changes as the viewer's pupil moves. The pupil then perceives a different image, including a different color balance. As a result, the brain tells the pupil to try and stay in one position instead of behaving as it normally would, which is to move around in a relaxed manner. The viewer, then, experiences fatigue. eMagin's near-eye system is unique because these eye movements make no difference. Also, the eye perceives the image as quite large.

According to Jones, "eMagin's OLED display can create a virtual image that appears to be a 19-in. diagonal image from 22 in. away. All of the other systems that are out there, with the exception of the large CRT systems, don't really do that. They create a 19-in. CRT that is perceived by the eye as being 2.5 to 3 ft. away. So the eye perceives a much smaller view. And as the pupil shifts and things change, you become fatigued after you watch it for a while."

The miniature, high-performance, virtual-imaging module targets near-eye applications, such as wearable PCs, wireless Internet appliances, portable DVD viewers, and digital cameras. The OLED-on-silicon microdisplay features over a million 12-µm pixels in an area smaller than a postage stamp. The bright 0.77-in. diagonal display runs at real-time video speeds and exhibits 256 gray shades at less than 400 mW, with a brightness of 200 candelas/m2. Because the OLED is built directly on a silicon chip, the drivers and other functions can be integrated in the display.

The company faced two challenges in the display's construction. First, it had to create a large image without having the image shift as the pupil moves. Second, its engineers had to eliminate the need for extremely precise positioning of the display relative to the eye.

"If you want to send an image directly to the eye and have the eye view it and you want a large field of view, you really would like to have that light coming out so you can recreate the image plane of a wide range of pupil positions," Jones says. "And that is one of the unique things the OLED does. It creates the type of light interference pattern that is easy to compress into a very small optical system that can be reasonably simple and quite compact."

If a viewer picks up a viewfinder that is based upon a reflector-type device, the image disappears quickly as it is moved a little bit one way or the other. If the pupil moves a bit, the image shifts. "The reason we picked OLED technology for this application several years ago was not just its Lambertian characteristics, but also the fact that it can depict colors with amazing gray levels," he adds.

Furthermore, the CEO explains, "We are able to produce all 256 distinct gray levels at the right intensity gradients. That is what a lot of technologies can't do. They feed in an 8-bit gray level, but what comes out is blurred, lacking distinct, discernible gray scale values." Jones says that eMagin will pursue solely near-eye applications—viewfinders—for camcorders and digital cameras.

Digital cameras are unique because they can let the user view a proof in an instant. With this technology, camera makers will get megapixel-plus resolution viewing right on the back of the camera. Camera users will be able to see something that looks like an 8- by 10-in. proof without needing to carry an 8- by 10-in. screen to see it.

"Normally when you look at viewfinders, you see little screens. Our technology will allow you to look into a viewfinder and see large, virtual screens," Jones concludes. "Your eye will see something that is ten times the virtual screen size. So, you will have a 19-in. CRT-like theatre in a package small enough to fit in your pocket."

TAGS: Components
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