Workstation Inspects OLED Displays
Brent D. Johnson
Organic light-emitting-diode (OLED) technology is one of the most significant advances in information display since the invention of liquid crystal displays (LCDs). Offering intense colors, sharp contrast ratios, rapid refresh rates and low power consumption, these devices are finding applications in car stereos, cell phones, personal digital assistants and a variety of microdisplays.
The results browser shows a close-up of a single display pixel stuck in the dark state, sometimes referred to as a cross.
As with any new technology, it must be tried, tested and proved before it can gain wide acceptance in the consumer arena. A rush to market in an attempt to beat the competition could be a costly mistake, with disastrous long-term effects. "The approach now is to be much more critical," said Arthur Harmala, vice president of sales and marketing for Integral Vision Inc.
The organic LEDs have kinks to be worked out before they can gain wide acceptance, he said. "Whether it's polymer, large-molecule or small-molecule OLEDs, they have a problem with fill factor and edge growth in getting a fully formed pixel."
Fill-factor issues, which involve defects in which the surface area of a pixel is not completely covered with emissive material, can cause problems with display uniformity and crosstalk. Edge growth is a type of fill-factor defect. Single-pixel, and sometimes subpixel, defects are critical factors that determine display quality.
Integral Vision developed its SharpEye inspection station to test organic LEDs, and developed and implemented algorithms that can detect, define and categorize functioning pixel defects of 1 pixel or less in size. The SharpEye uses megapixel CCD array cameras with 100 percent fill factor. Harmala said that the 3k x 2k-array Kodak 6.3i camera is a typical system, providing a large field of view and better pixel ratio for Nyquist sampling.
The Nyquist criterion postulates that you must, in a single dimension, sample at twice the frequency you are trying to reconstruct. In this application, therefore, at least four inspection camera pixels must be matched to each display pixel. "We do a 9:1 pixel ratio up to a 225:1 pixel ratio, depending on the process inspection. This can provide 3- to 5-µm resolution on the inspected display," Harmala said.
The company tested a system based on a 4k x 4k camera, with poor results. Harmala said that it is still too early in the development to get quality 4k x 4k imagers. He added that the Kodak 6.3i is one-quarter the price of the camera tested.
The station also works with LCD, microelectromechanical systems and other display devices.
EMagin is using the SharpEye workstation to test its 7/10-in. organic LED SVGA microdisplays, which are in devices ranging from the simple to the complex, such as games, medical devices and military equipment. "Counting defects is the trouble with OLEDs," said Olivier Prache, director of product development for eMagin. Defects are not as noticeable on a PC screen, but against a black screen, they can be very apparent, he said. "If it's a mission-critical system, you can't afford to have a minus sign change into a plus."
Three other companies are selling organic LED inspection devices, but Harmala said that they are not as advanced in terms of algorithms or machinery. Are there things he would improve about the SharpEye?
"You could always wish for higher throughput," he said, "but the perfect system doesn't exist."
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