Samsung and Stanford have detailed a new OLED display that can achieve more than 10,000 pixels per inch. This has many implications most notably in advanced virtual reality and augmented reality displays. Among other things it may eliminate the dreaded screen-door effect.
Remember, a commercial large-scale OLED television might have a pixel density of about 100 to 200 pixels per inch (PPI), whereas a mobile phone’s OLED display might achieve 400 to 500 PPI. You can start to imagine what 10K pixels per inch might mean.
In their coverage of this breakthrough, IEEE reminds us of the two underlying OLED technologies that are commercially successful:
In mobile devices and large-scale TVs. Mobile devices mostly used red, green and blue OLEDs, which companies manufacture by depositing dots of organic film through metal sheets with many tiny holes punched in them. However, the thickness of these metal sheets limits how small these fabricated dots can be and sagging of these metal sheets limits how large these displays can get.
In contrast, large-scale TVs use white OLEDs with color filters placed over them. However, these filters absorb more than 70% of light from the OLEDs. As such, these displays are power-hungry and can suffer “burn-in” of images that linger too long. The filters also limit how much the pixels can scale down in size. -Charles Q. Choi, IEEE Spectrum
The new display uses OLED films to emit white light between two reflective layers, one of which is made of a silver film, whereas the other is what researchers are calling a “metasurface,” described as a forest of microscopic pillars each spaced less than a wavelength of light apart. Square clusters of these 80-nanometer-high, 100-nanometer-wide silver pillars served as pixels each roughly 2.4 microns wide, or slightly less than 1/10,000th of an inch.
IEEE goes on to explain that each pixel in the metasurface is divided into four equal subpixels. “The nano-pillars in each subpixel manipulate white light falling onto them, such that each subpixel can reflect a specific color of light, depending on the amount of spacing between its nano-pillars. In each pixel, the subpixel with the most densely packed nano-pillars yields red light; the one with moderately densely packed nano-pillars yields green light; and the two with the least densely packed nano-pillars yield blue light.”
According to the researchers, emitted light reflects bounces among the display’s reflective layers until it finally escapes through the silver film out onto the surface. This build up of light, yields twice the luminescence efficiency of standard color-filtered white OLED displays, and higher color purity, the researchers say.
“If you think of a musical instrument, you often see an acoustic cavity that sounds come out of that helps make a nice and beautiful pure tone,” says study senior author Mark Brongersma, an optical engineer at Stanford University. “The same happens here with light — the different colors of light can resonate in these pixels.”
In the near term, one potential application for this new display is with virtual reality (VR). Since VR headsets place their displays close to a user’s eyes, high-resolutions are key to help create the illusion of reality, Brongersma says. This close viewing normally brings the “screen door” effect because you notice the gaps between the pixels. At 10K pixels per inch will it disappear?
Other research groups have developed displays they say range from 10,000 to 30,000 pixels per inch, typically using micro-LED technology, such as Jade Bird Display in China and VueReal in Canada. In terms of how the new OLED display compares with those others, “our color purity is very high,” Brongersma says.
It’s also more practical than you might think. Samsung is already working on a “full-size” display using the 10,000PPI tech, and the design of the corrugations makes large-scale manufacturing viable.
The scientists detailed their findings online Oct. 22 in the journal Science.