Transparent OLED microdisplays for AR glasses
Optical systems in AR glasses require a lot of battery power. Instead, Fraunhofer IPMS generates the virtual images on the micro OLED for direct viewing.
(Image: heise online)
At Display Week, the Fraunhofer IPMS from Dresden presented a new approach for superimposing the virtual image of OLED microdisplays on the real images in AR glasses without lossy optics. To this end, they developed a high-resolution semi-transparent CMOS backplane for OLED microdisplays through which the wearer of the glasses can see directly. They have grouped the non-transparent light elements into small blocks; beyond the blocks, the chip is transparent.
(Image: Ulrike Kuhlmann, c't)
Detour to the eye
Normally, the image in augmented reality (AR) glasses is guided from small micro-displays via optical fibers to the lens of the glasses, where it is decoupled again at a diffraction grating and directed to the eye. The compact AR displays achieve very high pixel densities of over 5000 dpi in some cases. Small LCD and LCOS projection ICs, micro OLEDs and, in future, micro LED displays are used as imaging displays. While LCD and LCOS require an additional light source, micro OLEDs and, in future, micro LED displays will generate the image directly on the CMOS chip.
However, the entire optical system consisting of the imaging chip, light guide and coupling optics is subject to losses, so in the end only a small amount of light reaches the eye from the lens. This drives up the energy requirement, which is unfavorable for compact mobile systems. The Fraunhofer scientists' semi-transparent micro OLED for looking through can dispense with most of the light-absorbing parts.
Clustered
In initial samples, the Fraunhofer scientists were able to achieve a light transmission of 22 percent for the entire system. A maximum transparency of 50 percent is possible, explained Philipp Wartenberg, who developed the semi-transparent OLED chip at the Fraunhofer Institute. The currently monochrome chip accommodates 7 x 4 clusters on an active area of 14.3 mm x 7.76 mm, each cluster is just under 1 millimeter in size and is 2.24 millimeters away from the next.
(Image: Ulrike Kuhlmann, c't)
The sub-pixels have an edge length of 3.5 micrometers, so the monochrome chip has over 7200 dpi. For color images, the Dresden scientists want to form groups of four red, green, blue and white sub-pixels (WOLED). Small lenses on the micro OLED combine the clustered sub-images into an overall image that floats in front of the chip or AR glasses when viewed through the chip at a distance of around two meters. Because the semi-transparent micro OLED chip is integrated directly into the field of vision of the glasses, the efficiency of the overall system is increased. We were able to take a look through the micro OLED at the joint stand of the German Flat Display Forum (DFF).
(uk)
