See-through OLED display goes eye-interactive
Micro-displays based on organic light-emitting diodes (OLEDs) achieve high optical performance with excellent contrast ratio and large dynamic range at low power consumption. Their direct light emission enables small-footprint and lightweight devices without additional backlight, making them suitable for mobile near-to-eye (NTE) applications such as electronic viewfinders or head-mounted displays (HMD).
Fig. 1: Cross-section of OLED-on-CMOS setup in bi-directional OLED microdisplay and functional demo
In state-of-the-art applications the micro-display typically acts as a purely unidirectional output device. With the integration of an additional image sensor, the functionality of the micro-display can be extended to a bidirectional optical input/output device. The major aim is the implementation of eye-tracking capabilities in see-through HMD applications to achieve gaze-based human-display-interaction.
While today’s mobile information systems such as smartphones and tablets are usually touch-controlled, micro-displays with state-of-the art pixel count but significantly decreased geometrical size, have found their way into consumer electronic products in the shape of electronic view finders in digital cameras. Micro-displays based on Organic Light Emitting Diodes (OLED) could have a very promising future for video and data display, especially if they can double up as an input channel. Now, OLED technology offers the possibility to integrate highly efficient light sources with photo detectors on a CMOS backplane. This enables fully integrated opto-electronic and smart applications based on silicon chips. One can realize micro-scale optical emitters and receivers on the same chip, e.g., in an array-type organization as “bidirectional OLED micro-display”, thus performing a device that presents and captures images in the same place and even at the same time.
This can be the foundation for a complete new class of devices for personalized information management: presenting information to the user while optically recognizing the user’s interaction. Implemented as augmented reality glasses that carry bidirectional micro-displays, such devices could feed visual information deliberately or unconsciously adapted to the context of operation, controlled through eye movement alone.
Bi-directional OLED microdisplay and optics
To achieve high-performance OLED characteristics on a standard CMOS process, a modification of the top-metal layer is necessary. The common requirements of an OLED-compatible top-metal layer are high reflectivity in the visible range of light, a smooth surface to prevent shorts in the OLED stack, and avoiding oxidation. A top emitting p-i-n OLED has a reflective bottom electrode and a transparent top electrode. Between these electrodes, an OLED stack with doped transport layers together with a triplet emitter system makes up a highly efficient and low voltage light emitter. The modified top-metal layer is used as bottom electrode and defines the shape and size of an OLED pixel. Below the bottom electrode there is space to integrate further drive circuitry. The photodetector device is realized by n-well diffusion in a p-substrate. By using this structure it is possible to realize a light emitting and a photo detecting device
with integrated driving circuits on a single CMOS chip.
The active area of the bi-directional microdisplay consists of nested display and image sensor (embedded camera) pixels surrounded by a second image sensor (frame camera) as well as driving and control circuitry – see Figure 1. The display and image sensor systems are electrically independent of one another, simply interacting via synchronization signals. A potential issue of such a bi-directional micro-display is optical crosstalk between display and camera. This crosstalk can be suppressed by operating display and camera time-sequentially.
The optical system consists of two non-spherical mirrors, a beam splitter and the micro-display – see Figure 2. The non-spherical mirrors have a dichroitic coating either reflecting visible light (380-780nm) in the display path or NIR light (780-1100nm) in the camera path. So the system allows the projection of a virtual image within the natural vision of the environment. The visible light of the OLED passes the beam splitter,gets reflected from the downside non-spherical mirror and is reflected by the bottom side of the beam splitter to the eye.
Fig. 2: Optical setup of bi-directional near-to-eye optics.
Additionally, the eye is illuminated by two NIR diodes emitting at a wavelength of 850nm. This wavelength leads to an improved contrast between pupil and iris within the captured image. The diodes are placed outside of the optical axis of the camera which creates a dark pupil image.
Following these principles, we were able to design a VGA bi-directional OLED micro-display into a binocular interactive see-through head-mounted unit. Here the embedded nested cam acts as an eye-tracking image sensor. The optics measure 45x35mm for a depth of 35mm. Figure 3 shows a captured eye-image, the optics provides a sharp and contrasted projection from the user’s eye to the embedded image sensor. For both eyes, the bi-directional optics are housed in an ergonomic spectacle frame that also integrates driver electronics. The display offers 640×480 pixels (VGA) for each eye, while the image sensor is 128×96 pixels. Transparency for the display is 50%, offering a virtual image at a distance of 750mm for a field of view of 20°x21.6°.
Table 1: Comparison of Google Glass and COMEDD data glass features
About the author
Uwe Vogel is Business Unit Manager for OLED Microdisplays and Sensors at the Center for Organic Materials & Electronic Devices Dresden (COMEDD) – He can be reached at uwe.vogel@ipms.fraunhofer.de
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