See-through OLED display with eye-interaction capabilities

　　Organic light-emitting diode (OLED)-based microdisplays have achieved high optical performance, with excellent contrast and large dynamic range, and low power consumption. They use a direct light-emitting mechanism and do not require an additional backlight, so they can be made very small and lightweight, making them ideal for mobile near-eye (NTE) applications such as electronic viewfinders or helmet-mounted displays (HMDs).

　　In many advanced applications, microdisplays are typically used as pure unidirectional output devices. After integrating additional image sensors, the functionality of microdisplays can be extended to bidirectional optical input/output devices. The main goal is to enable eye tracking in see-through head-mounted display applications to provide gaze-based human-display interaction.

　　While today's mobile information systems, such as smartphones and tablets, are typically touch-controlled, microdisplays with best-in-class pixel counts and significantly reduced geometric dimensions have found their way into consumer electronics, such as electronic viewfinders in digital cameras. OLED-based microdisplays will have a bright future in the field of video and data display, especially when they can also serve as input channels.

　　Now, OLED technology makes it possible to integrate highly efficient light sources and light detectors together on CMOS substrates, thus enabling fully integrated optoelectronic and smart applications based on silicon chips. We can realize micro-emitters and receivers on the same chip, such as "bidirectional OLED microdisplays" in an array-type structure, and ultimately a device can reproduce and capture images in the same place and even at the same time.

　　The above technology could become the basis for a whole new generation of devices for personal information management: reproducing information to the user while optically recognizing the user's interaction intentions. For example, augmented reality glasses with a two-way microdisplay could intentionally or unintentionally feed visual information adapted to the context of the operation and controlled by eye movements alone.

　　Bidirectional OLED microdisplay and optical components

　　In order to achieve high-performance OLED characteristics with standard CMOS processes, the top metal layer needs to be modified. Common requirements for an OLED-compatible top metal layer are high reflectivity in the visible range, a smooth surface to prevent short circuits in the OLED stack and avoid oxidation. The top light-emitting OLED has a reflective bottom electrode and a transparent top electrode. Between these electrodes, the OLED stack with a doped transport layer together with a triplet of emitter systems form a high-efficiency, low-voltage light-emitting tube. The modified top metal layer serves as the bottom electrode, which determines the shape and size of the OLED pixel. There is space below the bottom electrode for further integration of the driver circuit. The light detection device is realized by n-well diffusion in the p-substrate. With this structure, light-emitting and light detection devices with integrated driver circuits can be realized on a single CMOS chip.

　　The active area of ​​a bidirectional microdisplay consists of nested display and image sensor (embedded camera) pixels surrounded by a second image sensor (framing camera) and drive and control circuits, see Figure 1.

　　Figure 1: Cross-sectional view of OLED-on-CMOS device and functional demonstration in bidirectional OLED microdisplay.

　　The display and image sensor systems are electrically independent of each other and interact simply via synchronization signals. A potential problem with this bidirectional microdisplay is optical interference between the display and the camera. However, this interference can be suppressed by operating the display and camera in a time-sequential manner.

　　The optical system consists of two aspherical mirrors, a beam splitter, and a microdisplay (see Figure 2). The aspherical mirrors are two-color coated to reflect visible light (380nm-780nm) in the display path and near-infrared (NIR) light (780nm-1100nm) in the camera path. Thus, the system allows the projection of virtual images in the natural environment view. The visible light of the OLED passes through the beam splitter, reflects back from the aspherical mirror on the bottom side, and is then reflected to the eye by the bottom edge of the beam splitter. In addition, the eye is illuminated by two near-infrared diodes that emit a wavelength of 850nm. This wavelength improves the contrast between the pupil and the iris in the captured image. The diodes are placed outside the optical axis of the camera that forms the dark pupil image.

　　Figure 2: Optical principle of bidirectional near-eye optics.

　　Based on these principles, we were able to design a VGA bidirectional OLED microdisplay as a binocular interactive see-through helmet component. The embedded nested camera here is used as an eye tracking image sensor. The size of the optical assembly is 45mm×35mm and the depth is 35mm. Figure 3 shows the captured eye image, and the optical assembly provides a sharp and high-contrast projection from the user's eyes to the embedded image sensor. For both eyes, the bidirectional optical assembly is mounted in an ergonomic frame that also integrates the drive circuit. The display provides 640×480 pixels (VGA) for each eye, while the image sensor is 128×96 pixels. The display has a transparency of 50% and can generate a virtual image at a distance of 750mm with a viewing angle of 20°×21.6°.

　　Figure 3: Captured image of the eye (left) and the see-through head-mounted display in action (right).

　　Table 1: Performance comparison of Google Glass and COMEDD data glasses.