Stray-Light Mitigation for Under-Display Time-of-Flight Imagers

The next big thing in smartphone development will be under-display cameras. Conventional color cameras, implemented under-display, starting to appear for consumer products. For secure face-authentication, additional information in the form of the third dimension is required. The time-of-flight (ToF) measurement principle is based on measuring the phase and consequently the delay of emitted and reflected light pulses. Unwanted reflections of the display hinder the implementation of under-display ToF cameras. These reflections introduce errors and circumvent the processing of the depth information. In this paper, we present novel measurement principles enabling under-display Time-of-Flight imaging. In contrast to traditional continuous wave ToF imaging, our measurement principles remove the the display introduced stray-light reflection from the captured images. We present two independent approaches. One is based on the combination of continuous wave measurement with coded modulation. The second method is based on measurements with two modulation frequencies. These methods require a non-traditional approach for depth and amplitude calculation. Furthermore, design considerations are presented, such as calibration and temperature dependency. Our extensive evaluation considers the reflection suppression performance and the fidelity of the captured depth data. The results reveal the suppression of the stray-light by more than 80%. We evaluate our methods with a detailed description of the advantages and disadvantages of the different methods. This work is a starting point for the implementation of under-display ToF imaging.