Contents I. Introduction
The acquisition of three dimensional images of the environment or objects is desired in many applications such as surveillance, advanced driver assistance systems (ADAS) or goods inspection. With the improvements in CMOS processes in last years it is possible to fabricate well performing single-photon avalanche diodes (SPAD) which yield low dark count rate and high timing resolution without the need for custom and specialized processes. This allows fabrication of cost efficient sensors with on-chip electronics [1]. SPAD-based sensors for 3D measurement determine the distance based on the time-of-flight (TOF) of light. These systems consist of a light source which emits a modulated light beam which is reflected by the target and detected by the SPADs of the sensor. From the output of the SPADs the TOF and hence the distance is calculated [2]. Compared to other types of photo sensitive elements like intrinsic photon mixing devices, standard or avalanche photo-diodes, SPADs achieve sensitivity to individual photons [3] with high timing resolution in the picosecond range [1]. One potential application for these sensors are ADAS like electronic parking assistance, vision-based pedestrian detection systems or blind spot detection in vehicles [4]. Typically these sensors operate in the near infrared range of the electromagnetic spectrum which is invisible for the human eye [5]. However, especially in automotive applications they have to face high background light due to the radiation of the sun. This additional illumination is detected by the sensor and degrades the distance resolution of the measurement or, if the ambient illumination is too high, makes it impossible. In this paper the reasons for this are shown and concepts to reduce the sensitivity to ambient light based on the direct and indirect measurement technique are presented.
