  Event cameras output asynchronous events to represent intensity changes with
a high temporal resolution, even under extreme lighting conditions. Currently,
most of the existing works use a single contrast threshold to estimate the
intensity change of all pixels. However, complex circuit bias and manufacturing
imperfections cause biased pixels and mismatch contrast threshold among pixels,
which may lead to undesirable outputs. In this paper, we propose a new event
camera model and two calibration approaches which cover event-only cameras and
hybrid image-event cameras. When intensity images are simultaneously provided
along with events, we also propose an efficient online method to calibrate
event cameras that adapts to time-varying event rates. We demonstrate the
advantages of our proposed methods compared to the state-of-the-art on several
different event camera datasets.
