  Accurate calibration is crucial for using multiple cameras to triangulate the
position of objects precisely. However, it is also a time-consuming process
that needs to be repeated for every displacement of the cameras. The standard
approach is to use a printed pattern with known geometry to estimate the
intrinsic and extrinsic parameters of the cameras. The same idea can be applied
to event-based cameras, though it requires extra work. By using frame
reconstruction from events, a printed pattern can be detected. A blinking
pattern can also be displayed on a screen. Then, the pattern can be directly
detected from the events. Such calibration methods can provide accurate
intrinsic calibration for both frame- and event-based cameras. However, using
2D patterns has several limitations for multi-camera extrinsic calibration,
with cameras possessing highly different points of view and a wide baseline.
The 2D pattern can only be detected from one direction and needs to be of
significant size to compensate for its distance to the camera. This makes the
extrinsic calibration time-consuming and cumbersome. To overcome these
limitations, we propose eWand, a new method that uses blinking LEDs inside
opaque spheres instead of a printed or displayed pattern. Our method provides a
faster, easier-to-use extrinsic calibration approach that maintains high
accuracy for both event- and frame-based cameras.
