  We introduce a novel method for measuring properties of periodic phenomena
with an event camera, a device asynchronously reporting brightness changes at
independently operating pixels. The approach assumes that for fast periodic
phenomena, in any spatial window where it occurs, a very similar set of events
is generated at the time difference corresponding to the frequency of the
motion. To estimate the frequency, we compute correlations of spatio-temporal
windows in the event space. The period is calculated from the time differences
between the peaks of the correlation responses. The method is contactless,
eliminating the need for markers, and does not need distinguishable landmarks.
We evaluate the proposed method on three instances of periodic phenomena: (i)
light flashes, (ii) vibration, and (iii) rotational speed. In all experiments,
our method achieves a relative error lower than 0.04%, which is within the
error margin of ground truth measurements.
