Sub-frame timestamping of a camera network using a coded light signal

To extract the 3D displacements of structures using a camera network, it is essential to know the frame timestamps. However, technical constraints in outdoor conditions can make synchronous frame acquisition impossible. This study aims to reconstruct the timestamps of frames during post-processing to employ cameras without special camera feature. To achieve this, a LED panel that emits a coded light signal is placed in the field of view common to all cameras. The pixel intensity of the LED region in the sequential images is processed to reconstruct each frame's timestamp. The uncertainty of the timestamp reconstruction is numerically estimated to encode the light signal. Validation was conducted through indoor experiments and outdoor applications to monitor a tower crane. The uncertainty of the obtained timestamp is 1.65% of the frame interval under uncontrolled conditions. The proposed method can be used with cameras, even when synchronization is unattainable.