  Light-in-flight (LIF) imaging is the measurement and reconstruction of
light's path as it moves and interacts with objects. It is well known that
relativistic effects can result in apparent velocities that differ
significantly from the speed of light. However, less well known is that
Rayleigh scattering and the effects of imaging optics can lead to observed
intensities changing by several orders of magnitude along light's path. We
develop a model that enables us to correct for all of these effects, thus we
can accurately invert the observed data and reconstruct the true
intensity-corrected optical path of a laser pulse as it travels in air. We
demonstrate the validity of our model by observing the photon arrival time and
intensity distribution obtained from single-photon avalanche detector (SPAD)
array data for a laser pulse propagating towards and away from the camera. We
can then reconstruct the true intensity-corrected path of the light in four
dimensions (three spatial dimensions and time).
