As invisibility cloaking has recently become experimental reality, it is
interesting to explore ways to reveal remaining imperfections. In essence, the
idea of most invisibility cloaks is to recover the optical path lengths without
an object (to be made invisible) by a suitable arrangement around that object.
Optical path length is proportional to the time of flight of a light ray or to
the optical phase accumulated by a light wave. Thus, time-of-flight images
provide a direct and intuitive tool for probing imperfections. Indeed, recent
phase-sensitive experiments on the carpet cloak have already made early steps
in this direction. In the macroscopic world, time-of-flight images could be
measured directly by light detection and ranging (LIDAR). Here, we show
calculated time-of-flight images of the conformal Gaussian carpet cloak, the
conformal grating cloak, the cylindrical free-space cloak, and of the invisible
sphere. All results are obtained by using a ray-velocity equation of motion
derived from Fermat's principle.Comment: 11 pages, 6 figures, journal pape