We analyze light propagation in an unknown scene using projectors and cameras that operate at transient timescales. In this new pho-tography regime, the projector emits a spatio-temporal 3D signal and the camera receives a transformed version of it, determined by the set of all light transport paths through the scene and the time de-lays they induce. The underlying 3D-to-3D transformation encodes scene geometry and global transport in great detail, but individual transport components (e.g., direct reﬂections, inter-reﬂections, caus-tics, etc.) are coupled nontrivially in both space and time.
To overcome this complexity, we observe that transient light trans-port is always separable in the temporal frequency domain. This makes it possible to analyze transient transport one temporal fre-quency at a time by trivially adapting techniques from conventional projector-to-camera transport. We use this idea in a prototype that offers three never-seen-before abilities: (1) acquiring time-of-ﬂight depth images that are robust to general indirect transport, such as in-terreﬂections and caustics; (2) distinguishing between direct views of objects and their mirror reﬂection; and (3) using a photonic mixer device to capture sharp, evolving wavefronts of “light-in-ﬂight”.