Many problems in molecular reaction dynamics demand the simultaneous measurement of a particle's speed and angular direction; the most demanding require the measurement of this velocity in coincidence with internal energy. Studies of molecular reactions, energy transfer processes and photodissociation can only be understood completely if the internal energies and velocities of all products can be specified.
Product imaging approaches this goal by determining the three-dimensional velocity distribution of one state-selected product of the reaction.  For a reaction producing two products, because the speed of the unobserved sibling product is related to that of the measured product through conservation of momentum and energy, the internal state of the sibling can often be inferred.