  We derive photon counting statistics for an output field of a single-photon
wave packet interacting with a quantum system (e.g. a quantum harmonic
oscillator or a two-level atom). We determine the exclusive probability
densities for the output field by making use of quantum filtering theory. The
quantum trajectories for continuous in time measurements of the output field
(reflected and transmitted), are determined starting from a collision model and
difference filtering equations. We provide analytical formulae for quantum
trajectories associated with a two-dimensional stochastic counting process, we
describe their structure, and give a physical interpretation to them. Moreover,
we provide analytical expressions for probability densities for the times of
successive photon detections for a single-photon field scattered on a two-level
atom, for an arbitrary photon profile and any initial state of the atom.
