  We present a fully relativistic model for localized probes in quantum field
theory. Furthermore, we show that it is possible to obtain particle detector
models from localized quantum field theories that interact with a free quantum
field. In particular, a particle detector model is obtained when one traces out
over inaccessible degrees of freedom of the localized field. This gives rise to
a particle detector model, that is, a quantum degree of freedom that couples to
a free field theory in an extended region of spacetime. Moreover, we show that
the predictions of traditional particle detector models and fully relativistic
localized fields completely coincide to leading order in perturbation theory.
