  We investigate a class of microscopic systems in interaction with a
macroscopic system in thermal equilibrium, following the construction of
Dalibard, Dupont-Roc and Cohen-Tannoudji (DDC). By considering self-adjoint
operators as elements of Schwinger's Measurement Algebra (SMA), we construct
statistical mean values of the relevant observables as matrix elements in a
suitable operator basis, which correspond to the vacuum states of ThermoField
Dynamics (TFD).
