An open systems approach to calculating time dependent spectra

A new method to calculate the spectrum using cascaded open systems and master equations is presented. The method uses two state analyzer atoms which are coupled to the system of interest, whose spectrum of radiation is read from the excitation of these analyzer atoms. The ordinary definitions of a spectrum uses two-time averages and Fourier-transforms. The present method uses only one-time averages. The method can be used to calculate time dependent as well as stationary spectra.