A finite emittance model (CLSxBW) for calculating the flux of synchrotron radiation is described. CLSxBW accounts explicitly for the finite source size and divergence of the electron beam in the storage ring. Finite source effects are found to be very critical for the proper calculation of the linear polarization of the synchrotron radiation. Intercomparisons of CLSxBW with SCHWINGER (Bessy computer code) show agreement at 1 and 10 ppm for rectangular and circular detector apertures, respectively. Comparison of emitted power calculated with CLSxBW and power measurements on the white light beamline in BessyII using a broadband detector show excellent quantitative agreement.