AnexacttheorydescribingtheintensityPSDofphotonicsignalprocessorsbasedonmodulatedthermalsourceswithsmoothspectrum,first-orderdispersionandintensity-noise-limiteddirectdetectionispresented.Thetheoryisappliedtotheevaluationofthesignal-to-noiseratio(SNR)ofMicrowavePhotonicFilters(MPF)andofsys