The experimentally measured emission spectrum needs to be corrected for the peculiarities of the spectrophotometer, which includes the transmissive properties of the monochromators, and the detector's sensitivity as a function of wavelength. Typically, the instrument manufacturer will supply a correction file, which consists of a set of multiplication factors for each wavelength. After applying that to your measured spectrum, you will have the true emission spectrum. In a fluorescence resonance energy transfer experiment, to calculate the Forster distance Ro you must measure the overlap integral J, the area under the overlapping curves of the normalized emission spectrum of the donor and the absorbance spectrum of the acceptor. You must use the corrected emission spectrum for this purpose.