A new technique for analyzing excited state absorption (ESA) is introduced, in which the relative ESA spectrum is determined directly from fluorescence measurements. This represents, to our knowledge, the first application of the McCumber theory [D. E. McCumber, Phys. Rev. 136, A954 (1964)] to the calculation of excited state absorption spectra. The new technique is used to study ESA transitions from the 1G4 level in Pr3+ doped fluoride glass (ZBLAN). An ESA transition around 1380 nm is found, which shifts the peak of the gain curve for a 1.3 μm Pr3+ fiber amplifier about 10 nm to shorter wavelengths, and gives rise to negative gain for wavelengths longer than 1380 nm. Another ESA transition around 840 nm is found, which is an order of magnitude stronger than the 1 μm ground state absorption.