This paper describes a new method to image the flame index in partially premixed flames. To improve modeling of partially premixed flames, such as those in gas turbines and scramjets, there is a need to measure the flame index. CHEMKIN modeling of both premixed and non-premixed methane (CH4) / air flames determined that acetone (CH3COCH3) and nitrogen dioxide (NO2) were good tracers of methane and oxygen (O2). Using simultaneous acetone and NO2 Planar Laser-Induced Fluorescence (PLIF), flame index was measured in a CH4 / air co-flow burner with acetone and NO2 seeding. The application of the method to turbulent flames is discussed, as well as the signal-to-noise ratio.