Comparison between laser-induced fluorescence (LIF) and laser-induced incandescence (LII) images and axial intensity profiles derived from these images of a laminar ethylene-air diffusion flame distinguish the polycyclic aromatic hydrocarbon (PAH) and soot-containing regions along the axial flow streamline. Examination of the temporal evolution of the combined fluorescence plus incandescence signal along the axial streamline reveals the transformation of soot precursor material into solid carbonaceous soot. Bright field transmission electron microscopy (TEM) of thermophoretically sampled material shows soot precursor material and more generally nascent soot particles and their evolution towards solid carbonaceous soot. Corresponding dark field TEM images track the increasing crystallinity of the soot precursor material as carbonization proceeds with increasing integrated temperature-time history within the flame. Both bright and dark field TEM confirm interpretation of the optical signals as reflecting the overall material transformation from soot precursor material toward solid carbonaceous soot.