A combusting plume in an optically accessible direct-injection diesel engine was studied using simultaneous 2-D imaging of laser-induced incandescence (LII) and natural flame luminosity, as well as simultaneous 2-D imaging of LII and elastic scattering. Obtaining images simultaneously via two different techniques makes the effects of cycle-to-cycle variation identical for both images, permitting the details of the simultaneous images to be compared. Since each technique provides unique information about the combusting diesel plume, more can be learned from comparison of the simultaneous images than by any of the techniques alone.Among the insights gained from these measurements are that the combusting plume in this engine has a general pattern of high soot concentration towards the leading edge with a lower soot concentration extending upstream towards the injector. Also, the soot particles are found to be larger towards the leading edge of the plume than in the upstream region. Another significant result is that no liquid fuel droplets were found in or near the combusting region of the plume indicating that, in this engine at these conditions, the liquid has evaporated and the fuel in the combustion zone is gas phase.The simultaneous images also permit a direct comparison of the capabilities of LII and elastic scattering for diesel-engine soot distribution studies. In addition, the emission spectrum of LII in the engine is reported, and shown to consist only of broadband emission which resembles that of a gray body at 4500 K.