Multiple-injection strategies are widely used in DI diesel engines. However, the interaction of the injection pulses is not yet fully understood. In this work, a split injection into a combustion vessel is studied by multiple optical imaging diagnostics. The vessel provides quiescent high-temperature, high-pressure ambient conditions. A common-rail injector which is equipped with a three-hole nozzle is used. The spray is visualized by Mie scattering. First and second stage of ignition are probed by formaldehyde laser-induced fluorescence (LIF) and OH* chemiluminescence imaging, respectively. In addition formation of soot is characterized by both laser-induced incandescence (LII) and natural luminosity imaging, showing that low-sooting conditions are established. These qualitative diagnostics yield ensemble-averaged, two-dimensional, time-resolved distributions of the corresponding quantities. Not all of the quantities are measured simultaneously, but the distributions of all quantities can be compared precisely, due to the quiescent, well-controlled conditions in the vessel. Thereby, interfering signals are also analyzed.In particular, the ignition mechanism of the second injection is investigated. The images indicate that the mixture fields of the first and second pulse get into contact at a certain time during the injection event. After that, a relatively fast ignition of the second pulse is observed. These findings demonstrate that the second pulse is ignited by the combusting first pulse, rather than by an auto-ignition. Thus, the ignition mechanisms of the first and the second injection pulse are fundamentally different. This behavior is found for all investigated dwell times between the pulses.