A previous study by the authors has shown an efficiency benefit of up to 27 % for stratified operation of a high pressure natural gas direct injection (DI) spark ignition (SI) engine compared to the homogeneous stoichiometric operation. NOx and CO raw emissions in stratified operation were also found lower compared to the homogeneous stoichiometric operation. While best efficiencies appeared at extremely lean operation at lambda = 3.2, minimum HC emissions were found at lambda = 2. The increasing HC emissions and narrow ignition time frames in the extremely lean stratified operation have given the need to further investigate the mixture formation and flame propagation under these conditions. An optically accessible single-cylinder engine was used for a detailed analysis of the engine process. The mixture formation and the flame luminosity have been investigated in two perpendicular planes inside the combustion chamber. By quantifying the equivalence ratio using the laser-induced fluorescence (LIF) technique a correlation between the presence of an ignitable mixture at the spark plug and the indicated start of combustion (SOC) was shown (Friedrich et al. 2015, SAE Technical Paper 2015-24-2474). Through the investigation of the flame luminosity, the main reason for incomplete combustion and the according emissions was revealed. With this cause-and-effect comprehension, the combustion chamber has been modified to enforce an improved charge stratification. A geometric modification ensures deflecting the ignitable mixture to the spark plug and prevents the injection reaching the inner cylinder wall and crevices. Optical investigations serve determining an adapted design for an optimized mixture formation, validated by the 3D-CFD. The target design’s benefit will be verified at a single-cylinder research engine.