Ignition systems for large lean burn gas engines are challenged by large energy deposition requirements to ensure stable and reliable inflammation of the premixed charge. In this study, two different ignition systems are investigated experimentally: ignition by means of injecting a small amount of diesel spray and its subsequent autoignition is compared to the ignition with an un-scavenged pre-chamber spark plug over a wide range of engine relevant conditions such as methane equivalence ratios and thermomechanical states. The ignition behavior as well as the combustion phase of the two systems is investigated using an optically accessible Rapid Compression Expansion Machine (RCEM). Filtered OH-chemiluminescence images of the ignition and combustion were taken with a UV intensified high speed camera through the piston window. Pressure signals along with filtered photomultiplier signals of the total emitted light for three different radicals (OH, CH, C2) were recorded to study the effects of ignition system and operating conditions on the early combustion phase.Ignition with a pre-chamber spark plug was seen to accelerate the early combustion phase in the main chamber due to the turbulence generated by the flame jets, but following the dissipation of this initial turbulence slower flame propagation was observed. Higher temperatures and equivalence ratios (Φ) lead to increased turbulence generation of the flame jets as well as shorter delays between spark timing and entrance of the jets into the main combustion chamber. Similar trends with respect to temperature and pressure conditions were observed in the case of Diesel pilot ignition. However, the ignition delays of the pilot spray increased at higher levels of methane in the ambient gas. Generally, the flame/ignition jets generated by pilot injection provide a more stable and stronger ignition source than those achieved by means of pre-chamber spark plug ignition.