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[en] N-dodecane pilot-ignited lean-premixed natural-gas combustion in a rapid compression–expansion machine is investigated. The aim of this study is to characterize the combustion spectral footprint: identify the main sources of natural luminosity, characterize the temporal brightness evolution, and provide guidance for the evaluation of natural luminosity imaging acquisitions. Natural luminosity spectra in the range of 280–610 nm are acquired, 1D-resolved along the injector axis, using an imaging spectrograph and intensified high-speed camera. Four significant contributions to the natural luminosity are identified: Soot, OH* and CH* chemiluminescence, as well as overlapping broadband chemiluminescence of CO2*, CHO* and CH2O* species. The CH* chemiluminescence can only be detected at ignition and during the pilot-fuel combustion period. Similarly, first OH* and broadband luminosity are also detected at ignition. However, this luminosity additionally increases late in the cycle, when methane, enriched with diluted pilot-fuel, forms an extensive burnt zone with close-to-stoichiometry conditions. For the ignition delay detection, imaging of broadband luminosity has to be recommended since at ignition it shows a higher rise-rate than the OH* chemiluminescence. It is shown that, after ignition, in dual-fuel combustion, the coupling between the natural luminosity and heat release-rate is too weak to extract useful information.