Energy transition strongly leads researcher to find short-term alternative carbon-free fuelsto be used in internal combustion engines. An interesting and transient solution can be thedual fuel (DF) technology. It can offer significant reduction of carbon dioxide and pollutantemissions. In this paper, DF operation was performed in an optically accessed researchdiesel engine running at a constant speed of 1500 rpm. Substitute fuels (methane orhydrogen) were injected in the intake manifold in front of the entrance of the tumbleintake port. The objective was to compare the two DF modes using simultaneously Fast UVeVisible and Fast Infrared (IR) Imaging. We observed that IR camera was able to give deepercombustion information and allowed to grab the whole phenomenon for both DF hydrogenand methane, while identifying the combustion starting points. Also, IR acquisitions forpremixed hydrogen evidenced an unpredictably behaviour of the mixture, demonstratingthe necessity of a specific Diesel injection strategy to avoid hydrogen auto-ignition athigher engine load. Finally, a comparison of the measured emissions at the exhaust evidencedthat in DF mode (both with CH4 and H2) a significant reduction of particulate matteris achieved with respect to conventional diesel operation. In addition, when hydrogen isused as primary fuel, a more efficient combustion is obtained drastically reducing HC andCO2.