A developing practical approach of dual fuel combustion facilitates the fuel flexibility and provide a base to utilize diverse fuels in internal combustion engines. The abundance of natural gas and its clean burning property due to its lower C/H ratio has drawn the interest toward it. In addition, natural gas has high octane number and high auto-ignition temperature, which makes it a potential fuel to employ as a primary fuel in typical diesel engines with high compression ratio resulting in diesel like efficiency. In this study, DF-combustion involving methane (99.8% pure) as a primary fuel and diesel as a pilot fuel is subjected. In diesel-methane DF combustion, the combustion of lower reactivity fuel methane initiates through the energy provided by compression ignited pilot amount of high reactivity diesel fuel. The experimental investigation of DF-combustion was carried out by a natural luminosity optical method. The objective of this study is to visualize and characterize the DF-combustion at 1) varying methane lambda 2) varying intake air temperature 3) varying pilot amount. Experiments have been performed in a single cylinder, optically accessed Bowditch extension heavy-duty diesel engine and with a high-speed color camera. The camera recorded 512x512 pixels natural luminosity images with a temporal resolution of 0.11 ms at 1400 rpm, ~80% methane substitution rate and at I.M.E.P~10 bar. The optical results are accompanied with the corresponding cylinder-pressure, net heat release rate and ignition delay analyses. The results showed that decreasing methane lambda, the ignition delay (ID) increases because the presence of methane in the in-cylinder mixture affects the pre-ignition reaction activity of pilot. On the other hand, increasing the intake-air temperature decreases ID as it affects the physical and chemical properties of the mixture at the time of combustion. Moreover, with increasing pilot amount, affluent flame front propagation can be observed.