In-cylinder flame temperature and particulate concentration were measured in a Cummins single cylinder NH diesel engine by means of an optical radiation probe using the two-color theory. The radiation probe consists of a specially designed trifurcated fiber optical bundle and a sapphire rod window. A self-cleaning window was designed, which stays clean under steady state full load conditions. The engine was operated at various conditions for both standard cooled and simulated mini-cooled configurations. The heat release rate data and exhaust emissions, NO, NOx and CO are presented along with the radiant emission data.Increasing the coolant temperature gave slightly more exhaust soot production as well as in-cylinder soot production, because more late burning occurred at the higher coolant temperature. It is believed that increased late burning was caused by the delayed end of injection and lower injection rate perhaps due to thermal expansion effects in the injector. Experimental data were used to evaluate the potential radiation heat transfer. The averaged radiation flux to total heat flux ratio was approximately 11 percent, regardless the engine loads and speeds.