The instantaneous soot formation in a Diesel combustion chamber is generally measured using an auxiliary light source, typically a laser, which leads to use experimental engines with simplified geometries and reduced compression ratio.The proposed optical method use thermal radiations of soot particulates inside the flame to investigate their concentration and temperature. This method, based on the diffraction theory for small particles, is applicable to industrial Diesel engines without major modifications of their main characteristics. Measurements are performed at two locations inside the combustion chamber of an industrial D.I. Diesel engine. The presented results show a radial stratification of the soot concentration, soot particles beeing present primarily in the bowl location. With heavy loads, a second stage of soot generation and oxidization is observed.