Method 9 is a reference method established by U.S. Environmental Protection Agency (EPA) to quantify plume opacity by the certified observer. Later, digital Optical Method (DOM) was developed to quantify plume opacity from digital photographs for bright and dim conditions. However, there is a limitation for the use of Method 9 and DOM to quantify the plume opacity especially for dim condition. In this paper, DIM-DOM system was used instead of DOM to quantify plume opacity during dim condition. Opacity readings provided by DIM-DOM were compared with the opacity values obtained with the reference in-stack transmissometer of the smoke generator. The individual opacity error results demonstrated that DIM-DOM met Method 9’s requirements. The individual opacity values ranging from 0 to 100% compare well to the corresponding opacity results from the in-stack transmissometer results meeting USEPA’s Method 9 IOE requirement of ≤15%. These results are encouraging and indicate that DIM-DOM has the potential to quantify plume opacity during dim condition.