Photography is employed to objectively quantify opacity of fluids such as smoke plumes and dust via a method termed the Digital Optical Method (DOM™). The DOM™ quantifies the ratio of radiance values by means of a camera response curve obtained using objective measures. The radiance ratios are then used to calculate opacity of target fluids such as smoke plumes. The DOM™ quantifies opacity during both daytime and nighttime conditions with a much broader range of subject types, e.g., white, gray and black smoke plumes, and environmental conditions, e.g., non-blue-sky, building, and mountain backgrounds, than existing systems while not requiring human interpretation for any application. In one embodiment, the DOM™ quantifies opacity from digital photos using a pre-designed algorithm and an inexpensive digital camera. Very little training is needed to implement the DOM™ and it yields consistent objective quantitative results, while providing a permanent photographic record easily digitally archived.