Photography is employed to objectively quantify opacity of fluids such as smoke plumes and dust via a method termed the Digital Optical Method (DOM.TM.). The DOM.TM. 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.TM. 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.TM. quantifies opacity from digital photos using a pre-designed algorithm and an inexpensive digital camera. Very little training is needed to implement the DOM.TM. and it yields consistent objective quantitative results, while providing a permanent photographic record easily digitally archived.