Methods of developing a photosensitive film to a high degree of uniformity are demonstrated to be dependent upon the effectiveness of the mixing of the developing solution and the disruption of the formation of a concentration boundary layer during the developing process. Adaptation of the hydraulic brush developing technique and use of film and microphotometer calibration procedures based on a primary standard of measurement made possible the extension of the useful range of commercial films for photometric purposes to a density of over 2.5. These methods permit the use of photographic photometry to make measurements of irradiance varying over a range of 100 to 1 with an accuracy of better than 10% over the entire range.