In the title method, ?15 colored lines of const. optical d. (equidensities) are produced. Hexane, butane, MeOH, and liquefied natural gas, in pools of varying diam. or as a natural gas jet flame, are investigated without influencing combustion. Equidensities obtained from time exposures, instantaneous photographs, and a series of high-speed frames are evaluated. The relation between the optical d. of the emulsion and the spectral radiance (Ll) emitted by the flame in the visible region is derived. Equidensities obtained from time exposures can be related to time-averaged values of flame properties such as radiance, temp., flame shape, flame height, and soot concn. There is a characteristic dependence on fuel type. The min. sampling time for statistical combustion processes is easily measured.