Photometry was performed on 71 photographs of a model engine flame fired under simulated high-altitude conditions in a low pressure chamber to determine the flame brightness. The photographs were obtained in three colors covering the spectral range from 3600 to 8600, and at ambient pressures of 3-, 30-, 160-, and 760-mm of Hg. The surface brightness of the flame alternates by a factor of nearly 10,000 as the ambient pressure is lowered from sea level to 3 mm of Hg. On the other hand, the flame dimensions dilate by a factor of 10, so the total energy radiated by the flame drops only a factor of 100 over the pressure range covered. A discussion is given of the factors affecting the relative exposure of the flame under various conditions on the photographic emulsion in a ballistic camera. It is shown that at the ranges of interest, the flame will usually image either as a line or point. Author
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