The theory of the two path-length method for optically determining flame temperature is reviewed, and the requirements for quantitative photographic measurement are presented. Apparatus which includes a camera with the necessary combination of film and filters to accept light in a narrow wavelength interval, and several adjustable lamps to serve as standard sources, is described. The film record contains an image of the rocket exhaust jet adjacent to the images of the lamp filaments whose temperatures are known from a previous calibration with an optical pyrometer. Comparison of measured intensities with and without a reflector behind the flame yields the emittance and flame temperature. Some results of measurements at the exit regions of rocket nozzles affording nearly optimum expanison are presented. The utility of the method in rocket performance studies is discussed.
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