The sodium line reversal technique was used to determine the reversal temperature profile across the exit of small rockets. Measurements were made on one 73-kg thrust rocket, and two 23-kg thrust rockets with different injectors. The large rocket showed little variation of reversal temperature across the plume. However, the 23-kg rockets both showed a large decrease of reversal temperature from the axis to the edge of the plume. In addition, the sodium line reversal technique of temperature measurement was compared with an infrared technique developed in these laboratories.
M. Griggs and F. C. Harshbarger, "Measurements of Temperature Profiles at the Exit of Small Rockets," Appl. Opt. 5, 211-214 (1966)