Intensities of dispersed radiation in the region 500–3000A were measured with a compensated thermocouple mounted at the exit slit of a one-meter, normal incidence, vacuum monochromator. With 1-mm slits (17A band width), HeI at λ584 was found to be 0.025 microwatt over the exit slit. With a 1P21 tube coated with sodium salicylate and using 50-micron slits, the relative intensities of the first four members of the principal series of HeI at λ584, λ537, λ522, and λ516 were measured. The H2 spectrum from 850 to 3000A was also obtained with the thermocouple. The relative quantum efficiency of sodium salicylate was found to be constant in this spectral region and about 15 percent lower at λ584. Some of these measurements were made possible by a great increase of grating efficiency after an unintentional coating of the grating with a layer of evaporated platinum.
K. WATANABE and EDWARD C. Y. INN, "Intensity Measurements in the Vacuum Ultraviolet," J. Opt. Soc. Am. 43, 32-35 (1953)