Spectroradiometric calibration techniques in the far ultraviolet: a stable emission source for the Lyman bands of molecular hydrogen.

The problems associated with making accurate spectroradiometric measurements in the far uv region are sketched briefly. The equipment and methods that were developed for providing absolute sensitivity calibration of an Apollo 17 far uv spectrometer are described. The absolute reference standards were photoelectric diodes calibrated at the National Bureau of Standards. A complete vacuum optical facility, which included a premonochromator and stable uv light sources, was developed to calibrate the Apollo 17 instrument, and it has been used for a number of other tasks. Absolute radiometric calibrations between about 1200 A and 1700 A were performed with an absolute accuracy of +/-10%. The light source, which was designed to provide a very stable light output, is a low-pressure molecular hydrogen lamp in which the pressure is stabilized by thermal control of uranium hydride powder. Individual emission lines of the Lyman molecular band system of H(2) are used for calibration purposes. The lamp also copiously emits the 1215.7-A line (Lyalpha) of atomic hydrogen.