Lyman-alpha Hygrometer
Lyman-alpha Hygrometer

The Lyman-alpha Hygrometer measures the absorption by water vapor of vacuum-ultraviolet light at the Lyman-alpha wavelength of atomic hydrogen (121.6 nm). Air from outside the aircraft is brought to the absorption cell by a forward facing open-ended cone inlet (SCAI inlet) and exhausted through a scarf tube. The instrument ionizes the water molecules themselves as they pass through. Using a high intensity direct current discharge lamp, light at 121.6nm (called Lyman-alpha light) photodissociates water molecules producing excited OH radicals. Fluorescence is produced as the OH radical emits photons at 309nm. Making use of a phototube sensitive to this wavelength and a counter, the amount of OH can be calculated. The ambient air in the sample diminishes this signal by an amount proportional to the mixing ratio, so knowing the ambient pressure and temperature yields the mixing ratio from which the total water is determined. The instrument is periodically calibrated in flight by injecting a known amount of water vapor directly into the sample.