From AMS Glossary
A hygrometer based on the absorption of radiation by water vapor at the Lyman-alpha line, which is an emission line of atomic hydrogen at 121.567 nm.
Lyman-alpha radiation can be generated by a glow discharge in hydrogen, and detection is normally accomplished by a nitric oxide ion chamber. Two magnesium fluoride windows both at the radiation source and the detector bound the absorption path. Lyman-alpha hygrometers are used on aircraft and on meteorological towers for high-frequency humidity measurements. Inconveniences of the method, like drift of the source intensity or contamination of the windows, are overcome by special calibration techniques or by baselining the high-frequency output to the humidity values provided by a slower, but stable, hygrometer.