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Laser hygrometers measure the amount of water vapor directly via
absorption of laser light. If the initial strength of the laser and
the path length through which it travels is known, measuring the
diminution of its intensity will give an indication of the amount of
water vapor present. The JPL Laser Hygrometer uses this principle in
an open-path instrument; that is, the laser path is in the free stream
passing by the aircraft.
A tunable diode laser operating at 1.37mm is mounted in a window blank
(an aluminum panel which replaces the passenger window) on the port
side of the NASA DC-8. The laser and detector are mounted on a
circular aluminum disk in the upper rectangular 'arm' of the
instrument. Exactly 25cm away is a 0.5 inch diameter mirror from which
the laser beam is reflected. This gives a path length of 50cm- from
the laser, down to the mirror and back to the detector. The length of
the arms insures that the instrument is well outside the boundary
layer of the aircraft minimizing effects of the aircraft itself upon
the measurements.
Additional information available at
"http://ghrc.msfc.nasa.gov/uso/readme/c4djlh.html"
[Summary provided by Global Hydrology Resource Center]
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