There is described a simple photoelectric spectrophotometer, developed to determine the precipitable water in a vertical column of the atmosphere. A transmission replica grating is used as the dispersing element and an infra-red-sensitive photo-tube as the energy receiver. The photo-tube current is amplified by a negative feed-back d.c. amplifier and is read either by means of a 500 microampere ammeter or recorded by a high speed potentiometer recorder. Measurements are obtained by comparing the radiant flux in the 0.94µ water vapor absorption band with that at 1.01µ where no absorption occurs. A single-mirror heliostat is utilized to keep the spectrophotometer focused on the sun. An approximate calibration was obtained by plotting the spectrophotometer ratios against the total precipitable water values in a vertical column of the atmosphere as calculated from radiosonde moisture data.
NORMAN B. FOSTER and LAURENCE W. FOSKETT, "A Spectrophotometer for the Determination of the Water Vapor in a Vertical Column of the Atmosphere," J. Opt. Soc. Am. 35, 601-605 (1945)