An infrared hygrometer is described which utilizes a light beam as the principal sensing element. Composed of two narrow bands of infrared radiation, this beam traverses a 12‐in. path through a sample atmosphere containing the humidity concentration to be measured. Wavelength isolation is by means of germanium narrow band‐pass interference filters. One band, centered near a wavelength of 2.60 μ, is subject to attenuation by water vapor, the other located near 2.45 μ is not. Thus the ratio of transmitted band energies is sensitive to the concentration of water vapor in the path. This ratio is effectively maintained at unity through use of a servo‐operated glass wedge which moves to compensate for energy unbalance caused by changes in absorbing vapor. Wedge position is given electrical significance through use of an appropriate transducer, calibrated in terms of vapor density, grams per cubic meter. Included in the design is a dry‐gas purge standardization system whereby periodic checks of the zero point enable compensation for drift factors. Because of the potential use of the method in airborne applications, the instrument was used in tests to determine the character of pressure effects. These showed that vapor concentration (W
), total pressure (P
), and partial pressure (p
) of water vapor, may be related to fractional absorption (A
Conditions under which other, simpler relations may be used are discussed.