A hygrometer that determines the amount of precipitable moisture in a given region of the atmosphere by measuring the attenuation of radiant energy caused by the absorption bands of water vapor. The instrument consists of a collimated energy source, separated by the region under investigation from a detector that is sensitive to those frequencies that correspond to the absorption bands of water vapor. The basis for determining the water vapor concentration is Beer's Law: I/ I₀ = exp(−kx), where I is the light intensity after passing through the sample, I₀ is the incident intensity, x is the pathlength reduced to some absolute standard like STP, and k is the absorption coefficient. The most useful regions of the electromagnetic spectrum for this purpose lie in the ultraviolet and infrared regions. The most widespread application is the monitoring of very-high- frequency variations in humidity, as the time constant of a spectral hygrometer is typically just a few milliseconds. The use of spectral hygrometers remains mostly restricted to research applications. See Krypton hygrometer, Lyman-alpha hygrometer, differential absorption hygrometer.