A type of spectral hygrometer using at least two closely spaced wavelengths. One is chosen inside an absorption band, the other outside and not affected by water vapor absorption. Subtraction of the latter measurement from the first cancels all absorption effects except that by the water vapor. Despite this simple principle the technical realization is difficult due to the need for adjusting and maintaining the two wavelengths very precisely. Instruments based on this technique are differential absorption lidars operating on two fixed wavelengths, or, as a recent development, tunable diode lasers. The wavelength of these lasers can be changed or tuned continuously over a small wavelength range allowing for monitoring the shape and intensity of a whole absorption line and its vicinity.