Instruments using absorption of electromagnetic radiation to sense humidity. Ultraviolet (UV) hygrometers use a single wavelength absorbed by molecular water vapor, such as the Lyman-alpha line produced by a hydrogen discharge tube. Infrared (IR) hygrometers can also operate on a single absorption line if the source is an infrared-emitting tunable diode laser (TDL), though broad-band instruments, which integrate the absorption across many individual lines, are also used. These instruments utilize Beer's law to estimate the density of absorption across the path between the source and a detector (e.g., an ionization tube for ultraviolet, or a photodiode for infrared), which contains the air sample. In many cases, these hygrometers have sufficient speed of response that they can resolve rapid turbulent fluctuations of humidity.