ABSTRACT
Optical hygrometry is a classic method of humidity measurement. Detection and determination of the water vapor concentration take place through the measurements of optical properties. The first known measurements of the water vapor absorption in the infrared (IR) were undertaken by Fowle in 1912. However, optical methods had the greatest development only in the last 30 years, when there appeared the low-cost optoelectronic components such as light-emitting diodes (LEDs), laser diodes (LDs), photodetectors (PDs), and a fast data-acquisition and the data-processing technique. Optical hygrometers form a sufficiently large class of devices, which can be divided into subgroups on the basis of (1) the waveband used for the detection of water vapor (visible, infrared, and ultraviolet), (2) the fraction of light, which is detected after interaction with the sample (i.e., the transmitted, reflected, or absorbed fraction), and (3) spectral width of light (e.g., monochromatic, polychromatic, and reference light with continuous spectrum). All these types of optical hygrometers, including their construction, principles of operation, and approaches to practical realization are discussed in the present chapter.
