ABSTRACT
Absorption or colorimetric-based optical humidity sensors joined a large group of devices designed to measure the level of humidity in the atmosphere. Their functioning is based on detection of a humidity-induced absorption or color change in the sensing material. Therefore, the change in the intensity of radiation, measured at a particular wavelength, is the main index of concentration of water vapor in the air. This chapter gives a detailed description of these devices, including the sensor configurations that are acceptable for such measurements and the humidity-sensitive materials used in such devices. As it is shown in the present chapter, many different materials such as polymers, metal oxides, metal–organic frameworks, and salts can change their optical properties under the influence of moisture. However, the most common material used in the development of colorimetric humidity sensors is cobalt chloride, which shows a strong change in the absorption spectrum at hydration. In recent years, an increased interest has been also shown to humidity-sensitive materials based on photonic crystals. In most colorimetric-based fiber-optical humidity sensors the humidity indicators are located on either end of the fiber or between the two fibers. To measure the optical signal as a function of humidity, light usually is coupled into the sensor from one end of the fiber passing through the dye doped film and collected by the other fiber. Colorimetric humidity sensors can also operate in the reflecting mode if they are manufactured in configuration of optodes. Advantages and disadvantages of colorimetric humidity sensors are also discussed in this chapter.
