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Light can be characterized as a wave composed of oscillating electric and magnetic fields. Light waves, or electromagnetic waves as they are better called, are distinguished by their wavelength or frequency. Figure 4‐1 illustrates the typical sinusoidal nature of an electromagnetic wave and its associated wavelength. The length between successive oscillations of a wave is called wavelength (λ). The period of time that it takes a wave to go through an oscillation cycle is called frequency (v). Wavelength and frequency are related by the following expression:

Figure 4‐1 An oscillating electric field and its wavelength.

(4‐1)

where c is the velocity of light, 3.0 × 10⁸ m/s.

Literature describing continuous monitoring instruments often specifies the wavelength to characterize the spectral region used in the analytical method. Different units are often used for wavelength in different regions of the electromagnetic spectrum although the nanometer (nm) = 10⁻⁹ m has become the standard unit. Another unit, the Angstrom, Å = 10⁻¹⁰ m, has been used historically in the ultraviolet region. In the infrared region, both the μm = 10⁻⁶ m (also called 1 “micron”) and the wavenumber are commonly used by spectroscopists. The wavenumber is expressed as

(4‐2)

Note that the units of are given in terms of the number of wavelengths per centimeter, called reciprocal centimeters or wavenumbers. The wavenumber is essentially a measure of frequency, differing from v by the constant factor of the velocity of light. The wavenumber is calculated by calculating the reciprocal of the wavelength expressed in μm and multiply by 10⁴ to obtain wavenumbers in units of cm⁻¹.

The wavelength of light used in CEM instrumentation ranges from 200 nm in the ultraviolet to 20 000 nm in the infrared. Figure 4‐2 shows the regions of the electromagnetic spectrum and the wavelength bands where molecules interact with light energy. Instruments are designed to measure the effects of these interactions.

The infrared spectral region is particularly important for the measurement of gaseous pollutants, and many instruments have been designed to operate in the infrared region. The infrared region is separated into the near infrared (NIR), mid infrared (MIR), and far infrared. Although an ISO standard does exist (ISO 2015), the boundaries of these regions are not universally agreed upon; there exists some ambiguity as to where the mid infrared ends and the far infrared begins, depending upon the usage by instrument manufacturers, spectroscopists, astronomers, or others.