Fourier-transform spectrometers (FTS or FT spectrometers) have been replacing the dispersive instruments in many infrared and near-infrared applications over the last couple of decades. Their inherent advantages compared with the dispersive instruments are proven and well accepted by scientists and engineers working in the field of spectroscopy. This chapter provides a general overview of the types of spectrometers commonly used today, focusing on the differences in their operating principles. An analysis of an FT spectrometer's advantages over its dispersive counterpart is then presented.
1.2 Types of Spectrometers
Spectrometers can be categorized into three main types based on their principles of operation: dispersive, filter-based, and Fourier-transform instruments.
1.2.1 Dispersive spectrometers
As the name suggests, dispersive spectrometers generate spectra by optically dispersing the incoming radiation into its frequency or spectral components, as illustrated in Fig. 1.1. Common dispersive elements include prisms and gratings. Dispersive spectrometers can be further classified into two types: monochromators and spectrographs. A monochromator uses a single detector, narrow slit(s) (usually two, one at the entrance and another at the exit port), and a rotating dispersive element allowing the user to observe a selected range of wavelength. Figure 1.1 shows the simplified schematic of a monochromator.
A spectrograph, on the other hand, uses an array of detector elements and a stationary dispersive element. In this case, the slit shown in the figure is removed, and spectral elements over a wide range of wavelengths are obtained at the same time, therefore providing faster measurements with a more expensive detection system.