3.2: Diffraction Gratings and Tunable Filter Spectrometers

3.2.1 Dispersive devices

The most direct and commercially embraced methods of UV/Vis/NIR absorption spectroscopy are spatially dispersive techniques utilizing wavelength dispersive prisms or diffraction gratings. Dispersive methods of spectroscopy are advantageous in that these devices are easy to fabricate and simple to operate. Additionally, with technological improvements in both grating fabrication and detectors, these devices have steadily become more inexpensive while maintaining a high performance benchmark. In a static diffraction grating configuration, operating in either transmission or reflection mode, the spectrum of interest is broken down into its relative frequency components by angular separation. The intensity of each spatially discriminated frequency is recorded simultaneously using a focal plane array such as a CCD or photodiode array, as is shown in Figure 3.1.

Fig. 3.1: A static grating spectrometer with all spectral components detected simultaneously by a focal plane array.

Using a diffraction grating, the angular separation as a function of wavelength, ? _m( ?) , is determined from the basic grating relation:

where d is the grating period or pitch, ? _i