The scanning grating monochromator

What follows is a detailed discussion of the principles of operation and practical limitations of analysers based on scanning monochromators. Included in this discussion will also be the background information on NIR source and detector technologies, which are in fact common to all of the NIR spectroscopic technologies listed above. 

The diffraction grating monochromator is a specific example of multiple-beam interference effects. Interference between multiple beams can be generated by both division of 

The two examples shown demonstrate how an interference effect can be produced either by amplitude division of an incident beam, followed by retardation (achieved in this case by multiple reflection between the partially reflective parallel plates of a Fabry-Perot resonator) and recombination, or by division of the wavefront at the multiple equally spaced slits of a diffraction grating, again followed by recombination. 

The condition for the formation of an interference maximum at the point of recombination is that the contributions to the total irradiance at that point from all the slits of the grating should be in phase. This condition is met when the classical diffraction equation for normal incidence is satisfied  

One further point to mention about the design and use of concave holographic gratings concerns the exit focal plane. For a scanning monochromator it is of no great concern that 

---

Near-infrared Spectroscopy for Process Analytical Technology 119 5.3.2 The scanning grating monochromator and polychromator diode-array... 

A dispersive element for spectral analysis of PL. This may be as simple as a filter, but it is usually a scanning grating monochromator. For excitation spectroscopy or in the presence of much scattered light, a double or triple monochromator (as used in Raman scattering) may be required. 

In this section the mainstream NIRS technologies in current widespread practical use for PAC applications will be examined. These are scanning grating monochromators, grating polychromator PDA spectrometer, AOTF analysers and FTNIR analysers. 

Three major advances have moved the technique closer to equal utility with chromatographic methods for release testing. The first advance was the development of suitable hardware. While useful for food and agricultural products, early interference type filter instruments and scanning grating monochromators were neither sensitive nor quiet (instrument noise, not operating noise) enough for certain pharmaceutical applications. The development of stable, sensitive, and precise... 

Fluorometry and Phosphorimetry. Modem spectrofluorometers can record both fluorescence and excitation spectra. Excitation is furnished by a broad-band xenon arc lamp foUowed by a grating monochromator. The selected excitation frequency, is focused on the sample the emission is coUected at usuaUy 90° from the probe beam and passed through a second monochromator to a photomultiplier detector. Scan control of both monochromators yields either the fluorescence spectmm, ie, emission intensity as a function of wavelength X for a fixed X, or the excitation spectmm, ie, emission intensity at a fixed X as a function of X. Fluorescence and phosphorescence can be distinguished from the temporal decay of the emission. 

---