The scanning grating monochromator and polychromator diode-array

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 wave front 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 classic diffraction equation for normal incidence is satisfied  

Where in this expression a and b are the width and spacing between the slits of the grating, and hence d (= a + b) is defined as the grating constant. Here d and X are measured in the same units of length. The diffraction order is given by p, an integer value. 

In an analyzer based on scanning monochromator technology, one should be aware of a number of critical design issues  

The effect of multiple-order diffraction, if not corrected, is to introdnce a stray light signal at the detector, which badly affects the measnred linearity of the required absorbance signal. In the example above, if the required measurement is at 2000nm, and only 0.1% of stray light at lOOOnm reaches the detector, then appreciable absorbance nonlinearity is seen at absorbances above 2.0 AU. 

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

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