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Diffraction Grating

Theory of Operation

What is a Diffraction Grating?

Theory of Operation

Our NIR-O™ spectrometers, both the process and lab analyzer use a diffraction grating. The basic optical element of the Analyzer is a plane (flat) diffraction grating. A diffraction grating is essentially an aluminum-coated mirror with thousands of parallel and equally spaced grooves etched into its surface. (This surface is very delicate and should never be touched or cleaned.) As such, a grating is one of the most precise objects ever made. How gratings work is described in all good introductory college physics textbooks. Let us review a few of these facts, as they are important to the correct operation of the Analyzer. The operation of the grating is defined by the grating equation:

mλ = 2d(sin θ + sin φ)
m = the grating order integer
λ = the wavelength
d = the grating constant (lines per millimeter)
θ = the angle of the incident light (measured from the perpendicular to the grating)
φ = the angle of the diffracted light (also measured from the grating perpendicular)

This equation describes how white light is dispersed into its fundamental wavelengths (e.g., color for visible light). White light enters the monochromator through an entrance fiber in the multiplexer housing. The light is dispersed onto the output module fiber array by the grating, where it is directed to the detector module. A spectrum is recorded by rotating the grating and measuring the intensity of the light impinging on the detector.

The Analyzer is programmed to calibrate itself via the insertion of a NIST traceable rare-earth standard filter mounted on the filter wheel. Thus, the grating angles are accurately translated into the wavelength axis that is presented in a normal spectral scan.

It is essential, however, that the user understands the significance of the grating order integer, m. For zero order, m = 0, the angles of incidence and diffraction are equal but have opposite signs. This is the condition for a mirror – no dispersion occurs and white light is present at the exit fiber. Zero-order is only useful during the initial calibration process and, therefore, is of no concern. The spectrometer is designed to work in the first order, m = 1. However, under certain conditions, second order, m = 2, light may reach the detector. This second order light will have a negative impact on the photometric linearity of the resulting absorption measurements. An example of this is that 1000 nm light will appear naturally at 1000 nm in the first order but also at 2000 nm in second order and again at 3000 nm in the third order. If the instrument is equipped with an extended range InGaAs detector that is sensitive from 900 – 2150 nm, second order light will begin to appear at about 1950 nm since there is a 975 nm long pass filter permanently mounted in the lamp housing. This second order light is undesirable and needs to be filtered out. To prevent second order radiation from interfering with first-order analytical measurements, it is customary to insert an order sorting (long pass) filter into the optical path. This filter is provided in the lamp assembly in the form of a 1550 nm long pass filter (LP1550). For spectra recorded beyond 1900 nm, it is recommended to use the long pass option in the setup screen of the analyzer control software and insert the LP1550 filter at a point in the spectra that has no, or little, spectral information. The insertion point can be anywhere between 1550 nm and 1950 nm.

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