The basic optical element of the Analyzer is a concave holographic 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 freshman level college physics textbooks. We will 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:

1. Mλ = 2d(sin θ + sin ф)

• m = is the grating order integer
• λ is the wavelength
• d is the grating constant (lines per millimeter)
• θ is the angle of the incident light (measured from the perpendicular to the grating)
• ф is the angle of the diffracted light (also measured from the grating perpendicular)

This equation describes how white light is dispersed into its fundamental wavelengths (color for visible light). White light enters the monochromator through an entrance slit in the lamp housing. The light is dispersed onto the output module fiber array by the grating, where it is separated into the various channels of the spectrometer. A spectrum is recorded by rotating the grating and measuring the intensity of the light exiting the fibers.

The Analyzer is programmed to calibrate itself via the insertion of the appropriate standards into the light path. Thus the 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. For zero order, m = 0, the angle of incidence and diffraction are equal but have opposite sign. 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 first order, m = 1. However, under the proper 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 800 nm light will appear naturally at 800 nm in first order but also at 1600 nm in second order and again at 2400 nm in third order. If the instrument is equipped with a standard range InGaAs detector that is sensitive from 800 and 1600 nm even though only 1600 nm light is desired.

To prevent second order radiation from interfering with first order analytical measurements, it is customary to insert order sorting (long pass) filters into the optical path. These filters are provided in the lamp assembly; thus, long pass filter selection in the filter wheel setup should be left “unused”.