Grating options are described in Spectrometer Grating Chart. Information on grating efficiency curves and related data is also available.

Also, multiple spectrometer channels, each with its own grating, can be added to a Master channel for expanded wavelength range, multi-tasking or reference monitoring.

The WPI spectrometers are equipped with 1 of 14 different gratings that are fixed in place at time of manufacture. The grating and the desired wavelength range and the resulting optical resolution and signal that best fits your application must be specified when you order the system. If you cannot achieve the desired range with a single grating, or if your needs change, additional spectrometer channels may be purchased at a later date.
The dispersion of a grating is determined by the density of the grating's ruled or holographically etched grooves. The spectral range that is observed by our detector is determined by the path length of our optical bench, the length of the CCD array, and the asymmetry of our optical bench. As an approximation, the observed range will scale inversely with the groove density, e.g. 500 nm for a 600 line/mm grating, 250 nm for a 1200 line/mm grating, etc. However, the actual range varies as a function of wavelength, being generally shorter at higher wavelengths.

Glossary of key terms used in the Spectrometer Grating Chart:

Lines/mm. Groove density (ruled or holographically etched) of the grating; the greater the groove density, the better the optical resolution that will result, but the lesser the spectral range.

Spectral Range. The dispersion of the grating across the linear CCD array; also expressed as the "size" of the spectra on the CCD. When selecting gratings, you must choose a wavelength range with a width equal to the Spectral Range entry in the Spectrometer Grating Selection Chart. The grating's highest efficiency is within the range listed in the Best Efficiency (>30%) column. Consider: If you choose Grating #6 for an S2000 Series spectrometer, you are limited to a 200 nm spectral window within the 500-1100 nm range, the parameters of that grating's highest efficiency. For example, you can select 600-800 nm as the wavelength area of interest, or 575-775 nm, and so on.

Blaze Wavelength. The peak wavelength in the typical efficiency curve for a ruled grating. Also, for a holographic grating, the most efficient wavelength region.

Best Efficiency ( >30%). All ruled or holographically etched gratings optimize first-order spectra at certain wavelength regions; the "best" or "most efficient" region is the range where efficiency is >30%. In some cases, gratings have a greater spectral range than is efficiently diffracted. For example, an S2000 Series spectrometer with Grating #1 has a 650 nm spectral range, but is most efficient over a much narrower range -- from 200-575 nm. In this instance, wavelengths >575 nm will have lower intensity at the detector due to the reduced efficiency of the grating.


Spectrometer Grating Chart

Users must select a grating (including starting and ending wavelengths) and optical bench accessories for each spectrometer channel. Additional grating selection guidelines appear in the footnotes below.

Spectrometer system response depends on the grating and the detector. The grating efficiency ranges reported here are truncated to the response range of the CCD linear-array detector -- 200-1100 nm. Optimum system performance is between 220-1000 nm.

To see the efficiency curve of a specific grating, click on the Grating # in the far left column. To compare similar gratings, click on the entry in the Lines/mm column.

Also, please be aware that due to optical design limitations, systems configured with Grating #12 cannot be set above ~575 nm. In fact, although the efficiency of the grating is >30% to 700 nm, the optical design of the spectrometer prevents it from "seeing" wavelengths >~575 nm. However, Grating #11 can be set at wavelengths >575 nm and <800 nm, and will achieve comparable optical resolution (FWHM).

** The spectral range for Grating #8 will vary according to the starting wavelength range. The rule of thumb is this: the higher the starting wavelength, the lesser the spectral range.

*** The spectral range for Grating #13 extends beyond the response of the S2000's linear CCD-array detector (200-1100 nm). In fact, while the spectral range of a spectrometer configured with Grating #13 will span 300-2000 nm, the detector will "see" only the area from 300-1100 nm. There are two other considerations with Grating #13. First, though the grating has a very broad spectral range, it cannot be used to achieve very high resolution (<3.0 nm FWHM). Second, due to the grating's broad spectral range, second-order effects, which are characteristic of all gratings, are much more difficult to eliminate or reduce through the installation of order-sorting filters and the like.

1) Dispersion (nm/pixel) = Spectral Range of the Grating (see Choosing a Grating) divided by the Number of Detector Elements (2048 for S2000 or 1024 for S1024DW)

2) Resolution (in pixels) = value from slit size/fiber diameter chart
(see below)

3) Optical Resolution (in nm) = Dispersion (nm/pixel value from #1) x
Resolution (pixels value from #2)

EXAMPLE: S2000 spectrometer with Grating #3, 10-micron slit

   650 nm divided by 2048 = 0.32 nm/pixel x 3.2 pixels = 1.02 nm (FWHM)