Choosing a Grating
Each channel in an Ocean Optics spectrometer can be configured with one of 14 different gratings, which are fixed in place at the time of manufacture, covering the UV-VIS-Shortwave NIR. An Ocean Optics Applications Scientist can help you select the grating that yields the optimum wavelength range, optical resolution and signal for your application.
Grating options are described in the Spectrometer Grating Selection Chart. Information on grating efficiency curves and related data is also available.
Also, when specifying multiple spectrometer-channel systems -- a Master channel plus Slave channels -- users must select a grating for each spectrometer channel.
The dispersion of a grating is determined by the density of the grating's ruled or holographically etched grooves. The path length of the optical bench, the length of the CCD array, and the asymmetry of the optical bench determine the spectral range that is observed by the detector. Generally, the observed range will scale inversely with the groove density -- i.e., 600-650 nm for a 600 lines/mm grating, 300-325 nm for a 1200 lines/mm grating, and so on. For some gratings, the spectral range with Ocean Optics spectrometers will vary with the starting wavelength: the higher the wavelength, the lesser the spectral range.
Here's a glossary of key terms used in the Spectrometer Grating Selection 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 more truncated 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 200-270 nm spectral window within the 500-1100 nm range, the parameters of that grating's highest (best) efficiency. For example, you can select 500-750 nm as the wavelength area of interest, 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.
Unless purchasing a pre-configured spectrometer such as the S2000-UV-VIS, 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, detector and other factors. The grating efficiency ranges reported here are truncated to the response range -- 200-1100 nm -- of the CCD linear-array detector. However, optimum detector performance is between 220-1000 nm.
* The spectral range for Grating #6, #7, #9, #10, #11 and #12 will vary according to the starting wavelength range. The rule of thumb is this: the higher the starting wavelength, the more truncated the spectral range. For example, the spectral range for Grating #10 is 190 nm in the UV region (<360 nm) and 100 nm in the "red" region (>600 nm).
Also, due to design limitations, systems configured with Grating #12 cannot be set >575 nm. In fact, although the efficiency of the grating is >30% to 700 nm, the optical design of the spectrometer prevents the grating from responding to light at 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 #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 respond to light only in the region 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 use of order-sorting filters and the like.
This material taken from the Ocean Optics web site: www.oceanoptics.com