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.

· 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.

#	Intended Use	Lines/mm	Spectral Range		Blaze Wavelength	Best Efficiency (>30%)
#	Intended Use	Lines/mm	TR1000 Spectrometers	S2000 Spectrometers	Blaze Wavelength	Best Efficiency (>30%)
1	UV	600	500 nm	650 nm	300 nm	200-575 nm
2	UV/VIS	600	500 nm	650 nm	400 nm	250-800 nm
3	VIS/Color	600	500 nm	650 nm	500 nm	350-850 nm
4	NIR	600	500 nm	625 nm	750 nm	530-1100 nm
5	UV/VIS	1200	250 nm	300 nm	holographic/UV	200-400 nm
6	NIR	1200	200 nm	200-270 nm*	750 nm	500-1100 nm
7	UV/VIS	2400	<100 nm	100-140 nm*	holographic/UV	200-500 nm
8	UV	3600	50-70 nm	50-75 nm	holographic/UV	290-340 nm
9	VIS/NIR	1200	200 nm	200-300 nm*	holographic/VIS	400-800 nm
10	UV/VIS	1800	150 nm	100-190 nm*	holographic/UV	200-635 nm
11	UV/VIS	1800	>80 nm	120-160 nm*	holographic/VIS	320-800 nm
12	UV/VIS	2400	<100 nm	50-120 nm*	holographic/VIS	250-575 nm*
13	UV/VIS/NIR	300	800 nm	1700 nm^**	500 nm	300-1100 nm
14	NIR	600	450 nm	625 nm	1000 nm	650-1100 nm

** 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.

Choosing a Grating

This material taken from the Ocean Optics web site: www.oceanoptics.com

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