Choosing Gratings for Miniature Spectrometers

Find a product:

Each channel in an Ocean Optics miniature spectrometer can be configured with a grating of your choice. The grating is fixed in place at the time of manufacture. 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.

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 array and the asymmetry of the optical bench determine the spectral range 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 varies with the starting wavelength: the higher the wavelength, the lesser the spectral range.

The following is 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 array; also expressed as the "size" of the spectra on the array. 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, 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.

Spectrometer Grating Selection Chart for S2000-series, PC2000-series, S1024DW and USB2000-series Spectrometers

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 linear-array detector. However, optimum detector 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.

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

^{* The spectral range for Grating #6, #7, #9, #10, #11
and #12 varies 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
spectrometer's linear-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.

� 2001 Ocean Optics, Inc. All
rights reserved.

Terms of Use and Privacy Statement

Last Modified: Sunday, April 14, 2002}}

Contact an Ocean Optics Applications Scientist

Optical Resolution

System Sensitivity

Operating Principles