Ocean Optics - Inventor of the World's First Miniature Spectrometer
Ocean Optics - Inventor of the World's First Miniature Spectrometer
 

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Ocean Optics - Inventor of the World's First Miniature Spectrometer

Choosing a Grating & Wavelength Range: 
"USB" Optical Bench

Choose from among 14 gratings for each spectrometer. With each grating, you consider its groove density (which helps determine the resolution), its spectral range (which helps determine the wavelength range) and its blaze wavelength (which helps determine the most efficient range). Instead of the gratings rotating as they do in instruments such as scanning monochromators, our gratings are permanently fixed in place at the time of manufacture to ensure long-term performance and stability. A grating must be specified for each spectrometer. We offer ruled and holographic diffraction gratings. Both are polymer replicas of master gratings. There are trade-offs between these gratings: holographic gratings produce less stray light while ruled gratings are more reflective, resulting in higher sensitivity.

Grating Selection Chart
The chart below allows you -- with the help of our Applications Scientists -- to select the best grating. Bulleted items describe each column in the table.

  • Groove Density. The Groove Density (mm-1) of a grating determines its dispersion, while the angle of the groove determines the most efficient region of the spectrum. The greater the groove density, the better the optical resolution possible, 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. The spectral range (bandwidth) is a function of the groove density and does not change. When you choose a starting wavelength for a spectrometer, you add its spectral range to the starting wavelength to determine the wavelength range. For several gratings, the Spectral Range of a grating varies according to the starting wavelength range. The rule of thumb is this: The higher the starting wavelength, the more truncated the spectral range.
     
  • Blaze Wavelength. For ruled gratings, the Blaze Wavelength is the peak wavelength in an efficiency curve. For holographic gratings, it is 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, Grating 1 has about a 650 nm spectral range, but is most efficient from 200-575 nm. In this case, wavelengths >575 nm will have lower intensity due to the the grating’s reduced efficiency.
Grating Number Intended Use Groove Density 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 200-270 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 320-720 nm
12 UV-VIS 2400 50-120 nm Holographic VIS 260-780 nm*
13 UV-VIS-NIR 300 1700 nm 500 nm 300-1100 nm
14 NIR 600 625 nm 1000 nm 650-1100 nm
31 UV-NIR 500 200-1050 nm 250 nm 200-450 nm

* For applications >720 nm, please consult an Ocean Optics Applications Scientist

Grating Efficiency Curves
To see the efficiency curve of a specific grating, and to compare similar gratings, click on the Grating Number in the far left column of the table or click here.

Predicted Ranges & Resolutions
See a series of graphs to demonstrate the predicted Range and Resolution of your USB Spectrometer.

Grating Notes
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 300-1700 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.

 
Contact an
Ocean Optics 
Applications Scientist

Optical Resolution

System Sensitivity

Operating Principles

Choosing a Grating: "HR" Optical Bench

Choosing a Grating: "NIR" Optical Bench
 
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