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Choosing a Grating & Wavelength Range:
"USB" Optical
Bench
 You 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.
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 |
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 |
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.
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Applications Scientist
Optical
Resolution
System
Sensitivity
Operating
Principles
Choosing a Grating:
"HR" Optical Bench
Choosing
a Grating: "NIR" Optical Bench
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