Selection of the Grating
For each spectrometer channel a grating can be choosen from the following table. The selection is depending on the requirements of your application, i.e. spectral range, optical resolution and sensitivity.
Intended Use | Useable Range
(>30% Efficiency) | Spectral Range (nm) | Grating (lines/mm) | Blaze at
(nm) | # |
 | | | | | |
UV | 200-575 | 650 | 600 | 300 | |
UV | 290-340 | 50-75 | 3600 | holographic/UV | |
UV/VIS | 200-400 | 300 | 1200 | holographic/UV | |
UV/VIS | 200-500 | 100-140* | 2400 | holographic/UV | |
UV/VIS | 250-800 | 650 | 600 | 400 | |
UV/VIS | 200-635 | 100-190* | 1800 | holographic/UV | |
UV/VIS | 250-575* | 50-120* | 2400 | holographic/UV | |
UV/VIS | 320-800 | 120-160* | 1800 | holographic/UV | |
VIS/Color | 350-850 | 650 | 600 | 500 | |
VIS/NIR | 400-800 | 200-300* | 1200 | holographic/UV | |
NIR | 530-1100 | 625 | 600 | 750 | |
NIR | 650-1100 | 625 | 600 | 1000 | |
NIR | 500-1100 | 200-270* | 1200 | 750 | |
UV/VIS/NIR | 300-1100 | 1700** | 300 | 500 | |
*The spectral range of Grating # 6, # 7, # 9, #10, #11 und #12 varies with reference to the starting wavelength range. Rule: the higher the wavelength range the more truncated the spectral range.
For instance, the spectral range for Grating #10 is 190 nm in UV range (<360 nm) but only 100 nm for applications in the VIS range (>600 nm). Due to design limitations, spectrometers configured with Grating #12 cannot be set >575 nm. The efficiency of the grating is >30% to 700 m. However, in this case, the optical design of OOE spectrometers prevents the light from responding. In this case Grating #11 is recommended for use between 575 and 800 nm . It achieves a similar optical resolution (FWHM) as Grating #12. Please do not hesitate to ask our customer advisors to assist you in choosing the ideal configuration of your system.
** The useable spectral range in Grating #13 extends the spectrometer wavelength range (300 - 1100 nm). While the possible useable spectral range will span over 1700 nm, the detector can operate only in the range between 300 - 1100 nm due to its array. There are two other facts to consider with Grating #13: Since the grating is applicable in a very broad useable spectral range, it is impossible to achieve a very high optical resolution (< 3.0 nm FWHM). Furthermore, it is very difficult to eliminate or reduce so-called second-order-effects - even by means of order sorting filters. These effects are caracteristic of all gratings offered.
SLIT - Optional Entrance Slit
An optional slit can be installed in the input of the spectrometer. In this way the resolution of the spectrometer can be increased, if the cross-section of the connected glass fiber is larger than that of the slit. The smaller the slit the better the spectral resolution. The input slit also prevents that the wave length calibration depends on the length of the glass fiber core. If no slit is installed, the diameter of the connected glass fiber determines the optical resolution.
Optical Resolution Depending on Entrance Slit and Grating Selection
1) Resolution increases with an increase in the groove density of the grating, but at the expense of spectral range and signal strength; and
2) Resolution increases as the slit width or fiber diameter decreases, but at the expense of signal strength.
How to calculate the approximate Optical Resolution in nm (FWHM)
1. Determine the Spectral Range of the Grating. (see Table on page 5)
2. Divide the Spectral Range of the Grating by the Number of Detector Elements. The resulting value is the Dispersion. Number of the detector elements of the USB4000 is 3648.
Dispersion (nm/pixel) = Spectral Range of the Grating ÷ Number of Detector Elements
3. Determine the Pixel Resolution
| Slit size | USB4000 Bench | "HR2000" | "HR4000" |
| 5 micron slit | ~5.3 pixels | 1.5 pixels | 2.0 pixels |
| 10 micron slit | ~5.7 pixels | 2.0 pixels | 3.7 pixels |
| 25 micron slit | ~7.5 pixels | 2.5 pixels | 4.4 pixels |
| 50 micron slit | ~11.6 pixels | 4.2 pixels | 7.4 pixels |
| 100 micron slit | ~21.0 pixels | 8.0 pixels | 14.0 pixels |
| 200 micron slit | ~42.0 pixels | 15.3 pixels | 26.8 pixels |
4. Calculate the Optical Resolution (in nm)
Dispersion (step 2) x Pixel Resolution (step 3)
For example: USB4000 spectrometer with Slit 25 and grating 2
Dispersion (nm/pixel) = Spectral Range of the Grating ÷ Number of Detector Elements
Dispersion = 0,178 nm/pixel
Optical resolution = Dispersion (step 2) x Pixel Resolution (step 3)
Optical resolution = 1nm
L2 - Detector Lens for Increased Sensitivity
The cylinder lens L2 focuses the incident light on the CCD detector. The increase of sensitivity depends on the core diameter of the fiber and is approximately proportional to this. In most cases, it is recommended to use a fiber with a large core diameter and a collector lens in conjunction with a slit. This leads to a high degree of sensitivity at a moderate resolution.
UV - UV Coating for the Detector for Ranges < 360nm
The UV coating of the detector increases the sensitivity in the UV range down to 190 nm. The coating is necessary for all spectral ranges beginning at a wavelength of <360 nm. Owing to the coating, UV light is converted into visible light, for which the detector has a higher sensitivity.
OF1 - Order Sorting Filter (Fixed Installed)
At the entrance of the spectrometers an order sorting filter may be installed by factory. These filters are mostly used to prevent second order effects. They have to be selected depending on the spectral range of the spectrometer.
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