Introduction to the basic knowledge of spectrometer - Zhuoli Hanguang company ----- ( reproduced please indicate the source )
What is a spectrometer? The interaction between light and matter causes the electronic transition between the atomic and molecular energy levels of the substance, which causes the absorption, emission, and scattering of light to change in wavelength and intensity information. The instrument that detects and processes such changes is called spectrometer. Therefore, the basic function of the spectrometer is to separate/expand the complex color light in space according to different wavelengths, and obtain the original information such as the wavelength component and the intensity of each wavelength component with various photoelectric instrument accessories for subsequent processing and analysis.
As an important analytical method, the spectral analysis method plays an important role in scientific research, production, quality control and so on. Whether it is through absorption spectrum, fluorescence spectrum, Raman spectroscopy, how to obtain single-wavelength radiation is an indispensable means. Because modern monochromators have a wide spectral range (UV-IR), high spectral resolution (up to 0.001 nm), automatic wavelength scanning, and complete computer control functions are easily integrated into other high-performance automated test systems. The use of computers to automatically scan multi-grating monochromators has become the first choice for spectroscopy research.
When a composite light enters the entrance slit of the monochromator, it is first concentrated by the optical collimator into parallel light, and then dispersed by the diffraction grating into separate wavelengths (colors). With each wavelength leaving the grating at a different angle, the exit slit is reimaged by the focusing mirror. The exit wavelength can be accurately changed by computer control.
â— important parameters of the grating monochromator:
â—† Resolution The resolution of the grating monochromator R is a measure of the ability to separate two adjacent lines, according to the Roland criterion:
R=λ/Δλ
A practical definition in a grating spectrometer is to measure the full width at half maximum (FWHM) of a single line. In practice, the resolution depends on the resolution of the grating, the effective focal length of the system, the set slit width, the optical aberrations of the system, and other parameters.
R∠M·F/W
M-Grating Line Number F-Spectrometer Focal Length W-Slit Width ◆ Dispersion The dispersion of a grating spectrometer determines its ability to separate wavelengths. The inverse chromatic dispersion of the spectrometer can be calculated by changing the distance χ along the focal plane of the monochromator to cause a change in the wavelength λ, ie:
Δλ/Δχ=dcosβ/mF
Here, d, β, and F are the pitch of the grating groove, the diffraction angle, and the effective focal length of the system, respectively, and m is the diffraction order. As can be seen from the equation, the inverse dispersion is not a constant, it varies with wavelength. The variation may exceed 2 times over the wavelength range used. According to national standards, in this sample, the inverse of the chromatic dispersion of 1200 l/mm grating (typically 435.8 nm) is used.
â—† Resolution The resolution of the grating monochromator R is a measure of the ability to separate two adjacent lines, according to the Roland criterion:
R=λ/Δλ
A practical definition in a grating spectrometer is to measure the full width at half maximum (FWHM) of a single line. In practice, the resolution depends on the resolution of the grating, the effective focal length of the system, the set slit width, the optical aberrations of the system, and other parameters.
R∠M·F/W
M-Grating Line Number F-Spectrometer Focal Length W-Slit Width ◆ Dispersion The dispersion of a grating spectrometer determines its ability to separate wavelengths. The inverse chromatic dispersion of the spectrometer can be calculated by changing the distance χ along the focal plane of the monochromator to cause a change in the wavelength λ, ie:
Δλ/Δχ=dcosβ/mF
Here, d, β, and F are the pitch of the grating groove, the diffraction angle, and the effective focal length of the system, respectively, and m is the diffraction order. As can be seen from the equation, the inverse dispersion is not a constant, it varies with wavelength. The variation may exceed 2 times over the wavelength range used. According to national standards, in this sample, the inverse of the chromatic dispersion of 1200 l/mm grating (typically 435.8 nm) is used.
â—† Bandwidth Bandwidth is the wavelength width that is output from the spectrometer at a given wavelength, ignoring optical aberrations, diffraction, scanning methods, detector pixel width, slit height, and illumination uniformity. It is the product of the inverse dispersion and the slit width. For example, the monochromator slit is 0.2 mm, the grating inverted line dispersion is 2.7 nm/mm, and the bandwidth is 2.7 x 0.2 = 0.54 nm.
â—† Wavelength accuracy, repeatability and accuracy Wavelength accuracy is the scale of the wavelength determined by the spectrometer in nm. Generally, the wavelength accuracy varies with wavelength.
Wavelength repeatability is the ability of the spectrometer to return to its original wavelength. This embodies the stability of the wavelength driven machine and the entire instrument.
Zhuoli Hanguang's spectrometers have excellent wavelength drive and mechanical stability, and their repeatability exceeds wavelength accuracy.
The wavelength accuracy is the difference between the set wavelength of the spectrometer and the actual wavelength. Each monochromator checks the wavelength accuracy at many wavelengths.
â—†F/#
F/# is defined as the ratio of the focal length (f) to the minimum clear aperture (D) of the effective optics in the spectrometer. The light passing efficiency is inversely proportional to the square of F/#. The smaller the F/#, the higher the light passing rate.
â—† Wavelength accuracy, repeatability and accuracy Wavelength accuracy is the scale of the wavelength determined by the spectrometer in nm. Generally, the wavelength accuracy varies with wavelength.
Wavelength repeatability is the ability of the spectrometer to return to its original wavelength. This embodies the stability of the wavelength driven machine and the entire instrument.
Zhuoli Hanguang's spectrometers have excellent wavelength drive and mechanical stability, and their repeatability exceeds wavelength accuracy.
The wavelength accuracy is the difference between the set wavelength of the spectrometer and the actual wavelength. Each monochromator checks the wavelength accuracy at many wavelengths.
â—†F/#
F/# is defined as the ratio of the focal length (f) to the minimum clear aperture (D) of the effective optics in the spectrometer. The light passing efficiency is inversely proportional to the square of F/#. The smaller the F/#, the higher the light passing rate.
About grating
As an important optical splitting device, its choice and performance directly affect the performance of the entire system.
The grating is divided into a scribed grating, a replica grating, a holographic grating, and the like. The scribed grating is mechanically scribed on a thin metal surface with a diamond knives; the replica grating is replicated with a mother grating. The grooves of a typical scribed grating and a replica grating are triangular. The holographic grating is formed by laser interference fringe lithography. Holographic gratings typically include a sinusoidal groove. The scribed grating has the characteristics of high diffraction efficiency, and the holographic grating has a wide spectral range, low stray light, and can be used for high spectral resolution.
â—† How to choose a grating to select a grating mainly consider the following factors:
1. Grating engraving, the grating engraving line is directly related to the spectral resolution, the multi-spectral resolution of the engraved line is high, and the spectral coverage is small, and the two are to be flexibly selected according to the experiment;
1. Grating engraving, the grating engraving line is directly related to the spectral resolution, the multi-spectral resolution of the engraved line is high, and the spectral coverage is small, and the two are to be flexibly selected according to the experiment;
2, the blaze wavelength, the blaze wavelength is the maximum diffraction efficiency point of the grating, so the choice of grating should try to choose the blaze wavelength near the experimental wavelength. If the experiment is in the visible range, the blaze wavelength can be selected to be 500 nm;
3, the scope of use,
3. Grating efficiency, which is the ratio of monochromatic light diffracted to a given order to incident monochromatic light. The higher the grating efficiency, the smaller the signal loss. In order to improve this efficiency, in addition to improving the grating fabrication process, a special coating is also used to improve the reflection efficiency.
3, the scope of use,
3. Grating efficiency, which is the ratio of monochromatic light diffracted to a given order to incident monochromatic light. The higher the grating efficiency, the smaller the signal loss. In order to improve this efficiency, in addition to improving the grating fabrication process, a special coating is also used to improve the reflection efficiency.
◆Grating Equation The reflective diffraction grating periodically scribes a lot of fine grooves on the substrate. The interval between a series of parallel grooves is equivalent to the wavelength, and the surface of the grating is coated with a high-reflectivity metal film. Radiation interactions reflected by the surface of the grating trench produce diffraction and interference. For a certain wavelength, disappearing in most directions, only in a certain finite direction, these directions determine the diffraction order. As shown in Fig. 1, the grating groove vertically radiates the incident plane, the incident angle of the radiation and the grating normal is α, the diffraction angle is β, the diffraction order is m, and d is the groove pitch, and the interference is extremely large under the following conditions. Value: Mλ=d(sinα+sinβ)
The definition φ is half of the angle between the incident ray and the diffracted ray, that is, φ=(α-β)/2; θ is the grating angle with respect to the zero-order spectral position, that is, θ=(α+β)/2, which is more convenient. The grating equation:
Mλ=2dcosφsinθ
It can be seen from the grating equation:
For a given direction β, there may be several wavelengths corresponding to the order m λ satisfying the grating equation. For example, the first-order radiation at 600 nm and the second-order radiation at 300 nm and the third-order radiation at 200 nm have the same diffraction angle, which is why it is necessary to add a secondary spectral filter wheel.
The diffraction order m can be positive or negative.
Multiple wavelengths of the same order are spread at different β.
The radiation direction containing multiple wavelengths is fixed, and the grating is rotated to change α, and different wavelengths are obtained in the direction in which α+β is constant.
The definition φ is half of the angle between the incident ray and the diffracted ray, that is, φ=(α-β)/2; θ is the grating angle with respect to the zero-order spectral position, that is, θ=(α+β)/2, which is more convenient. The grating equation:
Mλ=2dcosφsinθ
It can be seen from the grating equation:
For a given direction β, there may be several wavelengths corresponding to the order m λ satisfying the grating equation. For example, the first-order radiation at 600 nm and the second-order radiation at 300 nm and the third-order radiation at 200 nm have the same diffraction angle, which is why it is necessary to add a secondary spectral filter wheel.
The diffraction order m can be positive or negative.
Multiple wavelengths of the same order are spread at different β.
The radiation direction containing multiple wavelengths is fixed, and the grating is rotated to change α, and different wavelengths are obtained in the direction in which α+β is constant.
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Number of raster lines (g/mm) | Inverted dispersion (nm/mm, @435.8nm) | Spectral bandwidth (nm, @100μm slit) | |||||||
Omni-λ150 | Omni-λ300 | Omni-λ500 | Omni-λ750 | Omni-λ150 | Omni-λ300 | Omni-λ500 | Omni-λ750 | ||
2400 | 2.7 | 1.4 | 0.9 | 0.6 | 0.27 | 0.14 | 0.09 | 0.06 | |
1800 | 3.6 | 1.8 | 1.1 | 0.7 | 0.36 | 0.18 | 0.11 | 0.07 | |
1200 | 5.4 | 2.7 | 1.7 | 1.1 | 0.54 | 0.27 | 0.17 | 0.11 | |
600 | 10.8 | 5.4 | 3.4 | 2.2 | 1.08 | 0.54 | 0.34 | 0.22 | |
300 | 21.6 | 10.8 | 6.8 | 4.4 | 2.16 | 1.08 | 0.68 | 0.44 | |
â—†Grating model specification comparison table
model | Grating line (g/mm) | Sparkling wavelength (nm) | Raster size (mm × mm) | Range of use (nm) |
Omni-λ150 series | ||||
5-180-H | 1800 | - | 32x32 | UV |
5-120-300 | 1200 | 300 | 32x32 | 200-600 |
5-120-500 | 1200 | 500 | 32x32 | 330-1000 |
5-060-500 | 600 | 500 | 32x32 | 330-1000 |
5-060-750 | 600 | 750 | 32x32 | 500-1500 |
5-030-500 | 300 | 500 | 32x32 | 330-1000 |
5-030-1250 | 300 | 1250 | 32x32 | 800-2500 |
Omni-λ300/500/750 series | ||||
1-240-H | 2400 | - | 68x68 | UV |
1-180-H | 1800 | - | 68x68 | UV |
1-120-300 | 1200 | 300 | 68x68 | 200-600 |
1-120-500 | 1200 | 500 | 68x68 | 330-1000 |
1-060-300 | 600 | 300 | 68x68 | 200-600 |
1-060-500 | 600 | 500 | 68x68 | 330-1000 |
1-060-750 | 600 | 750 | 68x68 | 500-1500 |
1-060-1250 | 600 | 1250 | 68x68 | 800-2500 |
1-030-500 | 300 | 500 | 68x68 | 330-1000 |
1-030-1250 | 300 | 1250 | 68x68 | 800-2500 |
1-030-1800 | 300 | 1800 | 68x68 | 1200-3600 |
1-030-3000 | 300 | 3000 | 68x68 | 2000-6000 |
1-006-D | 66.6 | 3140 & 10250 | 68x68 | 3000-25000 |
Other specifications grating | ||||
3-120-300 | 1200 | 300 | 38x50 | 200-600 |
3-120-500 | 1200 | 500 | 38x50 | 330-1000 |
3-060-1000 | 600 | 1000 | 38x50 | 660-2000 |
3-030-1250 | 300 | 1250 | 38x50 | 800-2500 |
3-015-500 | 150 | 500 | 38x50 | 330-1000 |
3-060-750 | 600 | 750 | 38x50 | 500-1500 |
6-120-500 | 1200 | 500 | 30x30 | 330-1000 |
7-060-500 | 600 | 500 | 18x18 | 330-1000 |
â—†Typical grating efficiency curve
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