Grating Spectrometer (Monochromator)
Grating spectrometer is a scientific instrument that decomposes the complex light into spectral lines. Through the spectrometer to capture the light information to the photographic film development, or computerized numerical display automatically display and analysis, in order to detect which elements contained in the article. Grating spectrometers are widely used in the fields of color measurement, chemical concentration measurement or radiometric analysis, film thickness measurement, gas composition analysis and the like.
Spectrometer Basics
Spectroscopy as an important means of analysis, in scientific research, production, quality control and so on, all play a great role. Whether it is penetrating absorption spectrum, fluorescence spectrum, Raman spectrum, 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 0.001nm) and automatic wavelength scanning, complete PC control functions can be easily integrated with other peripherals into a high performance automated test system , The use of computers to automatically scan multi-grating monochromator has become the first choice for spectroscopy.
When a beam of composite light enters the entrance slit of the monochromator, it is first condensed into parallel light by an optical collimator and then dispersed by a diffraction grating into separate wavelengths (colors). With each wavelength exiting the grating at a different angle, the exit mirror is re-imaged by the focusing mirror. Through the computer control can accurately change the exit wavelength.
As an important optical grating device, its choice and performance directly affect the overall system performance. In order to better help you choose the user, make a brief introduction here.
Gratings are divided into grating, copy grating, holographic grating and so on. Scratching grating is a mechanical engraving of a thin metal surface with a diamond knife; a copy grating is made from a master grating. The grooves of a typical scored and copied raster are triangles. Holographic grating is made by laser interference fringe lithography. Holographic gratings typically include sinusoidal grooves. Scratching grating has the characteristics of high diffraction efficiency, a wide range of spectral holographic grating, stray light is low, and can be made of high spectral resolution.
How to choose raster
The main consideration of choosing the following factors:
1, blaze wavelength, blaze wavelength is the maximum diffraction efficiency of the grating point, so the choice of grating should try to choose the wavelength of light in the experiment near the wavelength. Such as the experimental range of visible light, choose a blaze wavelength of 500nm.
2, grating lines, grating lines directly related to the number of spectral resolution, multi-spectral high-resolution reticle, less spectral coverage of broad spectrum, both flexible choice based on the experiment.
3, grating efficiency, grating efficiency is diffracted to a given level of monochromatic light and incident monochromatic light ratio. The higher the raster efficiency, the smaller the signal loss. To improve this efficiency, in addition to improving the grating production process, but also the use of special coating, improve the reflection efficiency.
Grating equation
Reflective diffraction grating is periodically scribed in the substrate a lot of fine grooves, a series of parallel groove spacing and wavelength equivalent to the grating surface coated with a layer of high reflectivity metal film. Diffraction and interference are caused by the radiation interaction reflected by the grating groove surface. For a wavelength, which disappears in most directions, it appears only in a limited limited direction, which determines the order of diffraction. As shown in FIG. 1, the grating grooves are vertically irradiated with the incident plane, and the normal incidence angle of the radiation and the grating is α, the diffraction angle is β, the diffraction order is m, d is the groove pitch, and the interference is extremely large under the following conditions Value: Mλ = d (sinα + sinβ)
Define φ as the angle between incident light and diffracted light, ie φ = (α-β) / 2; θ is the grating angle relative to the zero-order spectral position, ie θ = (α + β) / 2, which is more convenient The grating equation:
mλ = 2dcosφsinθ
From the grating equation can be seen:
For a given direction β, there may be several wavelengths corresponding to the level m to satisfy the grating equation. For example, a 600nm primary radiation and 300nm secondary radiation, 200nm tertiary radiation have the same diffraction angle, which is why the elimination of secondary spectral filter wheel meaning.
Diffraction order m can be positive or negative.
The same level of multi-wavelength in different β distribution open.
With multiple wavelengths of radiation fixed direction, rotating the grating, change α, the α + β constant direction to get different wavelengths.
Grating monochromator important parameters
The resolution R of the grating monochromator is a measure of the ability to separate two adjacent spectral lines, according to Roland's criterion:
R = λ / Δλ
A practical definition in a grating spectrometer is to measure the full width at half maximum (FWHM) of a single spectral line. In fact, the resolution depends on the resolving power of the grating, the effective focal length of the system, the set slit width, the optical aberrations of the system, and other parameters.
Rα M • F / W
M - number of grating lines F - focal length of the spectrometer W - slit width.
The dispersion of a grating spectrometer determines its ability to separate wavelengths. The down-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, F are grating groove spacing, diffraction angle and the effective focal length of the system, m is the diffraction order. As can be seen from the equation, the down-dispersion is not constant and varies with wavelength. In the wavelength range used, the change may exceed 2 times. According to national standards, in this sample, down-dispersion is used with an intermediate value of 1200 l / mm grating dispersion (typically 435.8 nm).
The bandwidth is the wavelength width of the 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 up-down dispersion and slit width. For example, the monochromator slit 0.2mm, grating down dispersion of 2.7nm / mm, the bandwidth of 2.7 × 0.2 = 0.54nm.
Wavelength accuracy, repeatability and accuracy
Wavelength accuracy is the scale of the spectrometer to determine the wavelength scale, in units of nm. In general, wavelength accuracy varies with wavelength.
Wavelength repeatability is the ability of the spectrometer to return to its original wavelength. This reflects the stability of the wavelength-driven machinery and the entire instrument. Zhuo Li Han light spectrometer wavelength drive and mechanical stability is excellent, its repeatability exceeds the wavelength accuracy.
Wavelength accuracy is the difference between the wavelength set by the spectrometer and the actual wavelength. Each monochromator checks the wavelength accuracy at many wavelengths.
F / #
F / # is defined as the ratio of the focal length of the spectrometer to the diameter of the collimated concave reflector. The light passing efficiency is inversely proportional to the square of F / #, and the smaller the F / #, the higher the light passing rate.
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