When selecting a diffraction grating for a Raman spectrometer there are four main considerations: spectral resolution, spectral range, blaze wavelength, and excitation wavelength.įigure 2: Ramacle ® software in measurement set up, highlighting ease of grating selection. Ramacle ® will then display the spectral range achievable with this grating and excitation laser both in wavenumber and wavelength. After selection, the grating turret will automatically move to the selected grating meaning there is no need for the user to manually exchange gratings. The gratings can be selected by the user from a drop-down menu in the Ramacle ® software, Figure 2, on the RM5 and RMS1000. To operate with multiple laser lines the spectrographs of the RM5 and RMS1000 can host up to five diffraction gratings so that a grating best suited for the laser’s wavelength and the users’ requirements. The RM5 Raman Microscope can host up to three lasers, and the RMS1000 Raman Microscope up to five with the option of additional external lasers. When analysing samples by Raman spectroscopy multiple excitation sources may be required to cover the users range of samples, for example, lasers in the UV, visible, or NIR regions. All Raman spectrometers will require at least one diffraction grating and will frequently be configured to contain more than one to allow the user optimum grating selection for their samples and excitation wavelength(s).įigure 1: Edinburgh Instruments RMS1000 (left) and RM5 (right) Raman Microscopes. In a Raman spectrometer diffraction gratings are used to separate the constituent wavelengths of the collected Raman scatter onto different pixels of the CCD camera for detection. In the figures below the grating dispersion curves are shown for the AvaSpec-2048.A diffraction grating is used to separate polychromatic light into its constituent wavelengths. When looking at the grating efficiency curves, please realize that the total system efficiency will be a combination of fiber transmission, grating and mirror efficiency, detector and coatings sensitivities. In the figures below their efficiency curves are shown. The spectral range to select in Table 2 depends on the starting wavelength of the grating and the number of lines/mm the higher the wavelength, the bigger the dispersion and the smaller the range to select. Table 2 illustrates how to read the grating selection table. Similarly, a higher resolution over a wide range can be achieved by using a dual or triple spectrometer.įor each spectrometer type a grating selection table is shown in the spectrometer platform section. Then master and slave(s) have different gratings. In order to cover a broader range, a dual or triple beam spectrometer can be chosen. Sometimes the specified usable range of a grating is larger than the range that can be projected on the detector. Further the user needs to indicate what wavelength range needs to reach the detector. The fiber optic spectrometer comes with a permanently installed grating that must be specified by the user. Gratings produced from laser constructed interference patterns and a photolithographic process are known as holographic gratings. The ruled gratings are physically formed into a reflective surface with a diamond on a ruling machine.
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