The slope of the triangular groove is usually adjusted to improve the brightness of the grating at a specific wavelength. Ruled gratings are produced by physically forming a series of triangular grooves onto the surface of a substrate, using a diamond tool mounted on a ruling engine. A brief description of Ruled, Holographic, and Blazed Holographic grating can be found within the following sections. Sarspec uses regularly three types of gratings in its spectrometers depending on the purpose of the application and required specifications. Which types of gratings are used in our spectrometers? When selecting the best grating for your spectrometer, take into consideration the grating efficiency – its maximum will, normally (also depending on the efficiency curve of the detector), correspond to the maximum sensitivity of the operational wavelength range (see Figure 2).įigure 2 – Absolute efficiency curve of two 600 grooves/mm gratings, maximum efficiency peaks at 300 nm (orange) and 500 nm (blue). In ruled and ruled holographic gratings, changing the groove angle (the angle created by the longer side of the groove and the plane of the grating) moves the diffracted radiation along the spectral region and changes the blaze wavelength. The overall shape of the grating efficiency curve (a plot of absolute or relative diffracted efficiency as a function of the diffracted wavelength) shows normally a single maximum, at the peak of maximum brightness (or blaze wavelength) and is generally a rather complex function of wavelength and polarization of the incident radiation and depends on the groove density, plane of polarization, shape, and angle of the grooves and the reflectance of the coating material. Grating EfficiencyĪs grating diffracts the incident radiation, it does not do so with uniform efficiency. Increasing the groove density leads to a higher optical resolution but also to a shorter wavelength operational range (see Figure 1).įigure 1 – Absolute efficiency curve of 3 gratings with 1800 (orange), 2400 (blue) and 3600 (black) grooves/mm. On the other hand, groove density also defines the operational range of wavelengths of the grating, being that a higher groove density will generate shorter operational ranges for your spectrometer. The increase of the groove density (grooves (or lines)/mm) results in an improvement of the optical resolution for a certain slit width. The groove density or groove frequency is usually expressed in nm/mm and determines the range of wavelengths that are dispersed by the grating into the detector. Detailed information about these two parameters can be found in the following sections. The two key parameters when choosing the right grating for your spectrometer are the groove density which allows you to set the operational wavelength range and optical resolution and the grating groove efficiency for the selection of the wavelength range where your system sensitivity is at its maximum. What are the key parameters when choosing the right grating for your spectrometer?Īll spectrometers manufactured by Sarspec are equipped with fixed diffraction gratings that can be selected by the user according to the wavelength range of interest.
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