Optical Resolution

Spectral (or optical) resolution is one of the most important characteristics of a spectrometer. It is related to the bandpass and determines the minimum difference in wavelength that can be separated by the optical elements of the spectrometer. There are three main elements that define the spectral resolution of a spectrometer:

The entrance slit sets the minimum spectral image size for a given wavelength that is formed on the detector and will cover a small number of pixels. For two spectral lines to be separated, it is necessary to disperse them over at least three pixels.

The diffraction grating determines the operational range of wavelengths of the spectrometer (how far the wavelengths are dispersed at the detector).

The detector determines the maximum number and size of pixels in which the spectrum can be digitized.

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When considering all the components that comprise the spectrometer (slit, diffraction grating and detector), is also important to take into account the core size of the optical fiber. When using optical fibers with a large core, spectral resolution can be improved by selecting a slit with a smaller size width. This reduces the amount of light that enters the spectrometer.

The form of the spectrum obtained from a monochromatic light source has generally a Gaussian profile that is shaped depending on the instrumental bandpass. The spectral resolution of a spectrometer can be measured at the full width half maximum (FWHM) of the peak of interest, by locating the two wavelengths where the intensity becomes half of its peak value.

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The entrance slit is the only optical element in Sarspec spectrometers that can be changed by the user (instructions to perform this change can be found here). In addition to the installed slit entrance (normally 10 um), Sarspec spectrometers are shipped with two additional entrance slit kit that includes two additional widths and allows the user to optimize the spectrometer for higher resolution (10 um) or higher efficiency (200 um). Additionally, for FLEX series spectrometers, the user can choose one of the two detectors available: 2048 pixels and 3648 pixels. In Table 1 are given the values for the optical resolutions obtained using the FWHM of several peaks of a calibration lamp, for the available entrance slits, diffraction gratings and detectors of the FLEX series of spectrometers.

Table 1 – The main parameters of all diffraction gratings that are available for installation in Sarspec spectrometers.