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. A lower value for optical resolution will correspond to a higher resolution.

Factors that Affect the Optical Resolution

There are three main elements that define the optical 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.

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 fiber with a large core, optical resolution can be improved by selecting a slit with a smaller size width. This reduces the amount of light that enters the spectrometer.

How do we Measure Optical Resolution

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 optical 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.

 

How to Change the Resolution of your Spectrometer?

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). Sarspec has five different slits available with widths of 10, 25, 50, 100 and 200 um, this gives the possibility to use the best resolution for each application. In addition to the installed slit entrance (normally 10 um) included with any Sarspec’s spectrometer there is the possibility to purchase individual slits or to buy a Slit Kit. Additionally, when selecting a spectrometer from Sarspec there are different options.  For FLEX series spectrometers, the user can choose one of the two detectors available: 2048 pixels and 3648 pixels. For extreme resolution requirements we strongly advice the X-RES spectrometer reaching resolutions up to 0.05 nm.

 

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