The optical resolution is defined as the minimum difference in wavelength that can be separated by the spectrometer.
For separation of two spectral lines it is necessary to image them at least 2 array-pixels apart.
Because the grating determines how far different wavelengths are separated (dispersed) at the detector array, it is an important variable for the resolution. For the higher lines/mm gratings the pixel dispersion varies along the wavelength range and gets better towards the longer wavelengths. The best resolution can always be found for the longest wavelengths.
The other important parameter is the width of the light beam entering the spectrometer. This is basically the installed fixed entrance slit in the spectrometer, or the fiber core diameter when no slit is installed..
Its image on the detector array for a given wavelength will cover a number of pixels.
For two spectral lines to be separated, it is now necessary that they be dispersed over at least this image size plus one pixel.
When large core fibers are used the resolution can be improved by a slit of smaller size than the fiber core. This effectively reduces the width of the entering light beam.
The influence of the chosen grating and the effective width of the light beam (fiber core or entrance slit) are shown in the table below.
||Grating (lines/mm) /
Slit size (mkm)
||1200 / 10
||0,12 - 0,08
||0,18 - 0,12
||1800 / 10
||0,08 - 0,05
||0,11 - 0,07
||2400 / 25
||0,06 - 0,03
||0,14 - 0,08
||3600 / 25
||0,03 - 0,02
||0,08 - 0,04
* The resolution in this table is defined as F(ull) W(idth) H(alf) M(aximum), which is defined as the width in nm of the peak at 50% of the maximum intensity.