Spectral resolution - simple illustrations

By increasing the density of the grating in the spectrometer, the spectral resolution will be improved.

In practice, and as a simplified rule of thumb, double the grating grooves and the dispersion will be doubled. Eg. 2 cm-1/pixel becomes 1 cm-1/pixel.

Eg. Swapping a 600 gr/mm grating for a 1200 gr/mm grating will double spectral resolution in this simplified treatment.

However, it is not possible to keep increasing the grating density ad-infinitum. As the grating density increases its working range will be restricted. Thus, a 2400 gr/mm grating will work in the UV and green end of the spectrum but no further, an 1800 gr/mm grating will work up into the red end, and a 1200 gr will work up into the NIR.

So, increasing the grating density will only take you so far in improving spectral resolution. So generally, the focal length of the spectrometer must also be considered.

Doubling the focal length will increase the spectral dispersion of a spectrometer.

Larger optics maintain throughput and sensitivity of the longer focal length system and will enable ultimately greater spectral resolution to be achieved as there are no restrictions.

In practice if you need to push the spectral resolution of a Raman measurement, a combination of longer focal length and good grating selection will provide the answer.

For more information on the technical considerations of spectroscopy, see the HORIBA Jobin Yvon optics for spectroscopy tutorial on this site.