This means that to achieve the same spectral resolution across this range, a Raman spectrometer will generally require higher density gratings (the dispersing element of the spectrometer) as you move from the red to the green. Typically, if you use a 600gr/mm grating in the NIR, you will need a 1200 or 1800gr/mm grating in the green to achieve a similar spectral resolution.
The use of higher diffraction gratings is not infinite and has fixed practical and physical limits, so if you need to achieve a higher spectral resolution you firstly can consider a higher grating, but then to really improve resolution dramatically it is better to look at a longer focal length Raman spectrometer.
The simple "rule of thumb" applies, double the focal length and the resolution will double. So for applications such as stress/strain in semiconductors, polymorphism, phase and subtle bonding effects, the use of a longer focal length will provide a far more detailed information on the sample. The LabRAM HR is an example of this - the long 800mm focal length of the spectrometer is ideally suited to the analysis of pharmaceutical polymorphs, where quite subtle differences are found, differences that would be difficult to observe on a smaller focal length instrument.
- For routine or process monitoring applications, often a standard spectral dispersion in the order of 1cm-1/pixel to 4cm-1/pixel is suited.
- For routine analysis and chemical ID, a dispersion of 0.5cm-1/pixel to 1 cm-1/pixel is generally well matched (see LabRAM ARAMIS)
- For measurements such as stress in semiconductors, for CNT and pharmaceutical polymorphs a dispersion of 0.3cm-1/pixel to 0.75cm-1/pixel is generally advantageous. (see LabRAM HR)
Click here for some simple diagrams of spectral resolution and dispersion.
High resolution and Medium resolution analysis of the same sample
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