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Spectroscopy

A 550 mm focal length spectrograph is provided as equipment in RAMANtouch/RAMANforce’s compact cabinet. It has a sophisticated design excellent in balancing spectral resolution, brightness and stability to demonstrate a high level of spectral performance. One can switch the 3 built-in gratings freely to adjust for a variety of spectral analyses.

Spectral resolution is compatible with a high level of brightness

“Substantial spectral resolution”, “Ensuring adequate brightness”, “Being as compact as possible”. In order to satisfy the above 3 requirements, a spectrograph with 550 mm focal length has been adopted. The spectral resolution is approximately 1.2 cm-1 (FWHM) when 785 nm excitation wavelength is applied using a 1200 gr/mm grating. Even the Raman peaks of carbon tetrachloride that are used to evaluate spectral resolution can be observed respectively at both 532 nm and 785 nm excitation wavelengths (right). 

It is possible to choose 3 gratings from multiple selections; such as 2400 gr/mm which has a higher spectral resolution, 600 gr/mm which strikes a balance between resolution and spectrum detection range, and 150 gr/mm which is useful for wide wavelength region measurements such as PL and reflectance spectroscopy. One can simply switch them by a single click of the software without any adjustments. 

Raman spectrum of CCl4 obtained with 532 nm excitation
Excitation wavelength532 nm
Grating2400gr/mm
Raman spectrum of CCl4 obtained with 785 nm excitation
Excitation wavelength785 nm
Grating1200gr/mm

(Reference) Spectral resolution and spectrum pixel resolution

Spectral resolution is the ability to separate and detect 2 adjacent Raman peaks. The spectral resolution of an instrument is determined by various factors, such as the line width of excitation laser and focal length of spectrograph (which are difficult to change), and also the slit width of spectrograph and ruling number of the grating, etc. (which can be changed by users.) The picture below shows the Raman spectra of carbon tetrachloride obtained by using different gratings. The higher the ruling number of the grating, the better the spectral resolution will be.

Raman spectrum of CCl4(600gr/mm)
Raman spectrum of CCl4(2400gr/mm)

The spectral resolution of an instrument is assessed by the full width at half maximum (FWHM) of a Rayleigh scattered light, i.e. line width of excitation laser (*1). This is because in principle Raman spectroscopy cannot resolve two adjacent peaks present within the line width of the excitation laser. Spectral resolution of RAMANtouch/RAMANforce is 0.67 cm-1 (typical value) when using 785nm excitation wavelength and 1200 gr/mm grating. On the other hand, the spectrum pixel resolution, which is the sampling interval of the spectrum, is approximately 0.4 cm-1/pixel.

Spectral resolution (FWHM of Rayleigh light)
Spectrum pixel resolution (sampling interval)

Spectrum pixel resolution is different from spectral resolution since it is not the ability to separate and detect 2 adjacent Raman peaks. Attention is needed as some manufacturers label their spectrum pixel resolution as their “spectral resolution”.

(*1) This includes broadening of line width derived from the instrument.