RAMAN-11 uses a spectroscope with a focal length of 500 mm to meet the following three requirements: sufficient spectral resolution, adequate brightness, and compactness. RAMAN-11 provides a spectral resolution of approximately 1.6 cm-1 (FWHM) when used with a 1,200 gr/mm grating, in addition to which, two other gratings, 2,400 gr/mm for higher wavelength resolution and 600 gr/mm for wide-range measurement, come with RAMAN-11 as standard components. These gratings can be switched from one to another automatically by using the software application.
Note: Definition of wavelength resolution
A combination of a long focal length of 500 mm and a grating for high resolution makes it possible to map peaks with a high precision of over 0.1 cm-1. Fitting analysis of Gaussian and Lorentz peak functions enables mapping of peaks at wave number precision over pixel resolution*. Usually, the data of a Raman image, consisting of some tens of thousands of spectra, are processed as fast as in several minutes.
RAMAN-11 uses a high-quality laser system with a narrow line width to maintain the high spectral resolution. A laser system with a wide line width causes a wide width of Raman peaks, having difficulty in mapping peaks and separating neighboring peaks.
Note: Definition wave number resolution
Some Raman microscopes use pixel resolution (unit: cm-1/pixel) as spectra resolution in specifications. Pixel resolution indicates a quantity of Raman light, which is scattered by a diffraction grating and illuminated to a CCD, in cm-1 per pixel. For example, when Raman light scattered in a range of 326 cm-1 to 800 cm-1 is illuminated to a CCD with 1,340 pixels, pixel resolution is obtained as follows. (See the figure on the right.)
(800-326) / 1340 = 0.35 cm-1/pixel
However, this value does not have anything to do with the spectra performance or spectra resolution. A Raman spectrometer with a pixel resolution exceeding the spectral resolution would be problematic, but such a product does not exist in the market. Caution is necessary when the spectral resolution presented in the specifications is high. You need to confirm whether the value presented is actually pixel resolution or not, because in the specifications of some spectrometers, spectral resolution is described in units of “cm-1,” instead of “cm-1/pixel,” while it is actually pixel resolution.
Illustration of pixel resolution
With a CCD with a large number of small-sized pixels, the value of pixel resolution can be minimized. However, this does not mean that spectral resolution increases.
