What Are The Factors Influence The Resolution of Raman Spectrometer?
The factors that affects the optical resolution of a Raman spectrometer have the below four items. However, due to the actual design of the instrument, these four factors have different permutations and combinations. Thus, the performance of a Raman spectrometer is not simply composed by measuring these four factors.
Focal length (the distance between the grating and the detector)
The longer the focal length spectrometer (interferometer), the higher optical resolution will be. A typical Raman spectrometer has 200mm (low / medium resolution) to 800mm (high resolution) focal length. In the long focal length design, a proper grating can also be operated in the low-resolution mode.
Grating
Typically, the number of grooves per millimeter (gr/mm) of a grating device, is the way to quantify the resolution. When the groove density is higher, generally, it means a higher optical resolution. Generally, the grating in a Raman spectrometer can be from the low-resolution 300 gr/mm to a high-resolution grating with 1800 gr/mm. Besides, there are some more professional grating, when can even achieve higher density, say, 2400 gr/mm, or 3600 gr/mm. Of course, there are certain limitations. One needs to consider the other factors into the design together.
Laser wavelength
In the above factors, the considerations are based on a unique wavelength of the laser source. However, Raman spectroscopy is the measurement of the energy-related method, say, Raman shift, or represents wavenumber in the unit of cm-1. This means, when the wavelength of excitation laser from infrared wavelength to ultraviolet wavelength, will decrease the optical resolution with the same grating. Therefore, the use of green laser, need higher density of grating, in order to reach the same resolution of a near-infrared laser source.
Detectors
Usually, most of Raman spectroscopy has only one single of detector. User is not able to do anything about it. However, a detector with smaller pixels size, higher pixel density, and sensitivity, brings the ability of better optical resolution.