Bandwidth is the wavelength width of a given wavelength, regardless of optical aberration, diffraction, slit height, scanning method, detector pixel width, etc. It is the product of the inverted line dispersion and slit width.
The optical resolution is defined as the minimum wavelength difference that the spectrometer can separate. To divide the two spectral lines, at least, to resemble the two-phase elements of a detector. The resolution depends on the resolution ability of grating, the effective focal of the system, the slit width, the optical aberration of the system and other parameters. The grating determines how far the wavelength can be separated from the detector (dispersion), which is a very important variable for the resolution. Another important parameter is the width of the beam into the spectrometer, which basically depends on the fixed incident slit or the incident fiber core diameter installed on the spectrometer (when the slit is not installed). Slit sizes are: 10, 25 or 50μmx1000μm (high) or 100,200 or 500μmx2000μm (high). At the specified wavelength, a slit is covered with a few elements as it is on the detector array. And if you want to separate two lines of line, you have to disperse them to this image size plus an image element. When the core diameter of the incident fiber is greater than the slit width, the resolution is determined by the width of the slit (effective width).
Spectrometer resolution can be approximated to the following metric: r∝m· F
where M is the grating line number, F is the spectrometer focal length, W is slit width.
Light source entrance. Slit area affects the intensity of light through. Slit width affects optical resolution.