Mirror-based spectrographs have aspherical or toroidal mirrors that guide the light from the entrance slit to the grating and on to the detector. Typically a minimum of two or three mirrors are used per spectrograph. Mirror-based spectrometers cover a wide spectral range and therefore a single spectrometer can be used with several excitation wavelengths. However, the imaging quality of mirror-based spectrometers is rather poor due to aberrations that can be introduced from the mirrors leading to distortions on the CCD detector. Also the light transmission rate of a mirror-based spectrometer is commonly only around 45 %. Due to these factors mirror-based spectrometers are typically used for the detection of single Raman spectra and strong Raman scatterer, but are not ideal for the fast generation of Raman images and the detection of weak Raman signals.
Lens-based spectrographs use an on-axis lens-system instead of mirrors to guide light through the spectrometer. The lenses are optimized for a certain spectral range and can therefore provide optimal Raman signal transmission (typically as high as 60 - 70%), spectral resolution and imaging capability. Therefore lens-based spectrometers are the preferred spectroscopic system for the generation of high-quality Raman images.