True confocal design
The biggest advantage of a true confocal Raman microscope, is the independent control of spatial and spectral resolution. This is achieved by placing a pinhole aperture in front of the spectrometer entrance slit. Variable pinhole apertures control the degree of confocality, while the entrance slit controls the spectral resolution of the spectrometer. The downside of this design are the difficulties encountered when trying to keep both apertures ideally aligned to maintain optimum performance.
In a simplified configuration the spatial resolution can be controlled by a combination of the entrance slit in one direction, and the spatial resolution of the CCD detector in the orthogonal direction. Spectrograph limitations lead to inferior performance when it comes to spatial resolution but by reducing the number of optics in the pseudo-confocal setup, the overall throughput is greatly improved.
Hybrid-confocal design (FlexFocus)
Since both, a high troughput and true confocal design offer obvious advantages, a Raman microscope can be equipped with a hybrid aperture array containing a set of pinholes and slits that can act as the confocal aperture and the spectrograph entranc. This Hybrid design combines the benefits of the two designs and allows on-demand access to a true confocal or high throughput setup.