Confocal Raman Microscopy

What is confocality?

In optical microscopy, confocality means that an illuminated sample spot and a pinhole aperture within the beam path both share the same focal point. In practical terms, instead of the entire sample, only a small part is illuminated by a point-shaped light source. The pinhole then blocks unfocussed light, thus increasing contrast and depth of field.

What is confocal Raman Microscopy?

This principle can be applied to Raman spectroscopy, thus enhancing spatial resolution along x,y- (lateral) and z-axis (depth) while also enabling depth profiling. Raman microscopes, however, may differ in their confocal design.

Traditional confocal light microscope
Differences between a traditional and confocal light microscope.
Pseudoconfocal hyprid
Differences between pseudo-confocal design and a hybrid setup.

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.

Pseudo-confocal design

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.

Abbildung 1: Rotes Spektrum: 10 seconds acquisition time, 50 µm pinhole. Blue spectrum 1 second acquisition time, 50 µm slit.