We have developed a model to describe the effect of refraction through a planar interface on the collection efficiency and depth of focus when performing confocal Raman microspectroscopy. The planar interface
introduces spherical aberration, which can substantially degrade the performance of the microscope, especially for large-numerical-aperture microscope objectives. This spherical aberration will increase
the range of focal depths spanned by the paraxial and marginal rays of the illuminating laser beam within the sample. In the collection path, it will also distort the scattering volume defined by the confocal
aperture; this results in a dramatic fall in the collected light intensity with increasing depth. We demonstrate that there is an optimum numerical aperture for collected light intensity at a given depth.
The prediction of this theoretical model is compared to empirical results obtained by mapping the stress distribution within the diamond anvil of a high-pressure cell. Both the collected Raman intensity
and the effective depth of focus are compared to the predictions from the theory.
Molecular Physics and Instrumentation Group, Department of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
Publication date: May 1, 2001
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