We have developed a model to describe the effect of refraction through a pl
anar interface on the collection efficiency and depth of focus when perform
ing confocal Raman microspectroscopy. The planar interface introduces spher
ical aberration, which can substantially degrade the performance of the mic
roscope, especially fur large-numerical-aperture microscope objectives. Thi
s spherical aberration will increase the range of focal depths spanned by t
he paraxial and marginal rays of the illuminating Laser beam within the sam
ple. In the collection path, it will also distort the scattering volume def
ined by the confocal aperture; this results in a dramatic fall in the colle
cted 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 h
igh-pressure cell. Both the collected Raman intensity and the effective dep
th of focus are compared to the predictions from the theory.