In light microscopy the transverse nature of the electromagnetic field prec
ludes a strongly focused longitudinal field component, thus confining polar
ization spectroscopy and imaging to two dimensions (xy). Here we describe a
simple confocal microscopy arrangement that optimizes for signal from mole
cules with transition dipoles oriented parallel to the optic axis. In the p
roposed arrangement, we not only generate a predominant longitudinally (z)
polarized focal field, but also engineer the detection scheme in such a way
that in a bulk of randomly oriented molecules, the microscope's effective
point-spread function is dominated by the contribution of those molecules t
hat are oriented along the optic axis. Our arrangement not only implicitly
allows for the determination of the orientation of transition dipoles of si
ngle molecules in three dimensions, but also highlights the contribution of
z-oriented molecules in three-dimensional imaging. (C) 2001 Society of Pho
to-Optical Instrumentation Engineers.