  The main advantage of two-photon fluorescence confocal microscopy is the low
absorption obtained with live tissues at the wavelengths of operation. However,
the resolution of two-photon fluorescence confocal microscopes is lower than in
the case of one-photon excitation. The 4Pi microscope type C working in
two-photon regime, in which the excitation beams are coherently superimposed
and, simultaneously, the emitted beams are also coherently added, has shown to
be a good solution for increasing the resolution along the optical axis and for
reducing the amplitude of the side lobes of the point spread function. However,
the resolution in the transverse plane is poorer than in the case of one-photon
excitation due to the larger wavelength involved in the two-photon fluorescence
process. In this paper we show that a particular arrangement of the 4Pi
microscope, referenced as 4Pi0 microscope, is a solution for obtaining a
lateral resolution in the two-photon regime similar or even better to that
obtained with 4Pi microscopes working in the one-photon excitation regime.
