How to compute the Nyquist rate with 2-photon excitation?
If in 2-photon excitation a pinhole is not used it is the excitation distribution which determines the imaging properties of the microscope and therefore the Nyquist rate.
The excitation *intensity* field is that of a widefield microscope, but since due to the 2-photon effect the effective excitation distribution is the square of the intensity distribution, the imaging properties are vastly different. The squaring operation makes the distribution more 'peaked', resulting in an improved resolution. It also causes the bandwidth (and with that the Nyquist rate) in x, y, z to be twice that of a widefield microscope at the same wavelength.
Importantly, the 3D shape of the band-pass area is very different: while the widefield area has a wedge at the center causing the large widefield blur cones, the 2-photon bandpass area has no such defects.
