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Allowing the illumination wavefronts to constructively interfere in the sample produces a main focal spot that is sharper in the z-direction by about 3-4 times (4Pi of type A).
A similar improvement is obtained if the lenses add their collected fluorescence wavefronts in a common point detector (4Pi of type B).
Doing both together is best, of course, and leads to a 5-7-fold improvement of resolution along z (4Pi of type C) [13, 14].
To date, 4pi has not encountered an SEM that cannot be modified to accept its Digital Image Acquisition System.
As a general rule, yes, the Spectral Engine II PCI card can be used without modification in either a PowerMac or a Windows box; however, 4pi will not support this switch for any card already in the field except on a case-by-case basis, and then only if 4pi tech support is notified first.
To use the 4pi system to acquire signals from a cathodoluminescence system, the 4pi system must be configured with 4 video channels.