Fig. 3.

Comparison of standard high numerical aperture (NA), adaptive optics (AO) and joint aperture (JA) confocal imaging. In each case, the figure shows the incoming wavefront, a focusing lens, and the converging wavefront to one sample location. In high NA imaging, lens aberrations induce more and more wavefront distortion as NA is increased. Thus, the imaging system reaches maximum collection efficiency at an NA value that is given by the specific imaging system. With adaptive optics, the incoming wavefront is predistorted in order to compensate for aberrations, enabling the use of very large NAs. JA imaging is conceptually similar to AO imaging, since each beam can easily be tilted individually in order to correct for aberrations. Note that, contrary to high-NA and AO methods, the relative phase across the NA does not need to be actively controlled, since it is already set by the active (illuminating) beam.