4Pi confocal microscopy with alternate interference.

We introduce 4Pi confocal microscopy with destructive interference of converging waves. Linear lobe deconvolution as well as nonlinear restoration of 4Pi confocal raw data with their point-spread functions (PSF's) leads to almost-identical images, irrespective of whether the 4Pi confocal PSF relies on constructive or destructive interference. Three-dimensional imaging of microtubules of a mouse fibroblast cell yielded an axial resolution near 100-nm in both cases. Moreover, restoration of 4Pi confocal images of the same object with alternate phases is introduced as a powerful test for (nonlinear) image restoration.