Briefly, a phase annulus is used as a circular illumination mask. A phase plate with matching circular modulator is in a conjugate aperture plane in the detection optics (typically the objective back focal plane). It attenuates and imparts a small phase shift to the un-diffracted 0th order light. Diffracted light (mostly) does not traverse the phase plate and is unattenuated – its interference with 0th order light in the image generates contrast.