A new scheme for synthesizing three-dimensional longitudinal spatial coherence function is proposed. By manipulating the irradiance of an extended quasi-monochromatic spatially incoherent source with a spatial light modulator, we generated a special optical field that exhibits high coherence selectively for the specified location along the optical axis of propagation and for the specified inclination between the two mirrors in the interferometer. The feasibility of the proposed principle is demonstrated by measuring a step height made by standard gauge blocks. The proposed scheme permits one to perform phase shift without recourse to mechanical movement. The quantitative experimental proof of the principle is presented.