- A longitude coherence control system needs a source that is temporally coherent and yet spatially incoherent. For this reason, a zone plate-like pattern is imaged onto a rotating ground glass. Generally the ground glass exhibits a strong directivity. It is far from an ideal uniform diffuser that is implicitly assumed in the theory. To solve this problem, we propose a new illumination system. In our system, the beam illuminating the zone plate-like pattern is focused on the test surface. We measured the longitudinal coherence functions in our optical system. The result shows that the longitude coherent function is better controlled in our interferometer.