This paper introduces a new technique for assessing the diversity of approximation of exact set. The diversity is assessed by the "exposure degree" of the exact set against the approximation set in the new technique where the projection of the approximation set on the exact set, not the approximation set, is used to cover the exact set. Therefore, the new technique still works well for the problems with 3 and more objectives, significantly improving the existing techniques. Computational results show that it assesses the diversity well. Rigorous analysis is made for the new method.