Compensation null test is a very important method in testing aspherical surface with high accuracy

in which a Zygo high accuracy digital interferometer is used. The design of a null lens is the key problem for compensation null test. In this paper

the principle of compensation null test is described. In connection with the being tested small aspherical mold surface with large NA

we developed a kind of null lens system made up of four thin lenses. The first two thin lenses form a system with very small NA mainly used to produce suitable spherical aberration; the other two form a large NA system

which together with that of the front two elements meets the null testing need. The wavefront variation of testing system due to the manufacture

test

assembly and adjustment errors is analyzed in detail. We are capable of measuring the aspherical surface to the accuracy of 0.05λ

which is enough to satisfy the testing of the mold surfaces of compact disc object lenses.