Digital Zone Plate Interferometer which can measure automatically both the shape of an aspherical mirror and the wavefront error, is investigated.
The zone plate is obtained from computer generated Fresnel zone plate hologram recorded on Kodak high resolution plate (type-IA). The focal length and the first order diffraction efficiency of the zone plate are 191mm and 15.4% respectively. The program of an automatical computer analysis is developed for interferometric fringe obtained in zone plate interferometer. The fringe contains the information of the wavefront error against the reference sphere.
The program consists of preprocessing part and analysis part. In the preprocessing part, interferometric fringe is digitalized, binarized, and then contoured at fringe centers after suppressing the noise by local averaging. In the second part, using the 2-dimensional gray level image information technique and the least square fitting method, the fringe orders are assigned, and then the wavefront error of the aspherical mirror under test is represented with Zernike polynomials. As the results, the coefficients of Zernike polynomials are obtained and the 3-dim plot of the wavefront error is accomplished.
The results thus obtained give the error information of the shape of the aspherical mirror surface under test. The error information of a surface is fitted to the general aspherical surface equation, and it is found that the surface under test is the general aspherical surface different from the conic surfaces. The over-all accuracy in the testing the aspherical surface with the present method is within λ/100.
