Inspection of misalignment factors in lens assembly

Abstract

The misalignment of a particular optic in lens assembly will induce aberrations and deteriorate the performance. For the purpose of failure analysis, there are requirements from optic manufactures for the development of measurement tool to address the misaligned element. This paper presents a method for the quantitative measurement and analysis of misalignment in lens assembly, which may figure out the misaligned element and its misalignment factors. Since there are several optical elements in lens assembly, and there are different misalignments, such as decenter, tilt etc, a multiparameter tool need to be employed in the analysis. Wavefront can be expressed with Zernike polynomials, which are selected for the analysis. We choose a positive lens assembly with four elements for the study. A point light source is placed in the front focus point of lens assembly; the collimated emergent wavefront is analyzed with Zernike polynomials. We use Zeemax to simulate the propagation of wavefront, calculate Zernike coefficients correspondent to various misalignment. The results show there is a group of Zernike polynomials correspond to each misalignment. Each polynomial increase/decrease progressively against the magnitude of misalignment. It is difficult to tell the misalignment only by the analysis of Zernike coefficients. To further address the misaligned elements, we present a concept - the contrast value of Zernike coefficients, which is a series of constant even though the magnitude of misalignment changes. The method and procedure is presented to measure the contrast value with the employment of dual directional wavefront sensing.