Research On Design Method Of Optical Freeform Surfaces Based On Manufacturing Constraints

In imaging optical systems,conventional spherical or aspherical surfaces can be replaced by the optical freeform surfaces to improve the image quality,reduce the number of optical components and simplify the system configuration.However,the surface structures of optical freeform surface are complex.The design results obtained by traditional freeform surface design methods,even if it may satisfy the imaging requirements in theory,have difficult in manufacturing and aligning process,which reduces the practicability of freeform surfaces.In order to avoid the detachment between the design and manufacture of freeform surfaces,the manufacturing constraints are introduced in the form of reference surfaces during the design process to control the shape and position of freeform surfaces,in order to reduce the difficulty of manufacturing while maintaining the optical performance.The whole design process can be divided into two steps,including construction and iteration.In both stages,the freeform surfaces are constrained according to the required geometric structure of optical system,and a cylindrical off-axis three-mirror imaging system was designed to verify the feasibility of the proposed method.The main works of this paper are as follows:(1)According to the constant optical path condition,the initial structures of all freeform surfaces are constructed on the basis of feature rays and feature points.In each sampling field,a reference point is selected to constrained the spatial distribution of the remaining feature points so as to control the shape and position of the surface.(2)The initial structure of freeform surface that obtained by construction are optimized by iterations,and the expression coefficients of surfaces are adjusted directly in the process of iteration.In this way,the image quality can be improved and the system configuration will not deviate from the original structure greatly,leading to a well-perform optical system.(3)The method's feasibility was validated by designing an off-axis three-mirror imaging system that operates at F/2.0 with a 200 mm focal length and a 4° × 4° field of view.The freeform surfaces were guaranteed to be distributed along a cylinder 150 mm in radius for reducing the difficulties of ultra-precision machining.