Freeform optical surfaces can be characterized as nonsymmetric surfaces and they can offer much more degree of freedom for optical design. This kind of optical surface can be seen as a revolution in the optical design and plays a key role in the next generation of high-performance optical systems. Another trend in imaging optics is to use phase elements (such as diffractive elements and metasurface). In specific, the flat or planar phase element can effectively reduce the weight and volume of the total system. Easier-alignment of the system can also be achieved. In this paper, the point-by-point design method are applied to the design of three kinds of nonsymmetric imaging systems: consisting of only geometric freeform surfaces, only flat phase elements, and both of them (the generalized case). The entire design process begins from an initial system using simple geometric planes. Both the geometric freeform surfaces and the phase profiles or functions are generated point-by-point based on specific design requirements. The design results can be taken as good starting points for further optimization. The dependence on existing starting points is significantly reduced and advanced design skills are not required. In addition, three typical three-mirror folding geometries are employed and designed using the proposed method for all the three kinds of systems under same system specifications. The imaging performance and system volume of the different systems after final optimization are analyzed and compared. The results offer insight on the selection of optimal system folding geometry and types of imaging element for the nonsymmetric system design tasks.
As a kind of radial basis functions, the Gaussian function has local feature and therefore has an excellent surface description ability. In this paper, we proposed the design strategy of freeform unobscured three-mirror system using Gaussian radial basis functions surface type and demonstrate a design example. A novel and high-accuracy surface fitting algorithm of Gaussian radial basis functions is proposed for the freeform surface fitting. Successive optimization strategy is employed for the system after surface fitting. The final example system works at the long wave infrared band and has an 8°×6° field-of-view with an F-number of 1.9.
In this paper, we demonstrated the design method of freeform unobscured reflective imaging systems using the point-bypoint Construction-Iteration (CI) method. Compared with other point-by-point design methods, the light rays of multiple fields and different pupil coordinates are employed in the design. The whole design process starts from a simple initial system consisting of decentered and tilted planes. In the preliminary surfaces-construction stage, the coordinates as well as the surface normals of the feature data points on each freeform surface can be calculated point-by-point directly based on the given object-image relationships. Then, the freeform surfaces are generated through a novel surface fitting method considering both the coordinates and surface normals of the data points. Next, an iterative process is employed to significantly improve the image quality. In this way, an unobscured design with freeform surfaces can be obtained directly, and it can be taken as a good starting point for further optimization. The benefit and feasibility of this design method is demonstrated by two design examples of high-performance freeform unobscured imaging systems. Both two systems have good imaging performance after final design.