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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.
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