Method for designing initial structure of off-axis multi-reflection free-form surface optical system

The invention relates to a method for designing an initial structure of an off-axis multi-reflection free-form surface optical system. According to the method, a double-seed curve expansion method isadopted to construct each free-form surface in the system, so that the construction precision of the free-form surface is greatly improved; and an imaging optical system composed of a plurality of free-form surfaces can be designed by introducing auxiliary virtual image points. In order to verify the reliability of the method, the initial structures of various different types of off-axis multi-reflection free-form surface imaging systems designed by using the method, results are obtained through tracing light, and the results show that the off-axis multi-reflection free-form surface imaging optical system designed by using the method is high in design speed, good in stability, high in efficiency and good in imaging quality; and an optical system of a vehicle-mounted head-up display is alsodesigned by using the met