Designing an off-axis freeform imaging system with a wide field of view based on an equal optical path surface extension method

An off-axis imaging system with a wide field of view (FOV) has significant applications and, combined with freeform surfaces, can achieve excellent optical performance. However, the initial structure of the off-axis freeform imaging system with a wide FOV is rare, and its optimization requires extensive experience, which brings a challenge to the system design. In this paper, a design method is proposed to effectively construct the initial structure of an off-axis freeform imaging system with a wide FOV. In this method, with the expansion of the FOV, the freeform surfaces of a system are extended according to the equal optical path condition. Then an iterative process exists for improving the imaging quality of the system. To verify the effectiveness of the method, an off-axis freeform imaging system with a 40^∘×1^∘ FOV is designed as an example. By using the proposed method, a good initial structure of the system is constructed, and after optimization, the system achieves good imaging quality.