Freeform imaging spectrometer design using a point-by-point design method.

In this paper, we present a novel design method of freeform imaging spectrometers. All the freeform surfaces including the grating surface are generated point-by-point. The rays coming from multiple wavelengths, multiple fields, and different pupil coordinates are all used, and the refraction/reflection as well as the diffraction of the grating are considered simultaneously in the design process. In addition, this method can be easily used in the design of special nonsymmetric configurations. The system designed by this method can be taken as a good starting point for further optimization. To demonstrate the feasibility of the proposed method, we successfully designed a compact freeform reflective imaging spectrometer working under the visible and near infrared band. High performance and small distortion have been achieved.