We address freeform aplanatic systems designs both as solutions of a differential system of equations and as limiting cases of SMD designs. We conclude that, in general, freeform aplanatic systems need at least 3 optical-surfaces.
Optical Design and Fabrication 2017 (Freeform, IODC, OFT). Freeform Optics 2017. Denver, Colorado United States.
Jose Infante Herrero