After the above process , the first unknown freeform surface Ω is generated , and the initial plane can be replaced . All the unknown freeform surfaces can be generated successively using this process , and the preliminary surface construction stage is completed . But the actual image points of the feature rays may be far from the ideal image points . Therefore , the iteration process is used to regenerate the freeform surfaces and reduce the deviations 27 . We take the system obtained after completing the preliminary construction process as the new initial system . When regenerating each surface in each iteration step , the coordinates of the points where the rays intersect with the initial surface are preserved as the coordinates of new feature data points , as shown in Figure 3c . The target points for the rays during iterations are generally their corresponding ideal image points . To accelerate the iteration process , an alternative negative feedback mode can be used . Here , the target point T i for each ray is determined on the basis of its ideal target point T i , ideal ( also the ideal image point ) as well as its actual image point T i * before the current surface is regenerated :