Research On The Design Method Of Freeform Illumination

Freeform surface illumination is an elegant way of lighting, which is remarkably different from the conventional surface illumination, due to its flexible spatial layout and high degrees of design freedom. With optical freeform surfaces in an illumination system, the structure of the system could be significantly simplified, the incident beams of the light source could be well controlled and some special illuminations could be easily produced. Moreover, some new illumination applications would be further explored, and both green lighting and healthy lighting could be achieved by using the freeform surface illumination. The freeform surface illumination could be a major breakthrough of nonimaging optics.This paper focuses on studying the design methods of freeform surface illumination, and aims to establish an effective theory and optical strategy for designing smooth freeform illumination. Based on the shortcomings of the existing design methods, this work goes through three main research phases, and finally proposes an effective design method for solving the problem of smooth freeform surface illumination, and establishes a mathematical model for the freeform surface illumination.In the first research phase, a method is proposed for design freeform surface for solving the illumination problem of collimated light. With this method, the inverse problem of freeform illumination design is converted into first-order partial differential equations, and a set of discrete data points of the freeform surface are obtained by mathematically solving these partial differential equations. A specific application of laser beam shaping is given to verify the feasibility of this design method. The continuity of the freeform surface designed by this method is strongly determined by the integrability of the mapping. Then, a design strategy of freeform lens arrays is proposed to obtain smooth freeform surface for laser beam shaping. With this design strategy, the continuity of the freeform surface in each freeform lens unit is ensured. Since freeform optical surfaces are often constructed only to pass though data points in most current CAD software packages and the given normal at the prescribed point can not be ensured, this paper also proposes a novel method to construct smooth freeform optical surfaces using unit tangent vectors of feature data points as constraints. Beams are controlled well with this new construction method, and better results can be obtained with fewer feature data points.In order to establish a more effective approach, an optimization method is employed in designing smooth freeform surface illumination using ray targeting in the second phase. Two main optimization steps are contained in this method. Based on the sequence ray tracing, the merit function of the first optimization step is constructed with the position deviation on the target plane for each ray to ensure the shape of the illumination pattern. The second optimization step employs the non-sequence ray tracing, and construct the merit function with the relative standard deviation of irradiance and the transmission efficiency of the illumination system to improve the optical performance of the illumination pattern. Due to the low design efficiency of the second step, an efficient evaluation method is introduced in this step to further improve the irradiance uniformity. With this new evaluation method, the irradiance uniformity can be significantly improved just by tracing several hundred rays based on the sequence ray tracing. The modified optimization design method is more efficient, and feasibility of this method is verified by theoretical simulation, manufacturing and experimental test. But, the results of this method are strongly determined by the initial design, and only some simple illumination tasks can be tackled by using this method.A design theory is established in the third research phase to deal with the problem of smooth freeform surface illumination design without assuming any symmetry based on the concept that this problem is similar to the problem of optimal mass transport. With this method, the freeform design is converted into a nonlinear boundary problem for the elliptic Monge-Ampere equation. For collimated beam and compact light source, a corresponding mathematical model is established for the freeform illumination, and a numerical method is given for solving this kind of mathematical model. Complex illumination tasks for laser beam shaping and LED illumination are easily tackled with this method, and smooth freeform surfaces are obtained. Also, feasibility of this method is verified by theoretical simulation, manufacturing and experimental test. This design method is more effective than other methods reported within the last ten years to the best of our knowledge, for tackling the problem of freeform illumination design.