Freeform optics constitutes a new technology that is currently driving substantial changes in illumination design. It liberates designers and engineers from restrictions of optical surface geometry, enabling them to achieve compact, lightweight, and efficient illumination systems with excellent optical performance. Freeform optics is currently used widely for both fundamental research and practical applications. This Review focuses on the design of freeform illumination optics, which is a key factor in advancing the development of illumination engineering. The design methods of freeform illumination optics can be divided into two groups: zero-´etendue algorithms based on ideal source assumption and design algorithms for extended light sources. The zero-´etendue algorithms include ray mapping method, Monge–Ampere equation method, and supporting quadric method. The algorithms for extended sources include illumination optimization, feedback design, and simultaneous multiple surfaces method. The characteristics and limitations of these design methods are illustrated and a summary of these methods with future outlooks is also given.

Design of Freeform Illumination Optics

In this letter, an ellipsoid–spherical combined light source structure is presented for the purpose of uniform flux and color mixing, when considering heat dissipation. To allow more rays couple into spherical chamber while keeping illumination uniformity, algebraic equations to estimate light spot size focused by ellipsoidal reflector are derived and then employed to decide input slot size of the chamber. Then Monte Carlo ray tracing simulation suggests that the most preferable output slot size is between 15% and 30% of the chamber radius when considering both uniformity and available test area. A prototype with a 150-W Xenon arc lamp is fabricated to demonstrate heat distribution for the system, and experiment about uniformity indicates that 89.4% of the test circle radius can reach 2% nonuniformity. Color mixing performance for the system is studied by equipped with three ellipsoidal reflectors to collect rays from different color LEDs, and experiment shows that the maximum root-mean-square error of the spectrum across 90% of output circle radius is 1.4%. Results demonstrate it is a structure with simplified design process and having less dependence with light source type.

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Design of Ellipsoid and Spherical Combined Light Source for Uniform Flux and Color Mixing

Through direct design methods, both coordinates and normals of discrete data points on a freeform surface are usually obtained, but traditionally the freeform surface is generated by only fitting the coordinates. In this paper, a novel generating method that fits both the coordinates and the normals is proposed based on the mathematical multiobjective optimization theory. This new method is suited for generating some freeform polynomial surfaces. Two design examples are introduced, and their optical performance is significantly improved when applying the new method compared with the traditional method. This new method is validated to be effective, and it is widely useful as long as the coordinates and the normals are provided in advance.

Generating optical freeform surfaces considering both coordinates and normals of discrete data points

A collimating lens for a light-emitting-diode (LED) light source is an essential device widely used in lighting engineering. Lens surfaces are calculated by geometrical optics and nonimaging optics. This design progress does not rely on any software optimization and any complex iterative process. This method can be used for any type of light source not only Lambertian. The theoretical model is based on point source. But the practical LED source has a certain size. So in the simulation, an LED chip whose size is 1 mm*1 mm is used to verify the feasibility of the model. The mean results show that the lenses have a very compact structure and good collimating performance. Efficiency is defined as the ratio of the flux in the illuminated plane to the flux from LED source without considering the lens material transmission. Just investigating the loss in the designed lens surfaces, the two types of lenses have high efficiencies of more than 90% and 99%, respectively. Most lighting area (possessing 80% flux) radii are no more than 5 m when the illuminated plane is 200 m away from the light source.

Collimating lens for light-emitting-diode light source based on non-imaging optics

An optimization method is proposed for designing an LED freeform lens which produces a uniform circular pattern with high energy efficiency. This method is composed of three main aspects: design of the initial guess, parameterization of the freeform surface, and construction of the merit function. The initial guess is created by solving an ordinary differential equation numerically. An approach of selecting optimization points is introduced for parameterization of the freeform surface. The merit function is constructed by use of the irradiance uniformity and the efficiency of the lens. Design examples are given, and the results show that the irradiance distribution is well controlled with a maximum uniformity (the relative standard deviation of irradiance, RSD) of 0.0122 and a maximum efficiency of 93.88%. This optimization method can be generalized to design freeform lenses with different lighting patterns or without rotational symmetry.

Optimized design of LED freeform lens for uniform circular illumination

The study related in this paper is the design of a ship-mounted LED of high brightness and distant spot. The freeform lens design obeying the edge ray principle and Snell’s law is presented first. Then, to fit the illumination requirement of the searchlight, we designed a freeform lens to collimate all the light rays coming from the LED. However, theoretical analysis proves that there is a critical angle for incident rays beyond which the rays cannot be collimated, and 55% is the light-efficiency limit for polymethyl methacrylate freeform lens. We then designed a combination of a freeform lens–coupled parabolic reflector that improved light efficiency to 70%. In this paper, the design of the freeform lens–coupled parabolic reflector is given in detail. In addition, tolerance analysis and the effect of manufacturing defect are presented.

Lens design of LED searchlight of high brightness and distant spot

In this paper, an illumination lens design for a LED-based pico-projector is presented. Different from the traditional illumination systems composed by lens group, the integral illumination lens consists of a total internal reflector (TIR) and a freeform surface. TIR acts as collimation lens and its top surface formed by a freeform surface reshapes the nonuniform circular light pattern generated by TIR to be rectangular and uniform. Diameter and height of the lens are 16 and 10 mm, respectively. An optimization method to deal with the problem of extended light source is also presented in detail in this paper. According to the simulation results of the final optimized lens, 77% (neglecting the effect of polarization) of the power of light source is collected on liquid crystal on silicon panel with a 16∶9 ratio and illumination uniformity achieves 92%.

Integral freeform illumination lens design of LED based pico-projector

Total internal reflection (TIR) lenses are optical components that are used to collimate the light or to generate a desired uniform illumination. However, most TIR designs do not pay attention to color uniformity, an increasingly important issue for the quality of lighting, and have a considerable dimension, which also limits their applications. In this study, we proposed an effective design principle of color mixing, and a phosphor-converted white LED module integrated with a compact modified free form TIR component was presented and optimized to achieve compact size and high angular color uniformity (ACU). Optimization results indicated that modified LED packages could achieve a smaller size in vertical height (∼0.52) within the same horizontal radius, compared with LEDs integrated with classic TIR lenses. Meanwhile, the enhancement of ACU with an optimum appropriate divergence half angle (65°) reached as high as 84% in terms of normalized standard deviation of yellow–blue ratio from 0.888 to 0.429.

Design of a compact modified total internal reflection lens for high angular color uniformity

LED's luminous efficiency increases rapidly in recent years, many novel illumination applications with LEDs are emerging. Based on the analysis of LED's characteristics, our optimization scheme and design methods for LED taxiway lamps are proposed. A novel LED lamp using in navigation of the taxiway of the airport is designed as an example. The simulation results demonstrate that the light utilization efficiency is 35%, which is much higher than the current standard with metal halide lamps. This also provides a new design concept for the next-generational LED navigation lamps.

A novel LED lamp for the middle line of the taxiway of the airport

Total Internal Reflection (TIR) lenses are optical components which are used to collimate the light. Desired uniform illumination within a limited beam angle is easy to be achieved by integrating LEDs with TIR lenses. However, most TIR designs do not pay attention to the color uniformity, an increasing important issue for the quality of lighting, and have a considerable dimension, which also limits their applications. In this study, a compact phosphor converted white LED module integrated with a modified freeform TIR component was presented and optimized to achieve compact size and high angular color uniformity (ACU). Optimal results of modified TIR components indicated that modified LED packages could achieve a smaller size in vertical height (~0.52) within the same horizontal radius, compared with LEDs integrated with classic TIR lenses. Meanwhile, the enhancement of ACU with an optimum appropriate divergence half angle (65 degrees) reached as high as 84% in terms of normalized standard deviation (NSD) of yellow-blue ratio (YBR), from 0.888 to 0.429. Phosphor converted white LEDs integrated with this modified TIR components can provide high quality lighting within a compact optical system.

Shuang Zhao

Papers

Lens design of LED searchlight of high brightness and distant spot

Integral freeform illumination lens design of LED based pico-projector

Design of a compact modified total internal reflection lens for high angular color uniformity

A novel LED lamp for the middle line of the taxiway of the airport

Angular color uniformity enhancement of phosphor converted white LEDs integrated with compact modified freeform TIR components