R. John Koshel

Simplex optimization method for illumination design

A modified simplex optimization method is developed for the design of illumination systems. The simplex method is a judicious choice for illumination optimization because of its robustness and convergence properties. To optimize the simplex method, its four parameters are adjusted dependent on the dimensionality of the space to converge with fewer iterations. This work is presented for the end game, when the optimizer is converging on a local optimum rather than searching for it. Up to a 37% reduction in the number of computations is realized. An example using a compound parabolic concentrator is compared between the standard and the modified simplex methods, providing over 22% improvement in the end game.

Field Guide to Illumination

The content in this Field Guide starts with traditional illumination in imaging systems, followed by the recent advances in computer-aided design of high efficiency nonimaging illumination optics, along with the modern source models that support these techniques. Sections on the illumination of visual displays are included as well as some important topics on architectural illumination.

Design of Efficient LightPipes for Illumination by an Analytical Approach.

We present the concept of principal sections of a lightpipe to analyze the propagation of light through the lightpipe by total internal reflection. Only the principal sections determine the acceptance angle and thus help in the identification of regions where the leakage occurs first. Use of principal sections for analysis leads to a significant reduction in the design effort. We present an analysis of several commonly used lightpipe configurations, e.g., straight and single circular bend, and different cross sections, e.g., elliptical and rectangular. This analysis leads to the maximization of throughput and transfer efficiency. The uniformity characteristics of elementary configurations and scaling factors for a lightpipe with a single circular bend are also discussed.

Modeling of the gain distribution for diode pumping of a solid-state laser rod with nonimaging optics

We investigate the absorption distribution in a cylindrical gain medium that is pumped by a source of distributed laser diodes by means of a pump cavity developed from the edge-ray principle of nonimaging optics. The performance of this pumping arrangement is studied by using a nonsequential, numerical, three-dimensional ray-tracing scheme. A figure of merit is defined for the pump cavities that takes into account the coupling efficiency and uniformity of the absorption distribution. It is found that the nonimaging pump cavity maintains a high coupling efficiency with extended two-dimensional diode arrays and obtains a fairly uniform absorption distribution. The nonimaging cavity is compared with two other designs: a close-coupled side-pumped cavity and an imaging design in the form of a elliptical cavity. The nonimaging cavity has a better figure of merit per diode than these two designs. It also permits the use of an extended, sparse, two-dimensional diode array, which reduces thermal loading of the source and eliminates all cavity optics other than the main reflector.

Non-edge-ray design: Improved optical pumping of lasers

We propose a novel design method for reflective nonimaging concentrators that is based on maximizing system performance beyond standard nonimaging metrics of transfer efficiency and proscribed illumination distribution. This new technique enables system operation characteristics to influence the algorithm, including nongeometrical parameters. This algorithm is termed non-edge-ray design (NERD) because we found that edge rays do not always give optimal illumination design. The case of a diode-pumped, Nd:YAG solid state laser is provided as a confirmation of the design utility. The pump-to-mode configuration, which includes a nonimaging pump cavity reflector, is the focus of this investigation. The merit function of the design process includes not only the transfer efficiency from an extended, 2-D laser diode array, but also the mode coupling of the absorption distribution within a laser rod to its desired output mode. Standard edge-ray design is shown to limit performance, with both numerical and experimental results. Two alternative pump cavity reflectors are developed and the improvements in the outputs from the lasers in TEM00 modes are presented. A periodic cavity sees over 8% improvement in optical efficiency at an output power of 10 W, while an averaging cavity provides nearly 5% improvement.

Characterization of lightpipes for efficient transfer of light

Lightpipes are used to transfer light from the source to a desired target. The lightpipe shape typically conforms to the necessary path, thus bending of the lightpipe is required. A number of different methods of bending the lightpipe have been developed, from linear, discontinuous bends to smooth, common circular bends to bends that expand or contract the cross-sectional size of the lightpipe over the path. In this paper we develop a set of parameters to describe the overall shape of an in-plane lightpipe section. These parameters include the thickness, radius of bend, index of refraction, and ratios of sets of these parameters. The transfer efficiency from the source to target is used to quantify the utility of parameterized lightpipes. Etendue is used to highlight the results. More complex lightpipes, such as those with several bends along their path, can be developed from the combination of parameterized sections. Finally, this parameterization can be used to automate the development of lightpipe geometry within optical analysis and design software.

Lit appearance modeling of illumination systems

In illumination systems the look and feel are often more important than objective criterion, such as uniformity and efficiency. The reason for this is two fold: the lit appearance often sells an item and substantial variation in the illumination distribution (up to 50%) over a broad region is not noticeable to an observer. Therefore, subjective criterion, such as the lit appearance, typically plays a crucial role in the development of an illumination system. Additionally, by using computer models to ascertain the lit appearance before manufacture of the system, it allows the designer to modify the system while not demanding investment to produce prototypes. I discuss methods of determining the lit appearance for illumination systems. This modeling includes the inclusion of material and surface properties, such as surface finish, spectral transmission, and internal scattering; the response of the human eye; and the amount of rays that must be traced. By archiving the ray data, animations as a function of position and angle can be developed. Examples are developed to highlight the utility of this technique. These examples include taillights for the automotive industry and a backlit LCD screen for a laptop. Animations of these models demonstrate their luminance.

Aspects of illumination system optimization

This paper focuses on the facets of illumination system optimization, in particular parameterization of objects, the number of rays that must be traced to sample properly its properties, and the optimization algorithm with the associated merit function designation. Non-interference ensures that the parameterized objects do not erroneously intersect each other or leave gaps during the steps of the optimization procedure. The required number of rays is based on a model developed for television cameras during their initial days of development. Using signal to noise ratio, it provides the number of rays based on the desired contrast, feature size, and allowed error probability. A lightpipe is used to highlight the nuances of this model. The utility of using system symmetry to increase ray count is also discussed. A modified simplex method of optimization is described. This algorithm provides quicker convergence than the standard simplex method, while it is also robust, accurate, and convergent. A previous example using a compound parabolic concentrator highlights the utility of this improvement.

Optics in 2007

This special issue of OPN highlights the most exciting research to emerge in the preceding 12 months in the fast-paced world of optics. “Optics in 2007” offers readers a unique opportunity to access, in a single source, summaries of cutting-edge optics research reported in the peer-reviewed press. The areas covered in 2007 include biophotonics, communications, holography, lasers, micro-optics, microscopy, nonlinear optics, photonic structures, plasmonics, quantum optics, slow light, solitons and ultrafast optics.

Methods of tolerancing injection-molded parts for illumination systems

Injection-molded optical components are used often for commercial illumination systems. This paper discusses methods of how to model the tolerance aspects of such components. Tolerance aspects include surface roughness, source-to-optic position and rotation errors, and surface slope errors. It is noted that all of these tolerance investigations cannot correctly account for errors in the injection-mold process. A method to model deformations induced in the injection-mold process is proposed. The method is based on the laser scan of an injection-molded part, which allows the rebuilding of the surface from the point cloud. This method, while quite accurate, is time consuming, so a second algorithm based upon approximation with a Harvey scatter model is developed that takes over an order of magnitude less in time. It is shown that the approximate model provides results within a few percent if comparisons are done in the far field. Near-field results require the rebuild method that uses the measured point cloud. Additionally, illumination systems comprising multiple interactions with the component surface (e.g., lightpipes) can use the approximate Harvey model.