When developing an LED illumination system, the designer is often restricted to a constrained working volume. This can lead to efficiency loss, thermal issues, and performance restriction. It becomes important to understand the etendue of the source and optics. Also, the optics should be designed so as to maximize the efficiency of the system. Along with discussion of these issues, a case example will be presented where incandescent position lights on the F-15 fighter are replaced with LED systems that have both visible and near infra-red functions.
KEYWORDS: Tolerancing, Systems modeling, Optical design, Manufacturing, Monte Carlo methods, Clouds, Computer aided design, Surface roughness, Algorithm development, Near field
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.
Techniques to improve source modeling are presented: filament flux weighing, depositions on the arc envelope interior, and electrode degradation. Filament sources provide more light from the center in comparison to the ends. Additionally, the helix interior is hotter due to increased absorption, and thus the flux emission is greatest here. These effects for linear filaments are modeled in software with the ancillary use of camera images of lit appearance. The result is that the source luminance is more accurately modeled. This technique, called flux weighting, is described and software examples using reflectors are presented and compared to those that do not use flux weighting. Software models of arc sources that employ camera images of the arc provide accurate representations of the source radiance. However, these models do not include arc source aging. Aging effects include degradation of the electrodes and the depositions on the interior of the envelope. These phenomena lead to a decrease typically in the luminance from the source. Camera images of the lit and unlit appearance of arc sources are presented and their effect on the arc output is discussed. Additionally, software examples using reflectors are presented and compared to those that do not use these techniques.
This paper discusses the reverse engineering of filament-based light sources for computerized optical analysis purposes, especially problems raised by source tolerances. The H12 automotive headlamp bulb is used as a case study. Notes on reverse engineering sources consist of: a statement of the challenges involved, our source-modeling methodology, and useful values and procedures pertaining to simulated sources and optical ray-tracing software. A multi-model approach is outlined and consists of: gathering tolerance information from specification sheets, modeling for source tolerances with eight key models, and output comparisons between nominal and toleranced versions of the H12 source. Recommendations for including source tolerances in non-imaging illumination designs conclude.
With the development of faster computers, the ability to design and optimize complex optical systems has been dramatically improved. This directly translates into faster product development cycles with less need to build costly prototypes. Systems using light pipes, faceted Fresnel lenses, and nonimaging optics demand non-sequential raytracing, generalized surface modeling, and scattering and/or ray-splitting off of surfaces. Addressing these issues slows computation, resulting in time constraints that, in the past, prevented the use of software codes to do much more than analyze complex systems. Now, a system’s radiometric performance can be evaluated in minutes instead of hours, allowing more exotic computer aided design and optimization techniques to be used. We present rules-of-thumb on how to design, optimize, and tolerance illumination systems. Examples of systems include faceted light pipes and Tailored Edge-Ray Concentrators that create uniform illuminance. Applications for such systems are broad and include automotive, appliance, and room lighting.
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