Designing and optimizing an edge-lit led lighting panel luminaire for illumination application

Abstract

1) A theoretical model that predicts the light coupling efficiency from circular LED light sources into a thin rectangular shaped optical light guide,

2) A batwing-shaped input beam can significantly improve light extraction efficiency and uniformity of the luminace appearance of the light guide panel compared to a Lambertian shaped input beam

3) A novel light guide design with a v-grove geometry at the coupling light guide surface together with prismatic light extraction features across the light extraction surface the edge-lit LED luminaire is shown to meets all three important criteria, including high system efficiency, a batwing shaped output beam distribution and uniform panel luminance.

This thesis study will contribute to a new area of inquiry for edge-lit panel luminaires used in the lighting applications. It will also build the relationship between human response and technology constraints. This dissertation work has the potential to expand our understanding of how to better designed lighting systems that not only meet the illumination requirements but also provides a pleasant lighting experience in the lighted space. This study also will contribute to research in the field of lighting design, architecture, and engineering, and will guide people to re-examine the way we use light in spaces. Unlike prior studies that simply focused on technology development and maximizing light extraction efficiency, this study took a holistic approach analyzing the technology to improve the system efficacy, beam distribution to cater the appliction’s needs and considered human subjects perception when optimizing the luminaire. The findings will make valuable contributions to the knowledge of manufacturers, lighting designers, architects, engineers, and optical designers and will provide tools and metrics to select their desired lighting experience.

Thin profile flat luminaires have attracted the attention of architectural lighting community because of their unique appearance. The edge-lit LED (light emitting diode) lighting panels are used in thin profile flat luminaires. These luminaires provide diffuse light and some of the commonly claimed benefits are thin profile and attractive, has the potential for energy savings, and has long service lifetime. For any area-emitting lighting system, three design criteria are often discussed: luminaire luminous efficacy (lumens per watt), light distribution, and visual appearance.

To ensure that these criteria are met, the system requires a comprehensive optical design and analysis. In addition it is important to gather and evaluate peoples’ subjective response to determine the visual appearance for luminance uniformity and attractiveness. One of the challenges for the edge-lit LED lighting panel luminaires is that traditionally they use a diffusive reflector that creates a Lambertian beam distribution, which may not be effective in many lighting applications. For example, in an open plan office lighting application, a bat-wing like distribution is often desired because a uniform illuminance distribution on the horizontal task planes can be created with reduced number of luminaires per space, and by eliminating the high angle beams with high intensities the discomfort glare can be reduced. However, with a bat-wing distribution, the luminaire efficiency is often reduced. Also, in conventional flat panel luminaires, the uniform appearance is created by carefully engineering the spacing of the extraction features and they usually emit a Lambertian beam distribution. In the case of other beam distributions, the uniformity hasn’t been sufficiently explored. Furthemore, it is important to quantify the uniformity of the flat panels and understand the acceptability range for occupants in the lighted space.

This study aimed to design and optimize an edge-lit LED lighting panel to achieve high luminaire efficacy, bat-wing output beam distribution, and uniform luminance across the flat panel and thus the appearance is pleasing. Therfore, the research questions addressed in this thesis are, what is the optical design, how to optimize and evaluate the targeted design criteria of an edge-lit LED lighting panel? In this context, the light guide plate (LGP) is one of the key components that needs proper design to achieve the criteria.

This dissertation work started with an extensive literature review to understand what past studies exist and thei approach addressing such criteria. Based on the findings a sample light guide panel was used for implementing promising designs and performed analysis. One of the criterion is high luminaire efficacy, that results from high coupling and high extraction efficiencies. The research question addressing luminaire efficacy is answered by addressing the coupling efficiency first. In doing so a theoretical model was developed and optimized for light coupling from an array of LEDs into the light guide using etandu matching theory. The model was validated using ray-tracing analysis and by conducting laboratory experiments. The theoretical model was developed using a cylindrical light guide as the base case and equating it to a square light guide, and stacking several such guides to form the rectangular light guide. The experiment results showed that the ray-tracing simulation results, and the laboratory experiment results agreed well with the results obtained using the developed mathematical model.

Next to maximize light extraction the input light beam was tailored. The required beam was achieved by creating a v-groove-shaped structure at the coupling surface that showed to increase light extraction and also the improved the uniformity of the beam across the light extraction surface of the light guide. The study showed that the beam profile of the extracted beam can be changed by the V-groove geometry at the coupling surface and the prismatic microstructure structure at the light extraction surface. Metrics for evaluating the panels apprerence and acceptability as uniform was gathered form past literature. This understanding is applied into this study to optimize the design of the edge-lit LED lightguide panel.

In this analysis it became evident that the light sources to coupling surface distance should maintain a minimum distance. Also by creating a v-groove surface geometry at the coupling surface it is possible to produce uniform luminance across the panel surface by distribution wiht uniformly spaced light extraction features to achieve high luminaire efficiency while offering a bat-wing shaped output beam distribution.

This thesis study contributed the following, new knowledge to the fields of optical light guide systems and lighting applications.