Lighting Research Center staff members and students presented six papers at the International Society for Optical Engineering (SPIE) conference, held July 9 through 11 in Seattle, Washington. The LRC made its presence felt at the conference, playing a role in planning and developing the session on solid-state lighting. "This event was a three-way collaboration between Rensselaer, Georgia Tech, and the University of California Santa Barbara," explains Dr. Nadarajah Narendran, Director of Research at the LRC. "We started these sessions in solid-state lighting at SPIE last year. We had what we thought was a good turnout then--about 60 people. This year we had more than160 people--it was standing room only. SPIE was very pleased about that, and so were we. There is a lot of interest in solid-state lighting now." The solid-state lighting session attracted over 40 technical papers from scientists around the globe.
In addition to serving as one of the conference chairs, Dr. Narendran authored or co-authored four of the papers presented by LRC staff and students. These papers ranged from studies in human factors to optics to metrics for solid-state lighting. They show the breadth and depth of the LRC's work in solid-state lighting, particularly in light-emitting diodes (LEDs).
Dr. Narendran presented "Color Rendering Properties of LED Light Sources," a paper he wrote with Lei Deng. The researchers conducted a laboratory human factors experiment to understand the color rendering properties of various LED-based reading lights. Human subjects rated their preference for a given scene illuminated by a test light source compared to an identical scene illuminated by a reference light source. In another experiment, the same subjects viewed and rated the scenes individually when illuminated by a test light source. The subjects preferred LED-based light sources to halogen and incandescent light sources for overall color appearance. Phosphor-based white LED light sources, however, were rated "poor" for the appearance of human skin tones. This study shows that RGB mix white LED light sources have the best overall characteristics for reading or task lights. Also, CRI has no correlation to peoples' color preference and should not be used as a target metric for color rendering properties in developing solid-state light sources because it could negatively affect overall performance. A better metric for quantifying light source color rendering and preference properties is long overdue.
Ramesh Raghavan presented "Refrigerated Display Case Lighting with LEDs," a paper he wrote with Dr. Narendran. Lighting in refrigerated display cases constitutes about 20% of the overall energy load of the refrigerator. Traditionally, retailers use linear fluorescent lamps in most refrigerated display cases, but this system has a number of drawbacks, including non-uniform illumination of objects, wasted light, and reduced light levels in cold temperatures. The researchers examine the possibility of using a distributed lighting system with LEDs instead of fluorescent lighting for refrigerated display cases. Because the LEDs are small, they can be configured into a linear array to form a distributed lighting system, which can minimize wasted light, provide a more uniform light on the objects, and create more visual appeal. This research compares the two lighting systems for uniformity of illuminance, flux used, and public preference.
Insya Shakir presented "Evaluating White LEDs for Outdoor Lighting Applications," a paper she wrote with Dr. Narendran. The researchers investigated the acceptability of different white LEDs for outdoor landscape lighting. They used a scaled model setup with a scene designed to replicate the exterior of a typical upscale suburban restaurant. The lighting replicated light levels commonly found in nighttime outdoor conditions. The model had a central dividing partition with symmetrical scenes on both sides for side-by-side evaluations of the two scenes with different light sources. While the researchers maintained equal luminance levels and distribution between the two scenes, subjects evaluated four types of light sources: halogen, phosphor white LED, and two white light systems using RGB LEDs. In this study, the RGB LEDs performed as well as or better than the most widely used halogen light source in this setting. A majority of the subjects found RGB LED and halogen light sources to be more inviting. The phosphor white LEDs made the space look brighter, but a majority of the subjects disliked them.
Dr. John Van Derlofske presented "White LED Sources for Vehicle Forward Lighting," a paper he wrote with Dr. Michele McColgan. With the advent of new higher lumen LED packages, and with the promise of even higher light outputs in the near future, automobile designers are beginning to consider using white LEDs as sources for all vehicle forward lighting applications. Through computer modeling and photometric evaluation, the researchers examine the possibility of using currently available white LED technology for vehicle headlamps. Obviously, optimal LED sources for vehicle forward lighting applications will be constructed with yet-to-be-developed technology and packaging configurations. By exploring currently available products, however, the researchers intend to begin a discussion on the design possibilities and significant issues surrounding LEDs in order to aid in the design and development of future LED sources and systems. The researchers consider such issues as total light output, physical size, optical control, power consumption, color appearance, and the effects of white LED spectra on glare and peripheral vision. Finally, they draw conclusions about the feasibility of current LED technology being used in these applications and make recommendations for technology advancements that may need to occur. For more information about this presentation, contact Dr. Van Derlofske at firstname.lastname@example.org.
Feng Zhao presented "Optical Elements for Mixing Colored LEDs to Create White Light," a paper he wrote with Dr. Narendran and Dr. Van Derlofske. The researchers conducted an experimental study to investigate the possible use of light guides as mixing elements for mixed-color white LED systems. They systematically analyzed for color mixing two types of light guides: one with a square cross section and the other with a circular cross section. Past literature suggests that square light guides are better color mixers than circular light guides. This study had two parts: a computer simulation using a commercial ray tracing software package and an experimental study verifying the results obtained from the simulation. In this study, beam uniformity--in terms of illuminance and color--did not improve significantly with the light guides. System efficiency dropped as a function of length. The measured results matched the simulation results well. Circular and square light guide geometries showed similar performance, contrary to what was suggested in previous literature. The researchers noted significant improvement of the illuminance and color uniformity when the output ends of the light guides were diffused, and this diffusion introduced only a small additional loss (6%) in system efficiency.
Patricia Rizzo presented "Color and Brightness Discrimination of White LEDs," a paper she wrote with Andrew Bierman and Lighting Research Center Director Dr. Mark Rea. Great strides have recently been made in the development of white LEDs, although perceptible variations remain in the color and brightness of nominally identical products. The researchers set out to examine color and brightness discriminability between different white LEDs when used as illuminants of colored and achromatic objects. They used a method of successive comparisons to assess discriminability rather than the more typical simultaneous (side-by-side) comparisons using a "same-different" response protocol. They developed three-dimensional tolerance zones based on discriminability in chromaticity (u', v') and luminance when illuminating the colored and achromatic objects. These tolerance zones could be used to establish specification tolerances for different lighting applications.
For more information about these presentations, contact Dr. Narendran at email@example.com, or Keith Toomey at firstname.lastname@example.org