Robert D. Gengelbach

Lens for uniform LED illumination: an example of automated optimization using Monte Carlo ray-tracing of an LED source

We present preliminary results of optimizing a T1 LED integral dome lens for efficient and uniform illumination. The automated optimization process is based on a geometrical model of the LED die radiation inside the encapsulant. A commercial nonsequential ray-tracing program is combined with an external global optimization engine to maximize both efficiency and uniformity. The LED model is based on surface radiance images of an LED package sectioned with a diamond saw, and validated by comparison to measured near-field irradiance patterns. The resulting lens design is compared with the original design based on calculated irradiance patterns. The optimized lens substantially improves calculated uniformity, and somewhat improves efficiency, at the expense of a more complex lens profile, including multiple ripples on the surface. Robustness of the new design is evaluated by ray-tracing the lens with the LED displaced by typical manufacturing tolerances for low-cost LEDs, resulting in significantly degraded uniformity. The results are still better than the original dome lens, but clearly indicate that manufacturing variabilities may place an upper bound on the uniformity achievable, at least with our preliminary approach.

Novel compact nonimaging collectors for LED arrays

Commercial adoption of non-imaging optics has been limited in part by high length-to-aperture ratios of classical designs. Previous low-aspect-ratio designs have been limited to particular angular ranges. We will present designs with length-to-aperture ratios of 0.6 or lower, with nearly ideal etendue-preserving performance and efficiency over a wide range of collection and output angles. The results described will include raytrace simulations, experimental results, and demonstration units.

Beam shape transforming devices in high-efficiency projection systems

Beam-shape transformers are used in high-efficiency projectors to match round source beams to rectangular LCD targets. Two common alternative approaches use (1) light- pipes and (2) lenslet arrays. Both approaches outperform the simple overfilling used in simpler systems. We review the two approaches conceptually and develop simple theoretical models which elucidate each approachs trade-offs between uniformity, efficiency, etendue, and compactness. For the light-pipe approach, we develop a detailed analytical theory for idealized, uniform sources, and we compare the predictions to ray-trace results. Finally, we compare the two approaches and find that the light-pipe approach offers similar performance with higher compactness. The lens array approach, however, may be favored in complex systems where multiple elements already result in long optical trains, or in systems where etendue conservation is not a priority.

Metal halide lighting systems and optics for high-efficiency compact LCD projectors

Compact LCD projectors require a high efficiency light source that has the smallest possible spatial extent. Further, they require optical systems that preserve the etendue. Current projector illumination systems have aberrations that produce a light beam whose etendue far exceeds the intrinsic etendue of the light source itself. As a result both efficiency and uniformity fall short of what is theoretically possible. We provide a theoretical framework for understanding these aberrations and the magnitude of their effect. We also present results showing the efficiency, uniformity, and other performance gains which are possible when these aberrations are corrected. This work also describes the performance of long-life, short-arc metal halide lighting systems that are able to increase screen brightness of compact projectors several fold without any increase in system power or heat. With these systems it has been possible to design and validate lamps operating at 50 Watts, producing > 3,000 lumens and having excellent lumen maintenance throughout their 4,000 hour life. The benefits of the combination of an improved etendue-preserving optical system and a short-arc metal halide lamp will be demonstrated.

Plastic optics in LCD backlighting

A precisely modeled acrylic lightpipe is combined with micro-structured films, reflective films, adhesives, and other mechanical components in the construction of a thin, high performance LCD backlight. We focus here on the manufacturing issues associated with the injection molded lightpipe.

Extension of tailored design method: illumination with a collimated annular source

Many commercially available sources can be modeled as collimated, annular sources. We use a tailored central-ray method to concentrate light from such a source onto a cylindrical target. The design method is a straightforward extension of methods previously describing for point sources. The device was developed and is now being used for a commercial application.

Modeling arc lamp collection efficiency for projection displays: ray tracing vs. measured results

Recent developments in arc modeling have stressed increasingly detailed representations of arc structure, but there is little or no published data on how well these arc models predict measured efficiency in a light collection system. We develop a highly simplified arc model consisting of two main components and a few minor components. We then compare the predictions of this arc model with actual measured collection efficiencies for an Ushio 350W short-arc metal halide lamp. The predictions of the simplified model are shown in this instance to be as good or better than much more complex models. The simpler approach may be more cost- effective in volume, and may have advantages for describing lamp variability.

Nonimaging optics in direct view applications

Direct view applications present a number of performance challenges for nonimaging optics. Specific requirements for direct view devices are introduced and several nonimaging components are evaluated in terms of these requirements. Results show that uniformity is the key issue for these applications. Two approaches to improving uniformity are presented.

Precision plastics and LCD display backlighting

A precisely molded acrylic lightpipe is combined with micro-structured films, reflective films, adhesives, and other mechanical components in the construction of a thin, high performance LCD backlight. We focus here on the manufacturing issues associated with the injection molded lightpipe.