John Van Derlofske

Computer modeling of LED light pipe systems for uniform display illumination

Computer modeling of distributive light pipe systems using light emitting diode (LED) sources to produce uniform illumination for liquid crystal displays (LCDs) is presented. Due to their small size, lower power consumption, and lower heat generation, LEDs are a natural source choice for display illumination. However, to be useful in display applications, LEDs must be made to produce uniform illumination over the display area. The conversion of an LEDs output flux distribution to one that is uniform over a given area can be accomplished with plastic, injection-molded light pipes. Illustrative examples of LED light pipe display systems are presented. These systems compare output coupling surface geometry for two LED input coupling scenarios; direct and indirect input coupling. Computer modeling via commercial software packages is used to optimize and analyze system designs. It is critical in these simulations to have accurate source models. Therefore close attention is paid to the LED source model. The final simulation results are presented and uniformity and total light output is compared. The implications of these results for display applications are discussed.Computer modeling of distributive light pipe systems using light emitting diode (LED) sources to produce uniform illumination for liquid crystal displays (LCDs) is presented. Due to their small size, lower power consumption, and lower heat generation, LEDs are a natural source choice for display illumination. However, to be useful in display applications, LEDs must be made to produce uniform illumination over the display area. The conversion of an LEDs output flux distribution to one that is uniform over a given area can be accomplished with plastic, injection-molded light pipes. Illustrative examples of LED light pipe display systems are presented. These systems compare output coupling surface geometry for two LED input coupling scenarios; direct and indirect input coupling. Computer modeling via commercial software packages is used to optimize and analyze system designs. It is critical in these simulations to have accurate source models. Therefore close attention is paid to the LED source model. The fmal simulation results are presented and uniformity and total light output is compared. The implications ofthese results for display applications are discussed.

Discomfort and Disability Glare from Halogen and HID Headlamp Systems

This paper investigates the effects of high intensity headlamps (HID), halogen, and blue filtered halogen lights on the drivers of those vehicles using them. Specifically, the studies reported measure the differences in impact on glare under conditions similar to those experienced by drivers in oncoming traffic at night. Both disability glare and discomfort glare are considered.

White LED sources for vehicle forward lighting

Considerations for the use of white light emitting diode (LED) sources to produce illumination for automotive forward lighting is presented. Due to their reliability, small size, lower consumption, and lower heat generation LEDs are a natural choice for automotive lighting systems. Currently, LEDs are being sucessfully employed in most vehicle lighting applications. In these applications the light levels, distributions, and colors needed are achievable by present LED technologies. However, for vehicle white light illumination applications LEDs are now only being considered for low light level applications, such as back-up lamps. This is due to the relatively low lumen output that has been available up to now in white LEDs. With the advent of new higher lumen packages, and with the promise of even higher light output in the near future, the use of white LEDs sources for all vehicle forward lighting applications is beginning to be considered. Through computer modeling and photometric evaluation this paper examines the possibilities of using currently available white LED technology for vehicle headlamps. It is apparent that optimal LED sources for vehicle forward lighting applications will be constructed with hereto undeveloped technology and packaging configurations. However, the intent here in exploring currently available products is to begin the 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. Considerations such as total light output, physical size, optical control, power consumption, color appearance, and the effects of white LED spectra on glare and peripheral vision are explored. Finally, conclusions of the feasibility of current LED technology being used in these applications and recommendations of technology advancements that may need to occur are made.

Discomfort Glare From Headlamps: Interactions Among Spectrum, Control of Gaze and Background Light Level

This paper discusses a laboratory study that examines the effects of headlamp spectral power distribution, level of ambient light, and gaze control on discomfort glare. The interaction among these variables is also described. The effects of these factors are important to nighttime driver safety and performance.

Headlamp Parameters and Glare

This paper discusses a field study which examines headlamp glare. It investigates the role of spectrum, size and illuminance. It ascertains the relative magnitude of each of these variables and its impact on glare from oncoming vehicles. Results of the study indicate that illuminance at eye level is the main factor that influences glare produced by the headlamp of an oncoming vehicle.

Optical elements for mixing colored LEDs to create white light

An experimental study was conducted to investigate the possible use of light guides as mixing elements for mixed color white LED systems. In this study two types of light guides, one with a square cross section and the other with a circular cross section, were systematically analyzed for color mixing. Past literature suggested that square shaped light guides are better color mixers than circular light guides. This study was comprised of two parts: a computer simulation using a commercial ray tracing software package; and an experimental study verifying the results obtained from the simulation. 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. Significant improvement of the illuminance and color uniformity was noted when the output ends of the light guides were diffused. This introduced only a small additional loss (6%) in system efficiency.

EVALUATION OF HIGH INTENSITY DISCHARGE AUTOMOTIVE FORWARD LIGHTING

An experimental field investigation is described that compares the off-axis visual performance of HID forward lighting systems with comparable halogen systems to determine the relative visual effects of HID lighting. This has been accomplished for European beam patterns; North American beam patterns are currently being examined. The goal of the investigation is to determine if the higher off-axis intensity levels combined with the spectral properties of HID lamps provide any benefits to visual performance over conventional tungsten halogen lamps. In this study three current production European headlamp systems, one HID and two halogen, are compared. These systems are used to illuminate a fixed scene. Subjects perform a visual tracking task, cognitively similar to driving, while, simultaneously, small targets located at various angles in the periphery are activated. Subjects release a switch upon detection and reaction times and missed signals are measured. From the results, comparisons are made among the HID and halogen systems in terms of reaction time to signals at different peripheral angles, and in terms of numbers of missed signals. The results are compared to a model that predicts visibility at nighttime (mesopic) light levels. Potential implications of the results on driving safety and on the development or refinement of forward lighting standards are discussed.

Design and optimization of a retinal flux density meter

We present the design and calibration of a retinal flux density (RFD) meter. This device can serve as a standard illuminance meter or can measure flux density on the retina, using both photopic and scotopic spectral responses. In addition, through post processing, the instrument can determine standard illuminances and RFDs at mesopic levels. This device is an extension of an earlier instrument, reported on by us, which replicated the spatial efficiency function of the human eye to determine retinal exposure. In this paper we describe the need for such a measurement device, detail its design, and report on its performance.

Strategies for Optimizing Headlamp Illumination and Visibility Along Curves

This document reports on a field study undertaken to measure peripheral visual performance under various headlamp conditions. The study include conditions that are typical of halogen and high intensity discharge (HID) headlamps and explored functions that could be incorporated in advanced forward-lighting systems (AFS). Targets of varying size were located at different locations along the edges of both left- and right-hand curves, and different headlamp illumination conditions were used. Reaction times and missed targets were measured. The authors found a benefit of increased peripheral illumination commonly found in HID headlamps and with AFS systems on peripheral target detection. These results are consistent with previously published studies. The authors describe how these data could be used to make predictions of peripheral visibility under arbitrary beam patterns as a function of target characteristics and headlamp intensity, and could help identify locations within beam patterns where increased illumination is likely to be beneficial when entering curves, and where it is not likely to benefit. Compared to configurations most closely matching the illuminance profile of halogen headlamps, the configuration with illuminance conditions most closely matching that of HID headlamps improved detection for targets located along a curve. The authors also conclude that once an asymptotic level of visual performance is achieved, there are diminishing returns from the increased illumination that could be gained by including AFS functionality.

Headlight Glare Exposure and Recovery

There is concern that the greater light output and increased beam pattern widths of some headlamp systems may be resulting in higher glare exposures to drivers for longer times. A set of experiments is described that examines how headlamp glare exposure affects recovery time and ratings of discomfort. Theoretical glare exposures were examined to study different aspects of glare, namely peak glare illuminance and total glare dosage. Glare exposures corresponding to representative tungsten halogen (TH) and high intensity discharge (HID) systems were also examined. It was found that the shape of the glare profile had a significant effect on recovery time. A larger dose of glare (product of illuminance and exposure time) results in a longer recovery time. It was also found that discomfort ratings are dependent on glare profile, with greater discomfort being proportional to larger peak illuminances. Surprisingly, no effect of glare duration or dosage was found on discomfort. It was also found that under nominal aiming conditions the glare exposures representing TH and HID systems produced similar recovery times. However, with 1° of misaim upwards, the HID exposure produced significantly longer recovery times.