New concerns regarding the unwanted effects of lighting our nighttime environment (i.e., light pollution), and new efforts to design energy efficient roadway lighting installations, are changing the requirements of roadway lighting. These factors necessitate the need for different luminous intensity distributions with greater optical control than have traditionally been produced by roadway lighting fixtures. New requirements affecting the optical design of roadway lighting are presented. An example of a nonimaging optical design solution that addresses these requirements is given. Conclusions are then made about the challenges that lie ahead in the optical design of roadway lighting fixtures.
An analytical model of flux propagation in light pipes, termed the flux confinement diagram (FCD), is further developed and applied. The construction of the FCD is reviewed. Non-planar surface geometries, non-constant cross-sectional geometries, and non-rectangular cross sectional geometries are examined with the FCD. It is shown that in the limit of a circular cross section the predictions of the FCD match the theory for large core fibers. Additionally, the angular propagation space defined by the FCD is explored further to describe the redistribution of propagating flux after a sudden change in geometry, such as a bend. This analysis is used to explore total internal reflection (TIR) at light pipe output surfaces. The implications of analytical modeling with the FCD on light pipe design are discussed.
An analytical model of light propagation in rectangular light pipes, termed the flux confinement diagram (FCD), is developed. Based on the edge ray concept of nonimaging optics, the FCD is a construction that describes the angular distribution of flux propagating in a light pipe depending on a light pipe's index of refraction and its geometry. With the FCD model, the angular "mode" of a ray can be defined at any plane in the system. The FCD model is developed here and used to describe flux input coupling, transport, and output coupling in light-pipe illumination systems. The practical example of a flux loss prediction due to a geometrical change (a bend) is examined using the FCD and compared to a ray-tracing analysis. We discuss how this model of flux propagation is a useful tool to aid in "first-order" design and layout of light-pipe illumination systems.
An analytical model of light propagation in rectangular light pipes is presented. Light pipe illumination systems are an efficient means of collecting, transporting, and distributing light. One area where light pipe illumination systems are successfully employed is in transportation display lighting, such as instrument panel illumination. In these applications the transportation industry takes advantage of injection molding to manufacture light pipe systems at relatively low costs. One historical drawback to using light pipe illumination systems is the design effort associated with iterative prototyping cycles and evaluation.
The model presented here is a graphical method of describing ray propagation in light pipes. The model describes ray direction vector space in spherical coordinates. With this model, the angular "mode" of a ray can be defined at any plane in the system. The angular mode propagation space describes input coupling, flux transport, and output coupling in light pipe illumination systems. The model of flux propagation described here is therefore a tool to aid in "first order" design and layout of light pipe illumination systems.
This study investigates illuminators composed of light emitting diode (LED) array sources and side-emitting light guides to provide efficient general illumination. Specifically, new geometries are explored to increase the efficiency of current systems while maintaining desired light distribution. LED technology is already successfully applied in many illumination applications, such as traffic signals and liquid crystal display (LCD) backlighting. It provides energy-efficient, small-package, long-life, and color-adjustable illumination. However, the use of LEDs in general illumination is still in its early stages. Current side-emitting systems typically use a light guide with light sources at one end, an end-cap surface at the other end, and light releasing sidewalls. This geometry introduces efficiency loss that can be as high as 40%. The illuminators analyzed in this study use LED array sources along the longitude of a light guide to increase the system efficiency. These new geometries also provide the freedom of elongating the system without sacrificing system efficiency. In addition, alternative geometries can be used to create white light with monochromatic LED sources. As concluded by this study, the side-emitting illuminators using LED sources gives the possibility of an efficient, distribution-controllable linear lighting system.
KEYWORDS: Optical components, Light sources, Light emitting diodes, Waveguides, Computer simulations, Monte Carlo methods, Ray tracing, Light sources and illumination, Optical simulations, RGB color model
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.
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.
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 LED's 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.
The design and optimization of a retinal exposure detector for measuring the total amount of light entering the human eye and falling on the retina is presented. The retinal exposure device was designed by first determining the spatial efficiency function of the human eye system. This is accomplished by combining the spatial response function of the average human eye with a standard facial cutoff function. The eye's spatial efficiency function is ascertained through ray trace analysis with optical modeling software of published theoretical and biometric wide-angle eye models. All major factors affecting light propagation, such as volume attenuation, the gradient index of the lens, aspheric surface curvatures, partial reflection, and vignetting are included in the simulation. A practical metering device that mimics the calculated total eye system spatial response function was designed and optimized by again employing optical simulation software. The final design consisted of baffles, a lens system, a decentered aperture, an optical diffuser, optical filters, and a silicon photodiode. The response of the prototype retinal exposure detector device design was shown to match that of the theoretical eye response to within three percent.
We discuss our experimental measurement and theoretical modeling of flux distributions from light sources used in waveguide illumination systems. We have constructed a computer-controlled goniometric detection system to map the intensity distributions of these light sources. As an example, we measure the intensity distribution of an incandescent light bulb. This light bulb is used in the waveguide illumination system of an automobile dashboard. The intensity distribution of this light bulb is not uniform. We present a model for this intensity distribution based on radiometric principles and the shape of the light bulb filament. This model describes the light bulb's intensity distribution as a function of the filament's projected area as seen from the detector.