This PDF file contains the front matter associated with SPIE Proceedings Volume 8835, including the Title Page, Copyright Information, Table of Contents, and the Conference Committee listing.

There is a need in the lighting industry to design and implement accelerated aging methods that accurately simulate the aging process of LED luminaire components. In response to this need, we have built a flexible and reliable system to study the aging characteristics of optical polymer materials, and we have employed it to study a commercially available LED luminaire diffuser made of PMMA. The experimental system consists of a “Blue LED Emitter” and a working surface. Both the temperatures of the samples and the optical powers of the LEDs are appropriately characterized in the system. Several accelerated aging experiments are carried out at different temperatures and optical powers over a 90 hour period and the measured transmission values are used as inputs to a degradation model derived using plausibility arguments. This model seems capable of predicting the behavior of the material as a function of time, temperature and optical power. The model satisfactorily predicts the measured transmission values of diffusers aged in luminaires at two different times and thus can be used to make application recommendations for this material. Specifically, at 35000 hours (the manufacturer’s stated life of the luminaire) and at the typical operational temperature of the diffuser, the model predicts a transmission loss of only a few percent over the original transmission of the material at 450 nm, which renders this material suitable for this application.

Generally in a white light-emitting diode (LED), a phosphor slurry is placed around the semiconductor chip or the phosphor is conformally coated over the chip to covert the narrowband, short-wavelength radiation to a broadband white light. Over the past few years, the remote-phosphor method has provided significant improvement in overall system efficiency by reducing the photons absorbed by the LED chip and reducing the phosphor quenching effects. However, increased light output and smaller light engine requirements are causing high radiant energy density on the remotephosphor plates, thus heating the phosphor layer. The phosphor layer temperature rise increases when the phosphor material conversion efficiency decreases. Phosphor layer heating can negatively affect performance in terms of luminous efficacy, color shift, and life. In such cases, the performance of remote-phosphor LED lighting systems can be improved by suitable thermal management to reduce the temperature of the phosphor layer. To verify this hypothesis and to understand the factors that influence the reduction in temperature, a phosphor layer was embedded in a perforated metal heatsink to remove the heat; the parameters that influence the effectiveness of heat extraction were then studied. These parameters included the heatsink-to-phosphor layer interface area and the thermal conductivity of the heatsink. The temperature of the remote-phosphor surface was measured using IR thermography. The results showed that when the heat conduction area of the heatsink increased, the phosphor layer temperature decreased, but at the same time the overall light output of the remote phosphor light engine used in this study decreased due to light absorption by the metal areas.

Radiant power emitted by high power light-emitting diodes (LEDs) have been steadily increasing over the past decade. High radiation, especially short wavelength, can increase the temperature and negatively affect the primary lens performance of high-power LEDs. In this regards, assessment of lens temperature during operation is important. Past studies have shown large errors when thermocouples are used for measuring temperature in high radiant flux environments. Therefore, the objective of this study was to understand the problem in using thermocouples to measure LED lens surface temperature and to find a solution to improving the measurement accuracy. A laboratory study was conducted to better understand the issue. Results showed that most of the error is due to absorption of visible radiant energy by the thermocouple. In this study, the measurements made using an infrared (IR) thermal imaging system were used as the reference temperature because the IR imaging system is unaffected by radiant flux in the visible range. After studying the thermocouple wire metallurgy and its radiation absorption properties, a suitable material was identified to shield the thermocouple from visible radiation. Additionally, a silicone elastomer was used to maintain the thermal interface between the lens surface and the thermocouple junction bead. With these precautions, the lens temperature measurements made using the J-type thermocouple and the IR imaging system matched very well.

LED light sources having multiple independently controllable color channels allow tuning of both the intensity and color output. Consequently, highly tailored lighting can be applied according to instantaneous user needs and preferences. Besides improving lighting performance, energy use can also be reduced since the brightest illumination is applied only when necessary. In an example application, low activity or vacant areas of a multi-zone office are lit by low power illumination, including colored light options, which can reduce energy consumption to 20-45% of typical full-time, fullbrightness, office-wide illumination. The availability of color also allows communication functions and additional aesthetic design possibilities. To reduce user burden in frequent switching between various illumination settings, an activity recognition sensor network is used to identify selected office activities. The illumination is then adjusted automatically to satisfy the needs of the occupants. A handheld mobile device provides an interactive interface for gathering user feedback regarding impressions and illumination preferences. The activity-triggered queries collect contemporaneous feedback that reduces reliance on memory; immediate previews of illumination options are also provided. Through mobile queries and post-experience interviews, user feedback was gathered regarding automation, colored lighting, and illumination preferences. Overall reaction was indicated by a range of response words such as fun, stimulating, very cool, very pleasant, enjoyed, good, comfortable, satisfactory, fine, energy saving, interesting, curious, dim, cave, isolated, distracting, and unfamiliar. Positive reaction from a meaningful, though not universal, fraction of users indicates reasonable application potential, particularly as personal preferences and control are accommodated.

The color rendition engine based on the statistical metric allows us to uniquely quantify the characteristics of color quality of illumination and assess the color rendition preferences. We now report on using the color rendition engine for revealing individual and cultural differences in color quality preferences of 205 American and Chinese subjects. Our study demonstrated that the majority of individuals preferred the color blend with the same statistical figures of merit on the average but with a much larger spread of blends for Americans. For both groups, the color rendition preferences depended on the object being illuminated. This was demonstrated by illuminating a set of common colored objects and three different paintings. We conclude that the color quality of lighting can be optimized and enhanced using the feedback to change the spectral power distribution of the illuminating source depending on the object being illuminated and on the preferences of an individual observer and a cultural group.

Optical design requires accurate characterization of light sources for computer aided design (CAD) software. Various methods have been used to model sources, from accurate physical models to measurement of light output. It has become common practice for designers to include measured source data for design simulations. Typically, a measured source will contain rays which sample the output distribution of the source. The ray data must then be exported to various formats suitable for import into optical analysis or design software. Source manufacturers are also making measurements of their products and supplying CAD models along with ray data sets for designers. The increasing availability of data has been beneficial to the design community but has caused a large expansion in storage needs for the source manufacturers since each software program uses a unique format to describe the source distribution. In 2012, the Illuminating Engineering Society (IES) formed a working group to understand the data requirements for ray data and recommend a standard file format. The working group included representatives from software companies supplying the analysis and design tools, source measurement companies providing metrology, source manufacturers creating the data and users from the design community. Within one year the working group proposed a file format which was recently approved by the IES for publication as TM-25. This paper will discuss the process used to define the proposed format, highlight some of the significant decisions leading to the format and list the data to be included in the first version of the standard.

A novel LED recycling scheme using double prisms is presented. Two identical triangular prisms with square bases, one cross-stacked on top of the other, are tight-fit into a mirrored light tunnel. The whole prism/light tunnel assembly is then mounted on top of a square LED source, whose emitting area is the same as that of the base plane of the said prism/light tunnel assembly. Each prism acts as a tapered-down light guide in one dimension, which selectively retro-reflects high angle light along that direction. The outer light tunnel serves as a mirrored wall that folds back any light that escapes outside the two prisms. For a given collection cone angle, the height of the two prisms is optimized using ASAP, a commercial ray-tracing software. Simulation and experimental results show promise in significantly increasing the brightness of the LED sources within the collection cone. Specifically for a 4x recycling ratio a 70% recycling gain in center illuminance has been achieved (i.e., illuminance measured in the forward direction). This scheme has advantages over previous recycling configurations due to its compactness and ease of mounting. For example, compared to Wavien's spherical reflector approach that has been previously published, the current recycling configuration is much smaller in size because instead of fitting a much larger mirrored reflector on top of the LED source, this time we're using a structure that has the same lateral dimensions as those of the LED source itself. Further improvement is also possible if optimization of various system parameters is carried out.

In the contribution we discuss the relationship between etendue and luminous flux of gobo projectors and its implications on illumination design and system architecture. On the basis of a given projector specification, a suitable LED type can be chosen and the number of needed LEDs and the necessary system etendue can be calculated. In a next step, we show how to design a suitable illumination system with primary and secondary optics and possibly including a homogenizer. Furthermore, implications for the design of the projection lens are discussed.

This study investigates the optical design of LED edge-lit curved light bar and applies to the design of automotive taillight. LEDs have been widely used in automotive lighting. However, the designs are mostly of direct backlight using arrays of LEDs with diffusive patterns above, which often causes problems such as low uniformity, glaring, and excess LEDs. Edge-lit light guide devices have been widely used in the back light models of LCD. However, the geometry of the lighting devices in the automotive lighting is often curved and non-rectangular, and the related literatures are very limited. This study addresses the design of edge-lit curved light bars and the optimization scheme for uniform light emitting. An automotive taillight with LED edge-lit light bar is used as an illustrated example. The light tracing software, TracePro is used to simulate the optical characteristics of the proposed design. 90° V-cuts are used as the optical features to distribute the light, and the lead angles of the V-cut are varied to achieve the optimum axial luminous intensity. A Fuzzy optimization scheme is proposed to manipulate the anchor spacing points which continuously varies the V-cut spacing along the light bar to satisfy the requirements of Society of Automotive Engineers (SAE) regulations and the illumination uniformity requirements. This study also proposes a solution to reduce the illuminance difference between the inner and the outer portions of the curved section of a light bar by varying bilaterial thickness of light bar cross section.

In this paper, we investigated the optical output characteristic of the light guide plate (LGP) with microstructures engraved by a CO2 laser, which is for edge-lit backlight of liquid crystal display or illumination. Especially, the laser-engraving method is suitable for the slim large-sized LGP used in LED TV backlight, and the engraved microstructure has polished surface to contribute to better optical efficiency. For seeking higher optical efficiency, we adopted a LGP with lenticular-arrayed surface (LAS LGP) as the experimental substrate. In order to investigate the effect of the lenticular-arrayed surface on optical output characteristic for the different kinds of the engraved microstructures, we used the laser to directly engrave the flat surface (opposite to the lenticular-arrayed surface) of the LAS LGP with different kinds of microstructures as experimental samples; each sample has one kind of the engraved microstructures on its flat surface. Similarly, we also engraved the same kinds of the microstructures on the flat LGP as a controlled group for comparison. The cross-section profiles of the engraved microstructures on both LAS LGP and flat LGP were measured by a laser confocal microscope. All the samples were further measured for spatial and angular luminance by the BM7 and Conoscope, respectively. In addition, both the experimental data and simulation results were demonstrated and compared to each other in this paper.

Sub-micron diffraction gratings have been used for two LED illumination applications. One is to create a transparent see through luminaire which can be used to illuminate and read a paper document or e-book. A second is a light sensor that can be used in a feedback loop to control a multicolor LED lamp. Optical design and experimental proof-of-principle are presented.

For a systematic approach to improve the white light quality of phosphor converted light-emitting diodes (LEDs) for general lighting applications it is imperative to get the individual sources of error for correlated color temperature (CCT) reproducibility and maintenance under control. In this regard, it is of essential importance to understand how geometrical, optical and thermal properties of the color conversion elements (CCE), which typically consist of phosphor particles embedded in a transparent matrix material, affect the constancy of a desired CCT value. In this contribution we use an LED assembly consisting of an LED die mounted on a printed circuit board by chip-on-board technology and a CCE with a glob-top configuration on the top of it as a model system and discuss the impact of the CCE shape and size on CCT constancy with respect to substrate reflectivity and thermal load of the CCEs. From these studies, some general conclusions for improved glob-top design can be drawn.

The emission-adaptive phosphor layer was fabricated by self-exposure method to realize the pc-white LED. The effect of phosphor layer forming parameters such as phosphor content, exposure intensity and exposure time on the phosphor layer shape and LED characteristics was investigated. Through the fabrication process of phosphor layer, two types of phosphor layer shape were fabricated by phosphor layer forming parameters. In the case of low exposure intensity and/or short exposure time with high phosphor content, the conformal coating shape phosphor layer which is imitative of the irradiance pattern of an LED chip was formed, whereas in the high exposure intensity and/or long exposure time with low phosphor content, the light distribution proportional shape phosphor layer which reflects the near-filed pattern of an LED chip was formed. Therefore the latter was superior to the former in the angular color homogeneity due to a strong similarity between the phosphor content distribution and the light intensity distribution of an LED chip in all directions. It was concluded that the object-oriented emission-adaptive phosphor layer closely similar to the light distribution proportional shape phosphor layer was formed in the condition of high exposure intensity and/or long exposure time with high phosphor content and resulted in high luminous efficacy, low correlated color temperature as well as high angular color homogeneity.

Although solid-state lighting (SSL) products are often intended to have product lifetimes of 15 years or more, the rapid change in technology has created a need for accelerated life tests (ALTs) that can be performed in the span of several months. A critical element of interpreting results from any systems-level ALT is understanding of the impact of the test environment on each component. Because of its ubiquity in electronics, the use of temperature-humidity environments as potential ALTs for SSL luminaires was investigated. Results from testing of populations of three commercial 6” downlights in environments of 85°C and 85% relative humidity (RH) and 75°C and 75% RH are reported. These test environments were found to accelerate lumen depreciation of the entire luminaire optical system, including LEDs, lenses, and reflectors. The effects of aging were found to depend strongly on both the optical materials that were used and the design of the luminaire; this shows that the lumen maintenance behavior of SSL luminaires must be addressed at the optical systems level. Temperature-Humidity ALTs can be a useful test in understand lumainaire depreciation provided that proper consideration is given to the different aging rates of various materials. Since the impact of the temperature-humidity environment varies among components of the optical system, uniform aging of all system components in a single test is difficult to achieve.

Solid-state lighting (SSL) luminaires containing light emitting diodes (LEDs) have the potential of seeing excessive temperatures during operation or during transportation and storage. Presently, the TM-21 test standard is used to predict the L70 life of SSL Luminaires from LM-80 test data. The underlying TM-21 Arrhenius Model is based on population averages, may not capture the failure physics in presence of multiple failure mechanisms, and does not predict the chromaticity shift. In this paper, Kalman Filter (KF) and Extended Kalman Filters (EKF) have been used to develop models for 70-percent Lumen Maintenance Life Prediction and chromaticity shift for a LEDs used in SSL luminaires. Ten-thousand hour LM-80 test data for various LEDs have been used for model development.

Color temperature constancy and color temperature maintenance are key issues in the context of the utilization of light-emitting diodes (LEDs) for general lighting applications. For a systematic improvement, it is imperative to understand how compositional, optical and thermal properties of the color conversion elements (CCE), which typically consist of phosphor particles embedded in a transparent matrix material, affect the constancy of a desired color temperature of a white LED source under operation. In particular, thermal stress, like a distinct thermal load of the CCEs under operation may also cause notable color shifts. In order to gain a better understanding of the thermal behavior of CCEs under operation, in this contribution we give by means of a combined optical and thermal simulation procedure a comprehensive discussion on the impact of different CCE shapes and sizes on their thermal responses.

Light flicker is a common but unwelcome phenomenon in conventional lighting applications. In solid-state lighting, driving or dimming methods also give rise to light flicker. AC LED products in today’s marketplace suffer from flicker, which stems from the arrangement of the micro-LEDs and the driving method. Research has shown that light flicker can be a health hazard to humans. Several solutions have been proposed to reduce light flicker in solid-state lighting applications; however, most have drawbacks in terms of power and other performance. This paper proposes a circuit design to reduce light flicker from AC LEDs while maintaining a normal power factor and high power efficiency. The circuit is composed of one resistive branch and one capacitive branch, and each branch drives a load which is made up of high-voltage LEDs. Percent flicker, power factor, and power efficiency were selected as three metrics, and their benchmarks were set to evaluate the performance of this circuit. Phase shift between the two branches was selected as a factor that could determine the circuit performance. The variations of percent flicker, power factor, and power efficiency as a function of phase shift were identified by theoretical analysis and were verified by experiments. The experimental results show that an optimal solution can be achieved for this circuit design at proper phase shift, where the benchmarks of the three metrics are reached.

The experimental excitations for luminescent glasses are usually monochromatic, but LED chips in applications have an emission bandwidth. In order to investigate the luminescence properties of rare earth ions doped glasses excited by broadband lights, a computational model was presented based on the dependences of excitation wavelengths on emission spectra, chromaticity coordinates and correlated color temperatures (CCTs). The simulations were carried out applying Ce/Tb/Eu co-doped calcium borosilicate glasses as examples. The results show that for the same CCTs, the center wavelengths of chips are different with the excitation wavelengths of fluorescence spectrophotometers.

Rare earth ions doped luminescence glasses are promising phosphor candidates in the fabrication of LEDs in the future due to their specific advantages such as higher thermal stability, higher transparency, compared to current commercial LEDs. While radiation patterns of luminescent glasses are different from current commercial LEDs fabricated by phosphors, luminescent glasses play roles both in emitting light and adjusting light distribution. In order to investigate radiation pattern of luminescent glasses, luminescence physical model of flat glasses doped with single rare earth ion was presented. Process of photons acting on rare earth ions and transporting in the luminescent glasses, and output light distribution from luminescent glasses were analyzed based on violet LED. At last, the simulating of radiation pattern for luminescent glasses based on Monte Carlo ray-tracing method was proved by experiment.

A complete adjustable and automated system has been developed on the base of three blocks: the first one is the mechanical and adjustable structure, which is composed by a mobile base and a rectangular arc, where the photodetector is located. The structure describes semi-spherical trajectories by means of two servomotors. Its lightweight and robustness constitutes two advantages which make it portable. The second stage corresponds to the automation of the structure movements. For controlling the motors and data acquisition, a microcontroller is employed that in turns helps keeping the cost of the overall system low. One of the servomotors is located on the lateral axis of the device; that displaces the sensor along a semi-circular trajectory of 160°, almost half meridians; the other one is located at the base of the illumination source, which enables it to realize an almost complete rotation around its axis. Finally, the last stage is formed by the graphical interface. The communication protocol between the data acquisition stage and the computer is USART. The graphical user interface (GUI) is developed using Visual C#. In the same window the data acquisition deployment and the data graph generator are given; the graphs can be shown in a polar or Cartesian formats showing more than one curve, if necessary, avoiding the use of additional software. The GUI keeps the low cost of the device, obtaining a comprehensive solution to generate the irradiance patterns of light sources.

For energy-saving, high efficiency and low pollution, the lighting of LED systems is important for the future of green energy technology industry. The solid state lighting becomes the replacement of traditional lighting, such as, light bulbs and compact fluorescent lamps. Because of the semiconductor characteristics, the luminous efficiency of LEDs is sensitive to the operating temperature. Besides increasing the luminous efficiency, effective controlling electricity and thermal characteristics in the design of LED lighting products is the key point to achieve the best results. LED modules can be combined with multi-grain process or through a combination of multiple LED chips. Accurate analysis of this LED module for the electrical, thermal characteristics and high reliability is the critical knowledge of modular design. In this report, we studied the electrical and thermal coupling phenomenon in solid state lighting systems to analyze their reliability. By experiments and simulations, we obtained the apparent variation of temperature distribution of LED system due to differences of their forward voltages and thermal resistances. These events may reduce their reliability. Besides, the evaluation of optical and chromatic properties was based on the variation of temperature distribution and current of LED system. This is the key technology to predict the optical and chromatic properties of LED system in use.

In recent years, with extensive use of InGaN LED, estimation of LED quality and improvement of LED reliability has become very important. In this report, the noise spectrum measurement techniques were used to estimate the reliability of InGaN LED devices and compare its reliability with its ESD tolerance test result. Experimental results show that the noise spectrum measurement more effectively distinguishes the LED device reliability than that of the current voltage curve measurement. EMMI, SEM and TEM images show that noise source and cause of failure of the LED device are attributed to poor quality of the SiO₂ and ITO interface.

One of the main advantages of multi-die array light-emitting diodes (LEDs) is their high flux density. However, a challenge for using such a product in lighting fixture applications is the heat density and the need for thermal management to keep the junction temperatures of all the dies low for long-term reliable performance. Ten multi-die LED array samples for each product from four different manufacturers were subjected to lumen maintenance testing (as described in IES-LM-80-08), and their resulting lumen depreciation and failure modes were studied. The products were tested at the maximum case (or pin) temperature reported by the respective manufacturer by appropriately powering the LEDs. In addition, three samples for each product from two different manufacturers were subjected to rapid thermal cycling, and the resulting lumen depreciation and failure modes were studied. The results showed that the exponential lumen decay model using long-term lumen maintenance data as recommended in IES TM-21 does not fit for all package types. The failure of a string of dies and single die failure in a string were observed in some of the packages.