KEYWORDS: Photobleaching, Optical components, LED lighting, Modeling, Light sources and illumination, 3D printing, Transparency, Transmittance, Time metrology, Process modeling
Photopolymer 3D printing of optically clear resins is a promising technology for producing custom optical elements for general illumination. However, the transparency of the final 3D-printed part may depend on secondary processes. Residual photoinitiator can result in a yellowish tint that can be photobleached after exposure of the 3D-printed part to a light source. The study was designed to understand the tradeoff between the spectral characteristics of the light source used for the photobleaching and the irradiance to which test samples were exposed on the rate of photobleaching. A total of 14 samples were tested at room temperature for 120 minutes under a combination of three light sources (xenon, phosphor converted white LED, and direct emission blue LED), and up to five irradiance levels for each source in the range 0.0025 to 0.2238 W/cm2. The results showed that for the white LED, irradiance can increase the magnitude of the photobleaching. In this study, the maximum chromaticity shift was equivalent to a 4-step MacAdam ellipse. These results seem to indicate that it is possible to expedite photobleaching by increasing the irradiance, although more testing is necessary to find an optimum value. The results for the blue LED tests (peak wavelength 450 nm) showed that this spectrum can be as effective or slightly better at photobleaching than the white LED tested for the same total irradiance. The samples exposed to the xenon light source resulted in increased yellowish tint, presumably because of additional oxidation on the surface of the sample. For these samples irradiated with the xenon lamp, the tint increased with increasing irradiance.
Today, the aluminum heat sink is one of the most expensive and heaviest components in an LED lighting product. Because manufacturers of LED lighting products face the pressure of having to reduce production costs, exploring ways to reduce manufacturing costs by way of using novel materials and manufacturing processes could be a potential solution. 3D-printed polymer-based composite heat sinks with suitable thermal properties and 3Dprinted metal heat sinks could help lighting fixture manufacturers reduce costs. 3D-printed heat sinks have the potential to be customized for increased functionality and visual appeal. This paper presented LED application-specific 3D-printed heat sink thermal performance characterizations. The heat transfer simulations showed that material with effective thermal conductivity values >15 Wm-1K-1 can potentially satisfy the thermal management requirements of a 50 W halogen equivalent MR-16 type LED integral replacement lamp. An experiment study was also conducted to evaluate the effect of Cu-plating of the PA-12 heat sinks. The Cu-electroplated heat sinks maintained an operating temperature below 105°C at the LED module case location for a thermal load equivalent to a 35W halogen equivalent MR-16 type LED integral replacement lamp. The 50μm Cu-electroplating thickness had equivalent performance to a 3D-printing material with an effective thermal conductivity of ~3Wm-1K-1. The 150μm Cu-electroplating thickness showed similar performance to a material with an effective thermal conductivity of ~6Wm-1K-1.
Vat photopolymerization and multi-jet modeling 3D printers using clear polymer resins have shown promise for making optically clear lenses for LED lighting systems. These clear resins are usually polymethyl methacrylate, acrylonitrile butadiene styrene, and polycarbonate-like photopolymers. One of the main requirements for such lenses in LED lighting systems is stable performance, i.e., maintaining transmitted light and chromaticity for an extended period (over 25,000 hours). A long-term aging study was designed and conducted to understand light transmittance properties as a function of time. The 3D-printed lens samples were exposed to elevated ambient temperature (~45 and 60°C) and short-wavelength optical irradiance (~0.20 and 0.4 W/cm²) with peak wavelength radiation ~450 nm and FWHM ~25 nm. Test samples were 3D-printed using three clear transparent resins and using vat photopolymerization and multi-jet modeling processes. The lens samples were removed from the aging setup at regular intervals and the transmittance was measured at room temperature. The measured time to 90% lumen maintenance (L90) and 70% lumen maintenance (L70) were affected more by optical irradiance change from 0.20 W/cm² and 0.4 W/cm² than ambient temperature change from 45°C and 60°C. The vat photopolymerization 3D-printed test samples used for the study showed higher relative transmittance degradation than the multi-jet modeling test samples used in the study for both irradiances and ambient temperatures.
Automated tools for the design of freeform illumination optics have enabled a new class of high-quality, high-efficiency luminaires for general lighting. Additive manufacturing takes this concept to the next level – allowing for completely custom luminaires to be designed and manufactured for very specific use cases. This paper looks at the optical designs created and manufactured for a Department of Energy project exploring the use of additive manufacturing for the lighting market. The subtle nuances of designing freeform optics for additive manufacturing as well as results of optical testing of material and surface quality will be discussed. Finally, comparisons will be provided between the simulated, as-designed optical performance and that of the measured parts.
During the past several years, the interest for 3D printing of lighting optics has been growing rapidly. Most optical prototypes have been 3D printed using transparent photopolymer resin materials. However, the literature has limited information about the optical efficiency and the accuracy of beam shaping of such 3D-printed lenses. Therefore, to better understand the status of 3D printing lenses, a total internal reflection (TIR) lens was designed for use in replacement MR-16 (multifaceted-reflector) LED integral lamps. Several lenses were 3D printed in our laboratory and by two manufacturers. These 3D-printed samples were tested and the results were compared with a commercially available injection-molded TIR lens. The process and results of this benchmarking study are presented in this paper. The goal of this investigation was to study how 3D printer and material combination, build orientation, and post-processing affect the optical performance of LED lamps. The results showed differences in optical efficiency and beam shape for the printed samples. The highest optical efficiency achieved by these prototypes was 75%. The 3D-printed lenses with post-processing had similar performance to the injection-molded lens in terms of optical efficiency and beam width. The results showed that the layer height and print orientation affected the optical performance of the 3D-printed lenses. Our final conclusion is that 3D printing can achieve similar performance to commercially available polymer TIR lenses when suitable print parameters and postprocessing are selected. Further studies are needed to identify the best build orientation and print layer height to minimize the light scattering that affects the lens performance.
This study characterized the thermal conductivity (κ) values of a few commonly available fused-filament fabrication (FFF) type 3D printing materials that have the potential to be used to 3D-print interior architectural wall panels. The materials included polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), and polyethylene terephthalate glycol (PETG). Three infill percentages (20, 60, and 98%) and three infill patterns (grid, zigzag, and honeycomb) were investigated in the study. The characterized thermal conductivity values revealed that these 3Dprinted material sample coupon thermal conductivity values ranged from 0.15 to 0.31 W m¯¹ K¯¹ and were comparable to gypsum plaster, drywall, and hardwood. Generally, lower infill densities (e.g., infill percentage ~20%) contributed to sample coupons with lower thermal conductivity values (e.g., κ~0.15 W m¯¹ K¯¹). Zigzag and honeycomb infill patterns generally showed lower thermal conductivity values (~30 to 60% lower κ-value) than grid-type infill patterns for a given infill density. 3D-printed sample coupons with PLA material indicated higher thermal conductivity values (~10-33% higher κ-value) when compared to ABS and PETG 3D-printed sample coupons. The study results also showed that 3D printing could fabricate components such as interior building panels with desired target thermal conductivity values. The findings also showed that by selecting the combination of 1) material, 2) infill pattern, and 3) infill percentage, constant or localized gradient thermal conductivity values could be engineered that are difficult to achieve with traditional interior building materials.
At present, solid-state light sources are more efficacious than traditional lighting technologies. To provide benefits in the target applications, this efficacy advantage at the light source has to be supplemented by the optical system used in the lighting system. In general, optical systems can be broadly classified as refractive or reflective based on the optical elements used in the lighting system. Usually, these secondary optic elements are made using injection molding (lenses) or casting and subsequent machining and polishing (reflectors) in large-scale productions. This aspect tends to reduce the use of unique or custom optical solutions in practical applications. Additive manufacturing, or 3D printing, has been successfully used to manufacture small- to medium-scale production volumes of customized solutions in other industries. This technology provides an opportunity to manufacture optical components that maximize the efficacy of a target application by creating unique optical components that facilitate the distribution of light in desired directions. In this study, an optical system based on reflective principles was designed to provide a Type V distribution on the target plane. The designed reflector system was 3D-printed and laboratory tested for total light output, intensity distribution, light output distribution, and optical efficiency. The test results were compared with Monte Carlo ray-tracing simulation results.
The abundance of commercial LED lighting fixtures in the marketplace has resulted in price erosion, forcing manufacturers to look for ways to lower manufacturing costs. 3D printing holds promise for providing new solutions that not only can increase the value of lighting but can potentially reduce costs. During the past few years, 3D printing has been successfully adopted in industries such as aerospace, automotive, consumer products, and medical for manufacturing components. For the lighting industry to adopt 3D printing for fabricating light fixtures, it has to show that different subcomponents of an LED light fixture, including thermal, electrical, and optical components, can be successfully made. Typically, optical components are either transmissive or reflective type. In both cases, the component’s optical properties affect fixture efficiency and beam quality. Therefore, the objective of this study was to understand how short-term and long-term optical properties are affected when using 3D printed optical components. In the case of transmissive optics, several optical elements were printed and aged at higher than ambient temperatures and their corresponding spectral transmissions were measured over time. Similarly, several reflective optical elements were printed and characterized for spectral reflectivity as a function of print parameters, including print layer height, print orientation, and the number of print layers before and after aging the parts at higher ambient temperatures. These results are useful for optical component manufacturers to understand the possibilities of using 3D printing to make high-quality optics for lighting fixture applications and for 3D printing material and printer hardware manufacturers to understand the requirements of optics for the illumination applications.
LED lighting systems using Power-over-Ethernet (PoE) technology have been introduced to the lighting market in recent years as a network-connected lighting solution. PoE technology can provide low-voltage direct current (dc) power and control information to LED lighting over a standard Ethernet cable. One of the commonly claimed benefits of the PoEbased lighting system is higher system efficiency compared to traditional line voltage alternating current (ac) systems. This is due to the fact that in the case of PoE systems, the ac-dc power conversion losses are minimized because the acdc power conversion takes place at the PoE switch rather than at all the LED drivers within the lighting fixtures. However, it is well known that power losses can occur as a result of increased voltage drop along the low-voltage cables. The objective of this study was to characterize a PoE lighting system and identify the power losses at the different parts of the system. Based on the findings, we developed a methodology for characterizing the electrical efficiency of a PoEbased LED lighting system and then used this methodology to characterize commercially available PoE-based LED lighting systems and compare their performance. The electrical efficiency characterization included both the system as a whole and each individual component in the systems, such as the power sourcing equipment, powered device, Ethernet cables, and LED driver. The study results also investigated the discrepancy between the measured and reported energy use of the system components.
KEYWORDS: Light emitting diodes, Thermal effects, 3D printing, Copper, 3D modeling, Resistance, Printing, LED lighting, Light sources and illumination, Aluminum
This study investigated the thermal properties of three-dimensional (3-D) printed components with the potential to be used for thermal management in light-emitting diode (LED) applications. Commercially available filament materials with and without a metal filler were characterized with changes to the print orientation. 3-D printed components with an in-plane orientation had >30 % better effective thermal conductivity compared with components printed with a cross-plane orientation. A finite-element analysis was modeled to understand the effective thermal conductivity changes in the 3-D printed components. A simple thermal resistance model was used to estimate the required effective thermal conductivity of the 3-D printed components to be a viable alternative in LED thermal management applications.
KEYWORDS: 3D printing, Printing, Light emitting diodes, Light sources and illumination, Solid state lighting, Additive manufacturing, Manufacturing, LED lighting, Optical components, Solid state electronics
Low energy use and reduced maintenance have made the LED, a solid-state light (SSL) source, the preferred technology for many lighting applications. With the explosion of products in the marketplace and subsequent price erosion, manufacturers are looking for lower cost materials and manufacturing methods. 3-D printing, also known as additive manufacturing, could be a potential solution. Recently, manufacturers in the automotive, aerospace, and medical industries have embraced 3-D printing for manufacturing parts and systems. This could pave the way for the lighting industry to produce lower cost, custom lighting systems that are 3-D printed on-site to achieve on-time and on-demand manufacturing. One unique aspect of LED fixture manufacturing is that it requires thermo-mechanical, electrical, and optical components. The goal of our investigation was to understand if current 3-D printing technologies and materials can be used to manufacture functional thermo-mechanical, electrical, and optical components for SSL fixtures. We printed heat sink components and electrical traces using an FFF-type 3-D printer with different filaments. The results showed that the printed heat sinks achieved higher thermal conductivity values compared to components made with plastic materials. For electrical traces, graphene-infused PLA showed low resistivity but it is much higher than bulk copper resistivity. For optics, SLA-printed optical components showed that print resolution, print orientation, and postprocessing affect light transmission and light scatter properties. Overall, 3-D printing offers an opportunity for mass customization of SSL fixtures and changing architectural lighting practice, but several challenges in terms of process and materials still have to be overcome.
KEYWORDS: LED lighting, Light emitting diodes, Light sources and illumination, Failure analysis, Buildings, Transmission electron microscopy, Resistance, Thermography, Safety, Lead
The concept of connected lighting systems using LED lighting for the creation of intelligent buildings is becoming
attractive to building owners and managers. In this application, the two most important parameters include power
demand and the remaining useful life of the LED fixtures. The first enables energy-efficient buildings and the second
helps building managers schedule maintenance services. The failure of an LED lighting system can be parametric (such
as lumen depreciation) or catastrophic (such as complete cessation of light). Catastrophic failures in LED lighting
systems can create serious consequences in safety critical and emergency applications. Therefore, both failure
mechanisms must be considered and the shorter of the two must be used as the failure time. Furthermore, because of
significant variation between the useful lives of similar products, it is difficult to accurately predict the life of LED
systems. Real-time data gathering and analysis of key operating parameters of LED systems can enable the accurate
estimation of the useful life of a lighting system. This paper demonstrates the use of a data-driven method (Euclidean
distance) to monitor the performance of an LED lighting system and predict its time to failure.
Recently, light-emitting diode (LED) lighting systems have become popular due to their increased system performance.
LED lighting system performance is affected by heat; therefore, it is important to know the temperature of a target
surface or bulk medium in the LED system. In-situ temperature measurements of a surface or bulk medium using
intrusive methods cause measurement errors. Typically, thermocouples are used in these applications to measure the
temperatures of the various components in an LED system. This practice leads to significant errors, specifically when
measuring surfaces with high-luminous exitance.
In the experimental study presented in this paper, an infrared camera was used as an alternative to temperature probes in
measuring LED surfaces with high-luminous exitance. Infrared thermography is a promising method because it does not
respond to the visible radiation spectrum in the range of 0.38 to 0.78 micrometers. Usually, infrared thermography
equipment is designed to operate either in the 3 to 5 micrometer or the 7 to 14 micrometer wavelength bands. To
characterize the LED primary lens, the surface emissivity of the LED phosphor surface, the temperature dependence of
the surface emissivity, the temperature of the target surface compared to the surrounding temperature, the field of view
of the target, and the aim angle to the target surface need to be investigated, because these factors could contribute
towards experimental errors. In this study, the effects of the above-stated parameters on the accuracy of the measured
surface temperature were analyzed and reported.
KEYWORDS: Light emitting diodes, Thermography, Geometrical optics, Temperature metrology, Magnesium, Infrared cameras, Epoxies, Reflectors, Ray tracing, Near field optics
The objective of this study was to understand how optical and thermal performances are impacted in a remote phosphor LED (light-emitting diode) system when the phosphor plate thickness and phosphor concentration change with a fixed amount of a commonly used YAG:Ce phosphor. In the first part of this two-part study, an optical raytracing analysis was carried out to quantify the optical power and the color properties as a function of remote phosphor plate thickness, and a laboratory experiment was conducted to verify the results obtained from the raytracing analysis and also to examine the phosphor temperature variation due to thickness change.
This study investigated the capability of a mathematical model in estimating the phosphor layer heat transfer of an LED system. The focus was on determining the temperature distribution based on light propagation in the phosphor layer. The mathematical model was built upon past work by Kang et al. and solved numerically with heat generation and transfer incorporated into the model. The model light propagation and heat generation was compared with past research and then used to simulate an experimental study in order to evaluate the solution from the present model and compare it with the temperature measurements of the experimental study. The solution to the temperature distribution using the mathematical model had good agreement with the experimentally measured temperature values using an IR thermal imaging camera. Then the model was used to predict the temperature distribution in the phosphor layer under different heat transfer conditions to provide insight that is difficult to observe in experimental studies due to practical limitations.
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.
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