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.
Specification-grade lighting jobs often require product customization. Custom products are expensive to manufacture because it involves engineering costs, changes in tooling, sourcing of special parts, modifications to manufacturing schedules, and often require low volume runs. Beyond the known benefits to redesign, prototype, test, and validate product change orders, emerging 3D printing technologies and materials offer different solutions to lighting manufacturers for functional parts. This presentation will describe potential scenarios where 3D printing can be used to provide the flexibility and speed-to-market needed by manufacturers of custom, high-end architectural lighting products to remain competitive.
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.
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.
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.
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.
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.
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.
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.
The organic light-emitting diode (OLED) is an area light source, and its primary competing technology is the edge-lit light-emitting diode (LED) panel. Both technologies are similar in shape and appearance, but there is little understanding of how people perceive discomfort glare (DG) from area sources. The objective of this study was to evaluate the DG of these two technologies under similar operating conditions. Additionally, two existing DG models were compared to evaluate the correlation between predicted values and observed values. In an earlier study, we found no statistically significant difference in human response in terms of DG between OLED and edge-lit LED panels when the two sources produced the same luminous stimulus. The range of testing stimulus was expanded to test different panel luminances at three background illuminations. The results showed no difference in perceived glare between the panels, and, as the background illumination increased, the perceived glare decreased. In other words, both appeared equally glary beyond a certain luminance and background illumination. We then compared two existing glare models with the observed values and found that one model showed a good estimation of how humans perceive DG. That model was further modified to increase its power.
Solid-state lighting (SSL) offers a new technology platform for lighting designers and end-users to illuminate spaces
with low energy demand. Two types of SSL sources include organic light-emitting diodes (OLEDs) and light-emitting
diodes (LEDs). OLED is an area light source, and its primary competing technology is the edge-lit LED panel. Generally,
both of these technologies are considered similar in shape and appearance, but there is little understanding of how people
perceive discomfort glare from large area light sources. The objective of this study was to evaluate discomfort glare for
the two lighting technologies under similar operating conditions by gathering observers’ reactions. The human factors
study results showed no statistically significant difference in human response to discomfort glare between OLED and
edge-lit LED panels when the two light sources produced the same lighting stimulus. This means both technologies
appeared equally glary beyond a certain luminance.
KEYWORDS: Light emitting diodes, Light sources and illumination, LED lighting, Light sources, Headlamps, Light, Halogens, Solid state lighting, Eye, CIE 1931 color space
In the past decade, there has been increased interest in energy-efficient lighting as energy resources become higher in
demand. Street lighting and outdoor lighting are applications that are rapidly changing from the incumbent high-pressure
sodium (HPS) to newer technologies such as light-emitting diode (LED) or induction-type lamps. There is evidence that
certain populations believe LED streetlights and area lights to produce more glare than HPS luminaires. A number of
differences exist between new and traditional light sources besides efficiency. These include spectral power distribution
(SPD), source luminance, beam intensity distribution, and the number of sources needed to achieve intended light levels.
Many field studies and laboratory studies have shown a relationship between glare and SPD, with most studies
suggesting that sources more weighted in short wavelengths have an increased likelihood of discomfort glare. A study to
assess the effect of different SPDs on perception of discomfort glare was conducted. Subjects were shown a white-light
LED array against a luminous background with one of three different SPDs (blue, white, or yellow). As well, different
intensities of light from the array and from the background were used. For the range of conditions evaluated, the
presence of any luminous background significantly reduced the perception of discomfort glare from the LED array. The
blue background reduced perception significantly less than the white or the yellow backgrounds. The implications for
solid-state lighting systems such as outdoor array lighting are discussed.
Lighting plays an important role in supporting retail operations, from attracting customers, to enabling the evaluation of
merchandise, to facilitating the completion of the sale. Lighting also contributes to the identity, comfort, and visual
quality of a retail store. With the increasing availability and quality of white LEDs, retail lighting specifiers are now
considering LED lighting in stores. The color rendering of light sources is a key factor in supporting retail lighting goals
and thus influences a light source's acceptance by users and specifiers. However, there is limited information on what
consumers' color preferences are, and metrics used to describe the color properties of light sources often are equivocal
and fail to predict preference. The color rendering of light sources is described in the industry solely by the color
rendering index (CRI), which is only indirectly related to human perception. CRI is intended to characterize the
appearance of objects illuminated by the source and is increasingly being challenged because new sources are being
developed with increasingly exotic spectral power distributions. This paper discusses how CRI might be augmented to
better use it in support of the design objectives for retail merchandising. The proposed guidelines include the use of
gamut area index as a complementary metric to CRI for assuring good color rendering.
The light-emitting diode (LED) is a rapidly evolving, energy-efficient light source technology that holds promise to
address the increasing need for energy conservation. However, the common belief that a high-efficacy light source
equates to lower energy demand in application is incorrect. Generally, when a new light source technology replaces an
existing light source in an application and claims energy savings, the inherent assumption is that all of the requirements
of the application are met. In the case of directional lighting applications, what matters ultimately is the amount of
luminous flux illuminating the task area. Therefore, when quantifying the performance of a luminaire, ideally one must
consider only the amount of flux reaching the task area and the total power demanded.
The objective of this paper is to introduce an alternative concept, application efficacy. This paper will demonstrate the
concept's usefulness and proposed metrics for three different lighting applications-under-cabinet task lighting,
refrigerated display case lighting, and outdoor parking lot lighting-and show how it better relates to energy demand.
Details of laboratory experiments and software analysis along with data are presented for the three applications.
Light-emitting diode (LED) technology is presently targeted to displace traditional light sources in backlighted signage.
The literature shows that brightness and contrast are perhaps the two most important elements of a sign that determine its
attention-getting capabilities and its legibility. Presently, there are no luminance standards for signage, and the practice
of developing brighter signs to compete with signs in adjacent businesses is becoming more commonplace. Sign
luminances in such cases may far exceed what people usually need for identifying and reading a sign. Furthermore, the
practice of higher sign luminance than needed has many negative consequences, including higher energy use and light
pollution.
To move toward development of a recommendation for lighted signage, several laboratory human factors evaluations
were conducted. A scale model of a storefront was used to present human subjects with a typical red channel-letter sign
at luminances ranging from 8 cd/m2 to 1512 cd/m2 under four background luminances typical of nighttime outdoor and
daytime inside-mall conditions (1, 100, 300, 1000 cd/m2), from three scaled viewing distances (30, 60, 340 ft), and either
in isolation or adjacent to two similar signs. Subjects rated the brightness, acceptability, and ease of reading of the test
sign for each combination of sign and background luminances and scaled viewing distances.
A field study was conducted at three clothing stores to validate previous laboratory findings indicating that colored
LEDs used as background display lighting could: 1) lower the power demand of accent lighting by up to 50 percent; and
2) provide greater attention capture and visual appeal than current lighting practice.
Blue LEDs provided a colored background for window mannequins by illuminating white backdrops. Eliminating
fluorescent general lighting and reducing the number and wattage of halogen accent lamps in the display windows
reduced the lighting power demand by up to 50 percent. During an eight-week period, more than 700 shoppers rated the
attractiveness, eye-catching ability, comfort, and visibility of four different lighting conditions. The results of this field
study showed that by introducing color contrast between the displayed objects and the background, the power demand of
the accent lighting could be reduced by up to 50 percent without sacrificing visual appeal, visibility, ability to capture the
attention of shoppers, and the ability to see the colors of the objects on display. Furthermore, the sales of the products on
display were not affected by the 50 percent reduction in lighting.
Two life tests were conducted to compare the effects of drive current and ambient temperature on the degradation rate of 5 mm and high-flux white LEDs. Tests of 5 mm white LED arrays showed that junction temperature increases produced by drive current had a greater effect on the rate of light output degradation than junction temperature increases from ambient heat. A preliminary test of high-flux white LEDs showed the opposite effect, with junction temperature increases from ambient heat leading to a faster depreciation. However, a second life test is necessary to verify this finding. The dissimilarity in temperature effect among 5 mm and high-flux LEDs is likely caused by packaging differences between the two device types.
This paper outlines two parts of a study designed to evaluate the use of light-emitting diodes (LEDs) in channel-letter signs. The first part of the study evaluated the system performance of red LED signs and white LED signs against reference neon and cold-cathode signs. The results show a large difference between the actual performance and potential savings from red and white LEDs. Depending on the configuration, a red LED sign could use 20% to 60% less power than a neon sign at the same light output. The light output of the brightest white LED sign tested was 15% lower than the cold-cathode reference, but its power was 53% higher. It appears from this study that the most efficient white LED system is still 40% less efficient than the cold-cathode system tested. One area that offers a great potential for further energy savings is the acrylic diffuser of the signs. The acrylic diffusers measured absorb between 60% and 66% of the light output produced by the sign. Qualitative factors are also known to play an important role in signage systems. One of the largest issues with any new lighting technology is its acceptance by the end user. Consistency of light output and color among LEDs, even from the same manufacturing batch, and over time, are two of the major issues that also could affect the advantages of LEDs for signage applications. To evaluate different signage products and to identify the suitability of LEDs for this application, it is important to establish a criterion for brightness uniformity. Building upon this information, the second part of the study used human factors evaluations to determine a brightness-uniformity criterion for channel-letter signs. The results show that the contrast modulation between bright and dark areas within a sign seems to elicit the strongest effect on how people perceive uniformity. A strong monotonic relationship between modulation and acceptability was found in this evaluation. The effect of contrast seems to be stronger than that of spatial frequency or background luminance, particularly for contrast modulation values of less than 0.20 or greater than 0.60. A sign with luminance variations of less than 20% would be accepted by at least 80% of the population in any given context.
Light-emitting diode (LED) technology is becoming the choice for many lighting applications that require monochromatic light. However, one potential problem with LED-based lighting systems is uneven luminance patterns. Having a uniform luminance distribution is more important in some applications. One example where LEDs are becoming a viable alternative and luminance uniformity is an important criterion is backlighted monochromatic signage. The question is how much uniformity is required for these applications. Presently, there is no accepted metric that quantifies luminance uniformity. A recent publication proposed a method based on digital image analysis to quantify beam quality of reflectorized halogen lamps. To be able to employ such a technique to analyze colored beams generated by LED systems, it is necessary to have contrast sensitivity functions (CSFs) for monochromatic light produced by LEDs. Several factors including the luminance, visual field size, and spectral power distribution of the light affect the CSFs. Although CSFs exist for a variety of light sources at visual fields ranging from 2 degrees to 20 degrees, CSFs do not exist for red, green, and blue light produced by high-brightness LEDs at 2-degree and 10-degree visual fields and at luminances typical for backlighted signage. Therefore, the goal of the study was to develop a family of CSFs for 2-degree and 10-degree visual fields illuminated by narrow-band LEDs at typical luminances seen in backlighted signs. The details of the experiment and the results are presented in this manuscript.
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