Yimin Gu

Life of LED-based white light sources

Even though light-emitting diodes (LEDs) may have a very long life, poorly designed LED lighting systems can experience a short life. Because heat at the p-n-junction is one of the main factors that affect the life of the LED, by knowing the relationship between life and heat, LED system manufacturers can design and build long-lasting systems. In this study, several white LEDs from the same manufacturer were subjected to life tests at different ambient temperatures. The exponential decay of light output as a function of time provided a convenient method to rapidly estimate life by data extrapolation. The life of these LEDs decreases in an exponential manner with increasing temperature. In a second experiment, several high-power white LEDs from different manufacturers were life-tested under similar conditions. Results show that the different products have significantly different life values.

A noncontact method for determining junction temperature of phosphor-converted white LEDs

The goal of this study was to develop a non-contact method for determining the junction temperature of phosphor-converted white LEDs as a first step toward determining the useful life of systems using white LEDs. System manufacturers generally quote the same life values for their lighting systems that the LED manufacturers estimate for a single LED. However, the life of an LED system can be much different compared with the life of an LED tested under ideal conditions because system packaging can affect system life. Heat at the pn-junction is one of the key factors that affect the degradation rate, and thus the useful life, of GaN-based white LEDs. The non-contact method described in this manuscript, combined with LED degradation rates, can be used to predict white-LED system life without affecting the integrity of the lighting system or submitting it to long-term life tests that are time-consuming. Different types of LED packages would have different degradation mechanisms. Therefore, as a first step this study considered only the 5mm epoxy encapsulated GaN+YAG Cerium phosphor white LED. The method investigated here explored whether the spectral power distribution (SPD) of the white LED could provide the necessary information to estimate LED junction temperature. Based on past studies that have shown that heat affects the radiant energy emitted by the InGaN blue LED and the YAG Cerium phosphor differently, the authors hypothesized that the ratio of the total radiant energy (W) to the radiant energy within the blue emission (B) would be proportional to the junction temperature. Experiments conducted in this study verified this hypothesis and showed that the junction temperature can be measured non-invasively through spectral measurements.

Performance characteristics of high-power light-emitting diodes

A laboratory experiment was conducted to investigate the performance characteristics of the currently available high-power LEDs under various drive conditions and ambient temperatures. Light output degradation and color shift properties as a function of time were measured for five types of commercial high-flux LEDs, namely, red, green, blue, and white from one manufacturer, and a different high-flux white LED package from a second manufacturer. The major difference between the two manufacturers’ products is that the first uses a single LED die per package, and the second uses multiple dies within its package. LED arrays were tested under normal drive current and ambient temperature, normal drive current and higher ambient temperature, and higher drive current and normal ambient temperature. Because each LED type has to operate at a particular ambient temperature, all were tested in specially designed individual life-test chambers. These test chambers had two functions: one, to keep the ambient temperature constant, and two, to act as light-integrating boxes for measuring light output parameters. Overall, the single-die green and white LED arrays showed very little light loss after 2,000 hours, even though the current and the ambient temperature were increased. However, the red LED array seemed to have a high degradation rate. The white LEDs had a significant color variation (of the order of a 12-step MacAdam ellipse) between them. However, the color shift over time was very small during the initial 2,000-hour period. For white LEDs to be accepted broadly for general illumination applications, the color variation between similar products must become much smaller, of the order of a 2-step MacAdam ellipse.

Spectral and luminous efficacy change of high-power LEDs under different dimming methods

Dimming is an important and necessary feature for light sources used in general lighting applications. An experimental study was conducted to quantify the spectral and luminous efficacy change of high-power colored and pc-white LEDs under continuous current reduction (CCR) and pulse-width modulation (PWM) dimming schemes. For InGaN-based blue, green, and pc-white LEDs, the peak wavelength shifts were in opposite directions for the two dimming schemes. The peak wavelength showed a blue shift with increased current, most likely due to band filling and QCSE dominated effects. InGaN LEDs exhibited red shifts with increased duty cycle, which is dominated by junction heat. AlInGaP red LEDs show mainly thermal-induced red shift with increased current or duty cycle. In addition, the luminous efficacy was always higher for the CCR dimming scheme at dimmed levels, irrespective of the LED type. Keywords: Light-emitting diodes (LEDs), white LEDs, mixed-color white LEDs, pulse-width modulation (PWM), continuous current reduction (CCR), peak wavelength shift, luminous efficacy

Characterization of thermal resistance coefficient of high-power LEDs

Heat at the junction of light-emitting diodes (LED) affects the overall performance of the LED in terms of light output, spectrum, and life. Usually it is difficult to measure junction temperature of a LED directly. There are several techniques for estimating LED junction temperature. One-dimensional heat transfer analysis is one of the most popular methods for estimating the junction temperature. However, this method requires accurate knowledge of the thermal resistance coefficient from the junction to the board or pin. An experimental study was conducted to investigate what factors affected the thermal resistance coefficient from the junction to the board of high-power LED. Results showed that the thermal resistance coefficient changed as a function of ambient temperature, power dissipation at the junction, the amount of heat sink attached to the LED, and the orientation of the LED with the heat sink. This creates a challenge for using onedimensional heat transfer analysis to estimate junction temperature of LEDs once incorporated into a lighting system. However, it was observed that junction temperature and board temperature maintains a linear relationship if the power dissipation at the junction is held constant.

Investigation of the optical properties of YAG:Ce phosphor

Recently, many studies have used optical ray-tracing analysis to investigate novel concepts of phosphor-converted white LEDs. Even though optical ray-tracing is a convenient tool, the accuracy of the results depends very much on the optical properties of the various components within the package used in the analysis. Presently, light transmission, reflection, and absorption properties of white LED phosphors are not very well quantified. Therefore, a laboratory study was conducted to quantify at different wavelengths of light the optical properties of a medium that has YAG:Ce phosphor mixed into epoxy. When short-wavelength radiation (blue light) strikes the epoxy-phosphor medium, some portion of the blue light is converted to longer wavelength radiation (yellow light). At a phosphor density suitable for creating a balanced white light, the amount of back-transferred and forward-transferred light, including blue and yellow light, are 53% and 47%, respectively. At a similar phosphor density, when green and red radiant energies strike the epoxyphosphor medium, most of the energy is not converted by the YAG:Ce phosphor because it is beyond the phosphors excitation region. In this case, nearly equal amounts of green and red radiant energy are transferred in the backward and forward directions. To demonstrate the usefulness of the results obtained in this study, an optical ray-tracing analysis of a remote phosphor white LED package was conducted. This analysis showed that the surface finish of the reflector cup of a reflective type remote phosphor white LED package does not affect extraction efficiency.

Improving the performance of mixed-color white LED systems by using scattered photon extraction technique

One of the methods for creating white light with light-emitting diodes (LEDs) is mixing radiations from several different colored LEDs. Mixed-color LEDs are expected to have greater luminous efficacy because they do not undergo down-conversion losses like phosphor-converted white LEDs. However, in reality mixed-color LED systems require extra optical elements to reduce spatial color variation and create uniform white light. Optical diffusing techniques commonly used for these purposes cause light loss, because some portion of the light is scattered back toward the LEDs where it is absorbed and lost. In 2004, a technique known as scattered photon extraction (SPE) was used to extract backscattered light from the phosphor layer of phosphor-converted white LEDs to increase overall light output. In this study, it was hypothesized that by using similar SPE optics with optical diffusers, the spatial color uniformity of mixed-color white LED systems could be improved without sacrificing the overall luminous efficiency. With this new approach, both a microsphere-doped diffuser and SPE optics were utilized. The experiments showed that the proposed setup did increase the spatial color uniformity of the mixed-color LED system with more than 79% overall optical efficiency. This study also demonstrated the effects of microsphere size, concentration, and diffuser thickness on spatial color uniformity and optical efficiency.

Estimating junction temperature of high-flux white LEDs

Performance of white light LEDs has improved significantly over the past few years. White LEDs are typically created by incorporating a layer of phosphor over the GaN-based blue emitter. Heat at the p-n junction seems to be the major factor that influences light output degradation in these devices. In an earlier paper, the principal authors of this manuscript demonstrated that the junction temperature of white LEDs could be measured from the (W/B) ratio, where W represents the total radiant energy of the white LED spectrum, and B represents the radiant energy within the blue emission peak. In that earlier study, the concept was verified using commercially available 5-mm type white LEDs. The goal of the study presented here was to evaluate whether the (W/B) ratio could be used to estimate junction temperature of new high-flux white LEDs. The results show that (W/B) ratio is proportional to the junction temperature of the high-flux white LED; however, the proportionality constants are different for the different white LED types.

White LED performance

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.

Design and evaluation of an LED-based light fixture

Interior spaces, such as conference rooms, require multiple forms of lighting to meet different task needs. Linear fluorescent lamp fixtures commonly produce the general ambient light, and reflectorized halogen lamp fixtures produce the directional lighting. Current lighting practice uses multiple light source technologies and fixtures to achieve the required illumination for the various tasks. However, many light fixtures can create an unappealing architectural design, especially in a small space, and multiple light source technologies can cause maintenance difficulties. The ideal scenario would be to create one fixture with a single type of light source that could meet a variety of lighting needs. The size, potential energy savings, and reduced maintenance benefits of white light-emitting diodes (LEDs) make them attractive for use in general illumination applications. Therefore, the goal of this study was to investigate whether a light fixture could be designed with white LEDs to provide different beam distributions. A commercially available ray tracing package was utilized to model, analyze, and optimize light fixture concepts created using LEDs and light guides. The fixtures were optimized for light output distribution and efficiency. While a rectangular-shaped light guide with simple diffused reflective surfaces provided the necessary cosine beam distribution, a more sophisticated surface treatment was required on the light extracting surface of the light guide to create the batwing beam distributions. The surface finish structures were in the form of micro-prism arrays. By switching LED arrays on and off to different light guides, various light levels and distributions could be achieved for different occasions. Further analysis showed that, when used in a typical conference room set up the fixture produced acceptable uniformity over the surfaces. Although the system provided the necessary beam distributions, the system efficiency was quite low, only 48%. Most of the light loss occurred in creating the batwing distribution. The overall system efficiency must be greater than 80% for this type of a fixture to become a viable solution.