M. Yazdan Mehr

Accelerated life time testing and optical degradation of remote phosphor plates

In this investigation the thermal stability and life time of remote phosphor encapsulant plates, made from bisphenol-A polycarbonate (BPA-PC), are studied. Remote phosphor plates, combined with a blue-light LED source, could be used to produce white light with a correlated colour temperature (CCT) of 4000 K. Spectral power distribution (SPD) and photometric parameters of thermally-aged phosphor plates were measured by Integrated-Sphere. Results show that thermal ageing leads to a significant decrease in the luminous flux and chromatic properties of plates. The photometric properties of thermally-aged plates, monitored during the stress thermal ageing tests, showed a significant change both in the correlated colour temperature (CCT) and in the chromaticity coordinates (CIE x, y). It is also observed that there is a significant decay both in the phosphor yellow emission and in the blue peak intensity. The decrease in the luminous flux is strongly correlated to the deterioration of the chromatic properties of the phosphor plates. The results also show a significant decay of CCT, postulating that the degradation of the remote phosphor plates affects the efficiency of light and the colour of emitted light as well. The decrease of CCT takes place with almost the same kinetics as the lumen depreciation.

Reliability and Lifetime Prediction of Remote Phosphor Plates in Solid-State Lighting Applications Using Accelerated Degradation Testing

A methodology, based on accelerated degradation testing, is developed to predict the lifetime of remote phosphor plates used in solid-state lighting (SSL) applications. Both thermal stress and light intensity are used to accelerate degradation reaction in remote phosphor plates. A reliability model, based on the Eyring relationship, is also developed in which both acceleration factors (light intensity and temperature) are incorporated. Results show that the developed methodology leads to a significant decay of the luminous flux, correlated colour temperature (CCT) and chromatic properties of phosphor plates within a practically reasonable period of time. The combination of developed acceleration testing and a generalized Eyring equation-based reliability model is a very promising methodology which can be applied in the SSL industry.

Colour shift in remote phosphor based LED products

In this paper, the thermal stability and the colour shifting of remote phosphor plates, made from Bisphenol-A polycarbonate (BPA-PC), are studied. In this study, the remote-phosphor and lens of BPA-PC samples of 3 mm thickness were thermally aged at temperature range 100 to 140 oC. Results show that thermal ageing leads to a significant decrease in the luminous flux and chromatic properties of plates. Lumen depreciation up to 30% reduction is extrapolated to temperatures lower than 100 °C. It is shown that the lifetime, defined as 30% lumen depreciation at 40 °C, is around 35 khrs. It is also shown that by increasing the temperature, the reaction rate becomes faster, inferring that lumen depreciation takes place at shorter time. Results also illustrate the colour shifting of white light towards yellow region.

Reliability and Lifetime Assessment of Optical Materials in LED-Based Products

Lumen depreciation is one of the major failure modes in light-emitting diode (LED) systems. It originates from the degradation of the different components within the system, including the chip, the driver, and the optical materials (i.e., phosphorous layer). The kinetics of degradation in real-life applications is relatively slow, and in most cases, it takes several years to see an obvious deterioration of optical properties. A highly accelerated stress testing (HAST) setup and a methodology to extrapolate the results to real applications are therefore needed to test the reliability of LED package and lens materials. Employing HAST concept in LED industry is inevitable due to the necessity of assessing the reliability of new products in a short period of time. This chapter aims at briefly explaining the degradation mechanism of optical components in LED package and how they contribute to the lumen depreciation of the LED package. The concept of HAST and the way the reliability of LED packages can be assessed will also be explained.

A review on discoloration and high accelerated testing of optical materials in LED based-products

Reduction of intensity of light output is one of the most common degradation modes in light-emitting diode (LED) systems. It starts from the failure of the various components in the system, including the chip, the driver, and optical components (i.e. phosphorous layer). The kinetics of degradation in real life applications is relatively slow and in most cases it takes several years to see an obvious deterioration of optical properties. Highly Accelerated Stress Testing (HAST) set-up and a methodology to extrapolate the results to real time applications are therefore needed to test the reliability of LED packages and lens materials. Using HAST concept in LED industry is inevitable due to the necessity of assessing the reliability of new products in a short period of time. This paper aims at briefly clarifying the degradation mechanisms of optical components in LED packages and explaining how they contribute to the depreciation of light output of the LED systems. The concept of HAST and the way the reliability of LED packages can be evaluated will also be discussed.

A POF based breakdown method for LED lighting color shift reliability

Color shift mechanism of LED lighting is complex due to its much more comprehensive structure, generally composed of LED die, phosphor, silicone, reflector, diffusers, and so on, each of them may contribute to the color shift during operation. This paper proposed a breakdown method for the color shift study and investigation on the LED based luminaire level products. For the downlight investigated in this paper, the LED packages part has the most important contribution in color shift, higher than 1sdcm even under the normal operation conditions of room temperature. Both humidity and temperature conditions can be used to accelerate the color shift of the mid power 5630 package and the humidity has a stronger acceleration. The reflective part contributes far less than 1 sdcm even when the reflective material crumbles after 4000hours of humidity test under certain conditions.