Color shift acceleration on mid-power LED packages
Introduction
Mid-power LED packages, whose electrical power rating ranges from 0.2 to 0.5 W are now widely used in many indoor illumination applications and in place of other high cost devices due to many advantages such as cost-effective, ease for simpler design for the distributed light system [1], [2], [3].
Several studies have been reported on the lumen output and degradation mechanisms of LED packages under different aging conditions [1], [2], [3], [4], [5], [6], [7], [8]. There are also some studies on the color shift mechanisms. Meneghini and Zanoni et al. investigated InGaN/GaN based high brightness LEDs with high temperature or dc or pulsed stress, results shown that thermal treatment can induce a worsening of the chromatic properties of the devices. In most of the cases, white LEDs submitted to stress tests show significant modifications of their spectral characteristics during stress time and, in particular, a decrease in the ratio between the intensity of phosphor-related emission and the intensity of the main blue peak. Consequently, after stress, the chromatic properties of the devices can shift toward a bluish light. The degradation of the chromatic properties is usually ascribed to the browning of the material used for the encapsulation of the phosphors/chip system. The degradation can be also correlated to the partial carbonization of the reflective surface of the package: this effect may introduce a decrease in the efficiency of the light extraction process due to the reduced contributions of the light reflected by the package to the overall emission. Both these processes can modify the spectral content of the light emitted by the LEDs and induce modifications in the relative intensity of the blue and yellow emission peaks [9], [10], [11], [12], [13], [14].
Davis et al. [15], [16], [17], [18], [19] examined chromaticity shift modes of the PAR38 lamps with four types of built-in LED packages, results shown the chromaticity shift is dominated by the characteristics of the LED packages. There are mainly four potential chromaticity-shift directions of a light source: blue shift, yellow shift, green shift and red shift. Huang et al. studied the color shift caused by the yellowing of package encapsulate for mid-power white light LED packages in the outdoor illumination applications with high humidity and high temperature (WHTOL) [20].
However, research on the color shift and mechanisms of mid-power LED packages for indoor illumination applications, especially for the color shift acceleration and prediction, is less published. To improve the light efficacy and miniature the package size for ease of secondary optical design as well as cost efficiency in many indoor applications, mid-power LED packages have reduced thermal and optical design compared to high-power LEDs: usually a Ag coated layer covering plastic housing is used but lack of dedicated thermal pad and optical element, which increased sensitivity of this type of LED packages toward environmental temperature. These characters will possibly induce a different degradation in color shift from other types. Since the color is important in some applications, although there is only one industry standard to define the color shift limit [21], [22], [23], it is important to predict the color shift for LED package designers and manufacturers.
The aim of this report is to investigate the color shift modes and mechanisms of mid-power LED packages caused by different acceleration stresses and propose an acceleration and prediction method for color maintenance. Temperature stress, humidity stress and current stress were experimentally designed and performed to accelerate the color shift of mid-power LED packages, and color shift mechanisms are discussed based on the color shift results obtained from measurements. A prediction method is proposed.
The experimental results and the proposed prediction method will be helpful in the mid-power LED package design and application, and also helpful in the color shift investigation and acceleration for the luminaire level products in which this class of LED packages is used.
The remainder of this paper is organized as follows: Section 2 presents the materials and methods for the experiment set-up and testing. Section 3 provides results and discussions on the designed experiment. Finally, concluding remarks are presented in Section 4.
Section snippets
Materials and methods
Samples used in the investigation are widely used mid-power LED packages, 5630 with dimension 5.6 × 3.0 × 0.96 mm3, from a leading manufacturer in the industry. LED packages were mounted on metal core based plates as shown in Fig. 1.
Samples were divided into 3 different groups, and each group was subjected to a different aging condition, 25 °C (room temperature), 85 °C, or 85 °C & 85%RH. 25 °C is used here as a reference temperature to simulate the daily use condition, 85 °C is a temperature lift of 60 °C
Results for the subgroup with loading current I = 75 mA
Chromaticity coordinates data was collected, u′ and v′ (loading current I = 75 mA) were normalized to their initial color points and shown in Fig. 3(a) and (b), respectively. Linear function model fitting demonstrates R2 (a scalar measure of model significance) is greater than 80% for all the aging conditions, which reveals a good linear relationship between color shifts (Δu′, Δv′) and aging time. Negative slope reveals a trend of decrease or degradation both in u′ and v′ under all the aging
Conclusions
Temperature stress, humidity stress and current stress were experimentally designed and performed to accelerate the color shift of mid-power LED packages and color shift mechanisms have been discussed based on the color shift results obtained from measurements. Conclusions could be drawn as below.
- 1)
Linear function fitting demonstrates a good linear relationship between color shift (Δu′, Δv′) and aging time almost for all the aging conditions. According to the investigation of SPDs, both blue
Acknowledgments
The work described in this paper was partially supported by the National High Technology Research and Development Program of China (863 Program) (No. 2015AA03A101). This work has also been accomplished within EMRP JRP ENG62 MESaIL which was carried out with funding by the European Union. Authors would like to give thanks to test group from Changzhou base of State Key Lab of Solid State Lighting for aging process support.
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