Color shift analysis and modeling of high power warm white pc-LED under high temperature and high humidity environment

This paper focuses on the color stability degradation analysis and modeling for the commercially available high power warm white phosphor converted LED (pc-LED) packages under both high temperature and high humidity conditions. During the experiment, 3 kinds of different high power pc-LEDs were subjected to both high temperature, and high relative humidity (85°C/85%RH, 75°C/75%RH, 65°C/90%RH) accelerated life test with a 350mA bias current. Experimental results show that the phosphor binder layer of the pc-LEDs can swell and the color stability of LEDs degrades significantly. High temperature and humidity will oxidize the phosphor particles and oxidized phosphor particles shift the phosphor emission spectrum to the lower energy wavelength. The redistribution and oxidation of phosphor particles inside the phosphor binder will cause dramatically degradation of colorimetric properties of LED package in a very short time. Also, a closed form color shift model is built to project the color shift distance forward and prognosticate the color stability of LED packages. In the proposed model, not only temperature and humidity are included, but also the package characteristics, such as phosphor particles diameter, phosphor binder thickness. Compared with the experimental data, it is found that the color shift model can predict the color shift distance of the LED packages in an acceptable range.