Comparison of the Changes in Luminescence Properties Between Cool White and Warm White LEDs at Varying Temperatures

Abstract

The changes of luminescence properties in cool white (CW) and warm white (WW) light-emitting diodes (LEDs) in response to operating temperatures are compared in this paper. In addition, the emission spectrum of blue LED chips was characterized under the same experimental conditions for cross-reference analysis. The yellow YAG (Y3Al5O12:Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ) and red nitrididosilicate (Sr2Si5N8:Eu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ) phosphors were synthesized in-house and packed at different concentration configurations to create LED samples of correlated color temperature 6500 K for CW LEDs and 2700 K for WW LEDs. The emission spectrum measurements showed drastic decrements of light emission intensities for both CW and WW LED samples at elevated temperature. Both CW and WW LED samples tend to have more bluish hue with increasing temperatures. Furthermore, the trend of color rendering index (CRI) changes of CW and WW LEDs has differed, a finding that is not commonly reported in other studies. The CRI of CW LED samples increased, whereas the CRI of WW LED samples decreased at elevated temperature. This is due to different shift directions of peak emission wavelength in yellow Y3Al5O12:Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> and red Sr2Si5N8:Eu <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> phosphors.