Solid State Lighting Systems for Horticulture: Impact of LED Degradation on Light Spectrum and Intensity

This paper reports on the degradation analysis of high-power light-emitting diodes (LEDs) for horticulture lighting applications. The study is based on the experimental characterization of state-of-the-art monochromatic commercial off-the-shelf LEDs from the same product-line and the same manufacturer. In this work we report the degradation kinetic of red (660 nm) and blue (450 nm) LEDs subjected to accelerated stress for a duration of more than 10’000 hours at three ambient temperatures. We describe how different electrical and optical degradation mechanisms take place with independent timing constant for the two group of LEDs. While red LEDs show a gradual degradation, blue LEDs are initially stable, but later suffer a sharp decrease of optical power due to silicone failure. The results of the degradation experiments are then used to analyze the spectral and intensity variation of red/blue horticulture lamps during their operating life. Finally the spectral and intensity variation of the lamps is compared with results reported in the literature for similar horticulture lighting, in particular the variation of the growth of strawberry and sweet basil is analyzed.