Using Luminescence to Reduce Ac LED Flicker hite alternating current (ac) LEDs operate Results directly off ac electrical mains rather than The experimental results showed that when  Wusing an external driver to direct current 80 (dc) as with traditional dc LEDs. With the elimination increases, the fl icker index of the resultant white of the driver, ac LEDs have the potential advantages decreases, with fl icker index extrapolated to of greater effi ciency, lower cost, and better reliability, reach 0.1 at 80 equal to 0.4 ms (see Figure 1). as well as a smaller system footprint. However, ac Based on the experiment and human factors results, LEDs suffer from more frequent and perceptible LRC researchers developed a set of luminescence fl icker than their dc counterparts, which could lead properties for slow-decay that can create a to undesirable light qualities. Some traditional white ac LED with minimal fl icker. This study showed light sources exhibit perceptible light fl icker, and that the mathematical model developed can reliably one method employed to counteract it is to add an predict the behavior of a slow-decay phosphor, external electronic circuit; however, such solutions which can be useful for scientists developing new often negate the ac LED’s advantages. Recent phosphors. The study showed that with suitable studies have reported that the luminescence emitted characteristics, slow decay phosphors can reduce from slow-decay phosphors, when applied to an ac fl icker from ac LEDs and create white light with good LED, can reduce the appearance of fl icker, but their quality. The human factors experiments verifi ed the mechanism and luminescence property requirements potential of slow-decay phosphors to improve the have remained unknown. acceptability of white ac LED light output. For values

greater than 0.22, 80 is acceptable. Experiments 0.5 0.4

In this study, LRC researchers investigated the 0.4 0.38

0.36

luminescence of slow-decay phosphors under CIE y 0.3 fl ickering excitation from an ac LED. A mathematical 0.34 0.32 model was then developed to predict the behavior 0.2 0.32 0.34 0.36 0.38 0.4 CIE x

of slow-decay phosphors, which considered the Flicker Index 0.1 Fl = 0.4135*exp(-3.598*τ80 ) following factors that affect the ac LED’s properties: R² = 0.9977 • the 80% decay constant of the luminescence ( ) 0 80 0 0.1 0.2 0.3 0.4 0.5 • the extraction efficiency of excitation light in the τ₈₀ (ms) phosphor layer (η ) ex Figure 1. Experiment results: fl icker index with chromaticity fi xed on the • the extraction efficiency of emission light in the blackbody locus, as in the inset. phosphor layer (η ) em 2

Acc = 7.3631*ττ80 ² + 5.6914* 80 – 1.6254 R² = 0.9989 1

More Information 0

Tan J and Narendran N. 2015. Defi ning phosphor -1 luminescence property requirements for white ac

Acceptability Rating LED fl icker reduction. Journal of Luminescence 167: 21–26; doi: 10.1016/j.jlumin.2015.05.064. -2 0 0.1 0.2 0.3 0.4 0.5 τ₈₀ (ms) Sponsor Figure 2. Acceptability ratings of stroboscopic effects under white conditions with different 80 values (+2: very acceptable; -2: very unacceptable). Federal Aviation Administration, research grant 13G009-2013

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