Nisa Khan M
Appl Opt. 2017 Sep 20;56(27):7539-7548. doi: 10.1364/AO.56.007539.
We present measurement and analysis of color stability over time for two categories of white LED lamps based on their thermal management scheme, which also affects their transient lumen depreciation. We previously reported that lumen depreciation in LED lamps can be minimized by properly designing the heat sink configuration that allows lamps to reach a thermal equilibrium condition quickly. Although it is well known that lumen depreciation degrades color stability of white light since color coordinates vary with total lumen power by definition, quantification and characterization of color shifts based on thermal transient behavior have not been previously reported in literature for LED lamps. Here we provide experimental data and analysis of transient color shifts for two categories of household LED lamps (from a total of six lamps in two categories) and demonstrate that reaching thermal equilibrium more quickly provides better stability for color rendering, color temperature, and less deviation of color coordinates from the Planckian blackbody locus line, which are all very important characterization parameters of color for white light. We report for the first time that a lamp's color degradation from the turn-on time primarily depends on thermal transient behavior of the semiconductor LED chip, which experiences a wavelength shift as well as a decrease in its dominant wavelength peak value with time, which in turn degrades the phosphor conversion. For the first time, we also provide a comprehensive quantitative analysis that differentiates color degradation due to the heat rise in GaN/GaInN LED chips and subsequently the boards these chips are mounted on-from that caused by phosphor heating in a white LED module. Finally, we briefly discuss why there are some inevitable trade-offs between omnidirectionality and color and luminous output stability in current household LED lamps and what will help eliminate these trade-offs in future lamp designs.
我们基于两类白光LED灯的热管理方案，对其随时间的颜色稳定性进行了测量和分析，热管理方案也会影响它们的瞬时光通量衰减。我们之前报道过，通过合理设计散热器配置，使灯具能快速达到热平衡状态，可以将LED灯的光通量衰减降至最低。虽然众所周知，由于颜色坐标根据定义会随总光通量功率变化，光通量衰减会降低白光的颜色稳定性，但基于热瞬态行为对颜色变化的量化和表征在LED灯的相关文献中此前尚未有报道。在此，我们提供了两类家用LED灯（两类共六个灯）的瞬态颜色变化的实验数据和分析，并证明更快达到热平衡能为显色性、色温提供更好的稳定性，且颜色坐标与普朗克黑体轨迹线的偏差更小，这些都是白光颜色的非常重要的表征参数。我们首次报道，灯从开启时刻起的颜色退化主要取决于半导体LED芯片的热瞬态行为，该芯片会随着时间经历波长偏移以及其主波长峰值下降，这进而会降低荧光粉转换效率。我们还首次提供了一项全面的定量分析，区分了由于GaN/GaInN LED芯片以及随后安装这些芯片的电路板发热导致的颜色退化，与白色LED模块中荧光粉发热导致的颜色退化。最后，我们简要讨论了为何当前家用LED灯在全向性与颜色及光输出稳定性之间存在一些不可避免的权衡，以及在未来灯具设计中哪些因素有助于消除这些权衡。
