Re: Introduction to LED Lighting Recording Is Up!

#133598

While Rick mentioned the effect of heat on the diode and the resulting acceleration of diode failure, there is an additional heat-related problem to LED’s. The diode at the heart of a multi-phosphor (white)LED’s (the ones that Rick has demonstrated) emit UV and near UV light, not visible light. The high-energy UV light photons are then absorbed by phosphor powders, on or over the diode, which, just like a fluorescent lamp, emit light in the human visible light spectrum. To get the broad spectral distributions of light like the ones Rick had used to illustrate today’s talk -those spectral power distribution graphs – the lamp must have multiple phosphors that emit light over several overlapping spectral regions. Together, they look “white”. These phosphors are often VERY heat sensitive and in the presence of heat near their reaction thresholds, the spectral emissions, or color of the light, deteriorate. That means that the color of the light produced by a lamp can change over time. Manufacturers specification sheets and warranties provide data for the 70% threshold of the total intensity of the lamp over time BUT NOT THE ABILITY OF THE LAMP TO MAINTAIN THE SAME COLOR OVER THE LIFE OF THE LAMP. The Xicato source marketed by LSI is the only manufacturer I have found so far that will warranty the color output of the lamp from lamp to lamp and over the life of the diode. This is because their phosphor array is separated from the diode surface and the phosphor powder mix is computer controlled to give exact color output by measuring the SPD of each diode as it is manufactured. Now, incandescent sources also color-shift slightly over their 3000 avrg life, but your aren’t spending $65 on a halogen MR-16, either. Color consistency from lamp to lamp and color consistency over the life of the LED has been a real problem form many manufacturers. Figure 7 in Druzik and Michalski’s “Guidelines for Selecting Solid¬‐State Lighting for Museums” shows that some manufacturer’s products can stay within ANSI specifications but some cannot. Don’t be disappointed by a less-expensive LED lighting source that looks different from lamp to lamp or shifts colors on your wall form lamp to lamp over very short periods of time. Those products ARE on your hardware department shelves!