March 29, 2012 at 7:50 pm #133592Elsa HuxleyMember
Click here to view the recording of today’s webinar: http://www.connectingtocollections.org/recording-community-webinar-introduction-to-led-lighting/
March 29, 2012 at 8:45 pm #133600Samantha SpringerMember
I have a follow up question about the LED strips for in-case illumination. Do these also require air flow to keep the temp below 130 degrees C to prolong the life of the bulbs? Or is the heat only around the transformer?
March 29, 2012 at 9:28 pm #133599Jenny Wiley ArenaMember
Hi Samantha, I asked Rick Kerschner your question and he replied:
Yes, LED strips need good ventilation as well. The heat is localized
around the LED chip, not the transformer. We learned this the hard way.
In the first generation LIA strips you may remember from the picture
that the through-hole LED’s were quite close together, in fact only 1/2″
apart. They began to dim significantly dropping to only about 30% of
original light output in less than two years. It was because the heat in
this particular brand of LEDs was not managed well at all and the heat
from one LED bled over to the next LED. Fortunately, the manufacturer
replaced them under warrantee. We also had LIA’s that were spaced 1 and
2 inches apart and they held their intensity much better.
You may remember that the next generation RIA strip produced about 2
years later contains fewer LEDs placed further apart. Although they are
brighter, they manage the heat better and have maintained their output
through the past 5 years.
March 29, 2012 at 10:39 pm #133598Dale KronkrightMember
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!
March 30, 2012 at 1:35 pm #133597Terri FoxMember
I recently completed the renovation of my museums entire lighting grid with both retrofits and the new LSI all in one LED fixture that Rick spoke of. During my many tests of the various retrofit bulbs, one of the bulbs I tested not only had the plastic casing fins start to melt, but it also began to ooze phosphor out of the back of the bulb. Having had many conversations with LSI and reading Druzik and Michalski’s Guidelines I was already aware of the color shifts and deterioration of light quality that would happen if the phosphor started to degrade. This particular bulb had been installed in an open back gimbal style fixture with plenty of air circulation. The melting of the casing happened over a period of two months. Be very careful using your existing can style fixtures with your LED retrofits and for now test everything.
March 30, 2012 at 1:56 pm #133596Richard KerschnerParticipant
Thanks to both Dale and Terri for commenting on the mechanism behind possible color shifts. I have a question for each. Dale, you mention how high-energy UV activates the phosphors. Do they completely absorb the UV or does some escape? The SPDCs show very little if any UV light omitted, so it must be absorbed. The LIA and RIA in-case fixtures that we used also were covered with a polycarbonate lens that would filter UV. I have always questioned whether the zero UV measured was a result of the LED’s simply not generating any UV or that any UV generated was filtered out by a covering lens. Perhaps with the white LEDs it is some of both?
Terri, did you observe a significant color shift over the two months that your rogue LED deteriorated?
March 30, 2012 at 2:03 pm #133595Richard KerschnerParticipant
Whoops – UV light emitted, not omitted.
March 30, 2012 at 4:47 pm #133594Terri FoxMember
The color shift was minimal in that particular bulb, but yes, especially after only a couple of months. I believe this was more noticeable because I was testing it near the Xicato. I have to be honest and say that I did not check on this everyday. But I did go from thinking that I had finally found the perfect light color match to completely unacceptable.
March 30, 2012 at 5:51 pm #133593Dale KronkrightMember
Great comments from Terri and questions from Rick. Rick, you are correct in deducing that the UV emissions are completely absorbed both by the phosphor powders and by the phosphor array enclosure and collimating lens, which are organic epoxide/acrylic. Additional protection comes from additional driver architecture. UV emitting diodes used in industry and security have to overcome their architecture to emit any long-wave UV. Note that there are no short wave UV LED’s. There have always been IR emitting LED’s, which is where the technology started, and these used in touch-screen technology, today. With the advent of new diode electron receptor materials, possible emission wavelengths got shorter and shorter. But because current is still flowing and because there is resistance to moving electrons across the anode of the diode, 15 watts of heat (infra-red) is still 15 watts of heat and that heat has to be dissipated somewhere – hence all the heat-sinks and fins in an effort to get it away from the phosphors. Using more expensive heat-stable phosphors is the option higher-priced lamps use. My guess is that most successful manufacturers will shift in this direction over the next 5-years.
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