-----Original Message----- From: E [mailto:drnano] Sent: Wednesday, August 25, 2004 12:27 AM To: wolfstone Subject: UV LEDs... Hello, I have been looking through some of your site pages -- very enjoyable. I would like to offer some comments on UV LEDs. They don't last very long. I have some Vaseline glass items and thought that one day I would like to illuminate them with UV. I had not gone very far with the idea because of the difficulty of doing it with fluorescent bulbs. So imagine my glee when I saw that UV LEDs were finally available. I had seen wooden bases with a bright white LED for sale on ebay, so I contacted the seller/manufacturer and asked him what he thought about selling some with UV LEDs for illuminating Vaseline glass doo-dads. He said that he would, but had not found any reasonably priced UV LEDs with a long enough useful life to make a commercial product. Hmmmmm. Well, I guessed that I would just have to find out for myself... So I ordered some blank bases and ordered some LEDs from several eBay sellers. Several sellers touted "long life" and quoted 100,000 hours of life. So maybe there WAS something to the limited life thing, after all. The LEDs came from China (HK) and were the typical 5mm clear plastic (epoxy) encapsulated items like most other LEDs. As soon as each batch arrived, I tested each LED in the batch and noted the output. Then I took 1 LED from each batch and connected it to a regulated wall wart through a resistor to run the LED at exactly 20 mA, the recommended operating current. My intent was to determine if and just how fast the LEDs deteriorated, while illuminating a couple of vaseline glass items. When the bases arrived, I installed 9 LEDs from one batch in a 3 X 3 grid in one base and set my baseball-sized chunk of Vaseline glass on top of it. WOW! It lit up the whole room. By now, the test LEDs had been on 24/7 for two weeks and the output of ALL of them was down 30%. At three weeks, they were down 50%! At six weeks the output was down 70%. At that point, the deterioration leveled off and continued a slow decline that reached 90% after about three months... where it has stayed ever since -- almost 2 years. It is very curious: To the eye, they are not much dimmer than they were when new. But as you (and others) state, the majority of the output is invisible. So, I guess that the decline in output is mostly in the UV area and the small amount of visible violet is about the same. The Vaseline glass doo-dads hardly glow at all any more. But there is still enough UV coming out of them to cause my corneas to fluoresced a little from 30 feet away in the dark. (This fluorescence is what causes you vision to go cloudy when black light falls on your eyes.) So what is going bad in the LEDs? Either the output of the actual chip is diminishing, or the epoxy encapsulent has broken down and now absorbs most of the UV. Or both. In the past, some small argon glow bulbs were rated at 1000 hours to half brightness because the short-wave UV that was absorbed by the glass of the bulb had changed the glass to the point that it began to block the long-wave UV, too. Now, there ARE some very expensive (and dangerous) UV LEDs that are mounted in a TO-39 metal can with a saphire lens on top. I imagine that they last quite a while. But they are not what we really want to play around with. Every time (what seems to be) a new seller pops up, I buy some UV LEDs from him in hopes that they will actually have a long useful life. And every time, the new LEDs poop out in a couple of months. As far as I can tell, ALL the UV LEDs for sale out of the East may well be from the same manufacturer, who has not corrected the problem. OR... it is a problem with the way that everyone makes them. So, for what it is worth, ALL of the UV LEDs that I have bought have lasted only a couple of months. I have resigned myself to replacing the LEDs in the wooden base every few months. So if you buy some with the intent of using them year after year in your props and effects, you may be in for a disappointment. Keep up the good work on your haunts and your site. Leigh Ellert -----Original Message----- From: E [mailto:drnano] Sent: Wednesday, September 08, 2004 12:39 AM To: wolfstone Subject: RE: UV LEDs... After writing to you, I revisited my test LEDs. There are seven different LEDs, each from a different ebay seller. The last one I bought was about a year ago, the first about two years ago. Every one is down to about 5% of its original absolute output, although some are making a lot more UV than the others. I say absolute output because the test setup I use does not normally distinguish between invisible UV and visible light. I use a black monocrystalline satellite-grade solar cell about one inch square connected to an digital milliammeter. The solar cell converts pretty much all photon energy from far infrared to deep UV to an electrical current that is roughly proportional to the amount of light that is falling on it. No doubt there are peaks and valleys in the response curve of the solar cell and variances in the conversion efficiency versus color, but it seems to be fairly linear and works well when comparing oranges to oranges. To measure the output of an LED, I run it from a regulated power supply and choose a resistor that allows exactly the Recommended Operating Condition current -- usually 20 mA. I let it run for a day or so and then measure its output with the solar cell, changing the angle and position of the LED relative to the solar cell to produce the highest reading on the meter. I note the current reading on the meter and date (and seller) on an Avery address label attached to the LED wire (flag style). Ever so often, I measure the output again and note it and the date on the label. As I mentioned before, this measurement does not distinguish between the visible deep blue and invisible UV that the LED is producing. Most LEDs produce light that is mostly one color, called the peak wavelength. All of the cheap UV LEDs I bought were advertised to have peak wavelength of around 400 nm to 395 nm. (UVA is generally considered to encompass wavelengths between 315 nm and 400 nm.) But, as with most things in the world, the reality of the LED emission looks like a standard distribution (bell-shaped) curve. Most of the energy is at the peak wavelength, but there is decreasing output on both sides of the peak. Some LEDs have a very sharp peak, but most of these cheap UV LEDs seem to have a sloppy, wide emission curve. What this means, as far as UV LEDs are concerned, is that while the peak wavelength might be at the edge of UV, it also emits in the deep blue on the "left" side of the peak and further into the UV on the "right" side of the peak. So, when I make an output measurement, the solar cell converts all wavelengths produced by the LED -- visible blue and invisible near UV -- to electrical current. I have considered various ways to measure only one or the other separately, but so far have not really actually done anything significant about it since to do it right looks like it will be fairly expensive. I said all of that so that I can say this: While all seven of the test LEDs still produce some visible light, only three seem to still be making any UV at all. Those three can still light up Vaseline glass a little, but the other four do not. When I first started the testing project, I did not think to try to see how much of the output was UV and how much was visible when the LEDs were new. So I don't know if the three LEDs that are still making some UV were making more than the others were when new or if they were all making about the same amount of UV and the four that don't make any now somehow lost the ability. In any case, this suggests that there are at least two different LED manufacturers involved and possibly one or two more based on a minute examination of the LED lead frame. However, there could be just one manufacturer that uses different (or older and newer) LED chips and lead frames based on availability. I dunno. I have ordered some more cheap UV LEDs from two different domestic sources (who turned out to have Chinese names) and will put a sample of each into testing. I'll let you know how it goes. Leigh