# Actual LED lumens maintenance. Not 100,000 hours.



## liveforphysics (Aug 12, 2008)

I'm an engineer in a datacenter. We have a company which ensures that they can save us money by swapping all of our T8 fixtures to LED fixtures. 

They feature a cool white bin of a CREE Q4. 

The basis of the cost savings mainly comes from labor time savings due to the complex process to change the bulbs in our fixtures. We have over 2,000 4 bulb fixtures placed above rows of servers and thousands of miles of cables. 
They drive each LED at 700mA.

They are claiming we would still have 90% lumens maintenance after 10 years of operation.

For some reason, I have this memory floating around in my head of some respected members of CPF doing lumens maintenance testing of LEDs, and finding results which were much different than the 100,000hr claims given by manufactures. 

I'm trying to avoid making a 3million dollar mistake if I go forward with this, and they are dim after a couple years.

I've searched Google and CPF, and CPF searches with google, and I can't seem to find anything but manufactures data on LED lifespan specs. 

Was I just dreaming that I read people performing tests that indicated 10-20% output drops after a few months of operation? 

Any input from folks who have insight on LED lumens maintenance testing?

Thanks!
-Luke


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## LukeA (Aug 12, 2008)

Cree specs 50k hours to 70%. If they're on 8 hours a day, then they'll last 17.1 years. Twelve hours a day is 11.4 years, 24 hours a day will be 5.7 years.


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## liveforphysics (Aug 12, 2008)

Thank you LukeA! 

50,000hrs to 70% when driven at 700mA, or 350mA? This is certianly quite a difference from the 10% at 10years they are claiming with their retro-fit product.

Those numbers seems closer to reasonable, but I seem to recall seeing some non-manufacture data done by a CPF member, perhaps McGizmo or someone similarly well respected who recorded big losses over a much shorter time span. 

-Luke


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## jirik_cz (Aug 12, 2008)

I recommend you to look on DOE Caliper test reports. I think that with good heatsinking 50000 hours is realistic claim. But some manufacturers make SSL retrofits with poor heatsinking and then the lifetime may be very short.


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## broadgage (Aug 12, 2008)

50,000 hours is not that impressive, premium brands of flourescent lamp often last several years of continous burning in good conditions.

Phillips produce ultra long life flourescents which are claimed to last 63,000 hours, and also claim 90% lumen maintenance at end of life.


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## jtr1962 (Aug 12, 2008)

With good heatsinking you can probably get 50,000 hours or more at 700 mA. While it's true that there are some fluorescent tubes which last this long, they're not that common, and probably not exactly cheap. With inexpensive commodity tubes figure you'll get about 20,000 to 25,000 hours.

A better design decision might be to drive the LEDs at 350 mA, or perhaps even lower. Although data for underdriving is sparse, if power LEDs are anything like 5mm LEDs then life will increase exponentially. In other words, it might not be totally unrealistic to expect 1 million hours to 70% brightness via severe underdriving (i.e. 100 mA). The downside of course is that you'll need 2 to 5 times the number of LEDs. I think this is where streetlight design is headed. The desire is to get the light emitter to last as long as the fixture, which by most accounts is around 200 years ( that's about 1 million hours if on 13 hours per day).


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## saabluster (Aug 12, 2008)

liveforphysics said:


> Those numbers seems closer to reasonable, but I seem to recall seeing some non-manufacture data done by a CPF member, perhaps McGizmo or someone similarly well respected who recorded big losses over a much shorter time span.
> 
> -Luke


I think it may have been Newbie. Not sure though. I think what you are remembering are tests that were done on the SSC P4. It showed a large loss of lumens in a very short time with it then leveling off some. It was speculated that although SSC's phosphor was very efficient, it was not to stable, and that was causing the reduction in output as well as a shift to blue. This is all just off the top of my head though. I don't remember ever seeing anything saying there was a problem with Cree's lumen maintenance.


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## SteveDavis (Aug 12, 2008)

I think it's best to consider this from the application standpoint. My understanding of datacenters is that they are well cooled and occupied very sparingly. That means that your flourescent fixtures are likely less efficient than normal from the cold, and lower life than normal from being turned on for short periods of time (look up the lifetime data on your tubes. I bet there's a big difference between 12hr starts and 3hr starts). That puts the application on the bad end for T8's, and on the good end for LEDs.

Cree rates mean lumen maintenance at 70% over 50,000 hrs for a junction temperature of 80 deg C. I believe this data is also at 350mA. Decreasing current or junction temperature will increase life, increasing these will decrease life. If the lighting company has excellent thermal management and the lights aren't on all that much, I don't think 10 years at 90% is unreasonable. I'd take a little time to figure out the average time that your lamps are on each on period, and how many hours a year that translates to. That will give you a good estimate of how long your tubes last, and how long the LED replacements will.

For more info on Cree lifetime, check here:

http://www.cree.com/products/pdf/XLamp_Reliability.pdf


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## monkeyboy (Aug 12, 2008)

It's all just speculation. No one has actually done a real world test of the lifetime of these particular LEDs due to the fact that they haven't been available for long enough. So you don't know if you're going to get 200,000h or 5000h. Anyone can maintain a T8 fluorescent system but this LED system ties you into one company and leaves you at their mercy. They can then charge what they like for maintenance or disappear off the face of the earth if it all goes horribly wrong.

There are factors other than lifetime also to consider;

1) Fitting cost. To produce the amount of light required for an office will require a *LOT* of Cree emitters. 3 million dollars is a large amount to spend in one go for dubious long term savings.

2) Colour rendering. Cree Q4 LEDs can't match the CRI of tri-phosphor "daylight spectrum" fluorescent tubes. So maybe that would be a better upgrade in producing a healthier working environment?

3) In 5 years time, Cree Q4 will be obsolete. There will probably be silicon based LEDs with better efficiency and better CRI for one tenth of the cost.

IMO, It's not worthwhile switching to indoor LED lighting for at least another 5 or 10 years. Definitely not on that scale anyway. The first people to adopt a new technology are always the ones to get ripped off.


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## Oznog (Aug 12, 2008)

The lifetime depends very very strongly on how low the die temp is maintained within the device. Unfortunately, heatsinking is the most expensive and difficult part of the design and it's easy to skimp on. It's unlikely they'd be able to use a fan. The traditional enclosures in the ceiling don't have a good way for air to flow through at all. 

People won't notice anything wrong right away and the sale goes through.

Note that lumen maintenance affects efficiency too. What was 100lumens/watt initially becomes 70 lumens/watt by the end. That changes the bottom-line for the efficiency goals.


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## jtr1962 (Aug 12, 2008)

monkeyboy said:


> IMO, It's not worthwhile switching to indoor LED lighting for at least another 5 or 10 years. Definitely not on that scale anyway. The first people to adopt a new technology are always the ones to get ripped off.


I tend to agree with that assessment. LEDs at this point in time make quite viable replacements for incandescent-based light. However, I think it will be at least a decade before LED becomes cost-effective to replace T8 fluorescent systems. Right now efficiency is about the same, color rendering is the same or worse, and cost is much higher, even factoring in the longer life of LEDs. LED needs to be markedly better in at least two out of those three criteria before wholesale T8 replacement makes sense.


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## asdalton (Aug 12, 2008)

liveforphysics said:


> For some reason, I have this memory floating around in my head of some respected members of CPF doing lumens maintenance testing of LEDs, and finding results which were much different than the 100,000hr claims given by manufactures.



The largest discrepancies were for 5-mm white LEDs, which are not designed to dissipate heat. They show degradation of output after thousands of hours--shortened to hundreds or even tens of hours if overdriven (which in practice they often are).

The original "100,000 hour" claim may have been carelessly transferred from the performance of underdriven monochromatic LEDs. Phosphor degradation is a weak point for white LEDs, and colored LEDs have no phosphor.

Properly driven, properly heatsinked power LEDs (Luxeon, Cree, Seoul, etc.) should last many thousands of hours easily. But I still wouldn't believe any "100,000 hour" claim without data.


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## JohnR66 (Aug 12, 2008)

Don't do it! BAD idea! A T8 system can give you 90 to 100 lumens per watt with good electronic ballasts. Q4s can match that, but as I understand the die temp can't be kept cool enough in normal operation to keep the LED that bright. The system is likely very expensive and you probably will give up some light to save money.

One 32 watt 48" T8 tube can put out nearly 3000+ lumens and the bulb costs a couble bucks. A single die LED puts out 200 lumens under ideal conditions and is more expensive. It would take 15 of them to equal the output of the fluoro tube.

I like LEDs a lot, but for general lighting, they don't make the grade. In some directional, color and low power uses, LEDs can be perfect, but metal halide and Flouro are still the kings for efficient lighting at high lumens now.


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## pril (Aug 12, 2008)

My .02$

Led is future without doubt but your question is all about money. 

If you can sign contract (with good standing company and its life expectation for next 10+ years and with business insurance) with guaranty for power consumption, light durability, quality etc then this is ok, otherwise you should be very careful. 

In this second case you have to do some cost analysis. You probably have exact data (power consumption and maintenance costs etc) for current system. You can easy compare your current costs with oferred costs for new system. Maybe costs analyze shows small difference for next two years - then is better to wait. 

Maybe LED technology is not mature yet so you can try with small testing area for this. You can also compare other benefits from each technology (light quality, resistance to frequent switching etc.). 

I understand your dilemma: wrong decision can cost you thousands of dollars. For this money is reasonable to order independent report (created by professionals for light systems) that will compare both systems. Maybe you can find someone right here …

Primoz

(Sorry, English is my second language)


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## blasterman (Aug 12, 2008)

I'll agree with most of the LED posts above in that we need more data, and that solid state retrofit kits are for the most part bad news.

However, I'll happily 'kick the dog' and offer the advice that you should be dumping T8's and moving to T5's anyways. T5s *are* the state of the art in fluorescent design and in some cases displacing metal halide. 4100K T5's with a CRI greater than 90 are exceptional for data centers and offices.


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## jtr1962 (Aug 12, 2008)

Just going through some rough numbers:

Typical four-foot T8 tube = 2850 lumens
Four tube instant start ballast typical ballast factor: 0.89
Four tube instant start ballast typical power input: 110 watts
Output of 4-tube fixture = 4 x 0.89 x 2850 = 10146 lumens
Typical fixture efficiency: 80%
Actual light output of 4-tube T8 fixture = 0.80 x 10146 = 8117 lumens
Overall system efficiency = 8117/110 = *73.8 lm/W*

Now let's do the numbers for Cree Q4's driven at 700 mA

Typical output: 170 lumens
Typical Vf: 3.5 V
Number of LEDs per fixture needed to match output of 4-tube T8 fixture: 48
Power input to LEDs = 48 x 3.5 x 0.7 = 117.6 watts
Power input from AC line (assuming 95% efficient LED ballast) = 123.8 watts
Overall system efficiency = *65.9 lm/W*

Going through the same thing but driving the LEDs at 350 mA instead requires about 82 LEDs and has an overall system efficiency of around 82 lm/W.

Bottom line is that LEDs probably need to improve to at least 150 lm/W at higher currents such as 1 amp before replacing T8 tubes will begin to make much sense, at least from an efficiency standpoint. There may be mitigating factors where difficulty of changing lamps, low temperature, or frequent starts make LED a better choice to replace T8 tubes right now, but for most general linear fluorescent lighting, I think we're at least 5 years away. LEDs will probably be viable for replacing CFLs in 2 or 3 years however.


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## jtr1962 (Aug 12, 2008)

blasterman said:


> 4100K T5's with a CRI greater than 90 are exceptional for data centers and offices.


All of the common T5s I've seen have a CRI of 86 regardless of color temp. There just don't seem to be lower or higher grades. T8s generally come in 2 commodity grades-one with CRI in the high 70s and the other in the mid 80s, with >90 available for a premium, and generally only for CCTs around 5000K-5500K.


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## blasterman (Aug 12, 2008)

> Typical four-foot T8 tube = 2850 lumens


 
That's higly optimistic in a non VHO tube - trust me. The main problem with T8's (and spiral CFLs to be fair) is strikeback absorption, which can suck a huge percentage of the light's emission. You need a very efficient and optimized reflector, one that basically you can see yourself shaving in to hit that efficiency. The big advantage with T5, and linear CFL is thinner tubes and less strikeback. Hence, it's easier to optimize a reflector and actually use all the photons you generate. 

Regardless, T5 is the king right now for commercial fluorescent fixture design and efficiency, and T8 is only being sold for legacy fixtures. T5 bays are even displacing HID right now, and doing so at an industrial scale. 

I had a site that was selling 90+ CRI T5's, but it isn't working now, so 85CRI / 4100k is your best bet in any tube size. It's a good working color temp.


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## 2xTrinity (Aug 12, 2008)

About the only advantage that could be offered by LEDs is if they are on motion detector control. LEDs are much "happier" being short-cycled than any kind of fluorescents, and if the area that needs to be lit is only occasionally occupied, running these on motion detectors woudl both save power (due to the light being completely OFF much of the time) also, LEDs have the potential to be more optimally focused, and save money by ONLY throwing light where it's absolutely needed. However, probably no commercial LED products on the market today live up to these hypothetical potentials.

I still believe T5 is probably a better way to go. Many of these fixtues have motion detectors as well, and the best way is probably to set this but with a somewhat longer delay before turnoff -- that way oy're not cycling them every few minutes, but they will NEVER be on for hours at a time with nobody around. You may also consider high-output fixtures. Efficiency is slighly lower, but since each tube has double the brightness, and the tubes themselves are physically smaller, you can light the datacenter to acceptable illuminances with fewer total tubes, leading to less money spent during maintenance.


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## jtr1962 (Aug 12, 2008)

blasterman said:


> That's higly optimistic in a non VHO tube - trust me. The main problem with T8's (and spiral CFLs to be fair) is strikeback absorption, which can suck a huge percentage of the light's emission.


That's raw tube lumens. I accounted for fixture efficiency later in my calculation (I think 80% is reasonable for an average T8 fixture). I do know that T5 fixtures using highly reflective mirrors can exceed 95% fixture efficiency. For the LED fixture I assumed 100% since Crees emit all of their light over a half sphere (and hence no need for optics of any kind). Diffusers would obviously hurt all fixtures equally, so no need to account for that here.


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## liveforphysics (Aug 13, 2008)

Thank you so much for all the excellent info!

It seems you folks are telling me much of what I had also thought about the performance relationship between T8/T5 vs LED. For our specific application, a big part of the appeal to LED tech was the labor involved to change bulbs. This enviroment is very risky to change bulbs in, due to the potential for bumping a ladder or man-lift into some part of the mostly overhead data infrastructure.

saabluster- You also recall seeing some large output drops in the infancy period of LED lifespans from tests that may have been from Newbie? This is exactly the sort of thing which causes me concern. I'm fairly certian you would not see evidence of the early phosphor failure by reading the SSC manufacture's spec sheets. I can't help but question the manufacture spec sheets with 10's of Kilohour output durability estimates for products with a development to production time measured in months. The thermal management consists of mounting 16 x Q4's on a little aluminum strip that mounts into the existing fixture, and replacing the ballast with a 700mA current supply. The product really seems like the sort of thing that you could DIY in your garage in a few hours for about 40$ over the cost of the LEDs, yet they each sell for a small fortune. They carry 10 year warrenty on manufacture defects ONLY. I get the impression that if they all become dim in 2-3 years, we are entirely on our own.

In our application, we always have techs handling server issues on the datacenter floors 24/7, so our lights are never switched off. When you have 400,000 servers, it requires continous babysitting to keep things running smoothly. 

I personally think CRI is important, but I seem to be alone on this in the datacenter lighting field. 

There are really only two factors that seem to most strongly come into play. Reduction of maintenance time, and hence risk to incidents which effect server infrastructure, and power usage.

Does anyone have any non-manufacture supplied data on lumens maintenance for CREE products? 


Thank you for being such an amazing group of people!

-Luke


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## Juctuc (Aug 13, 2008)

About lumens: Most of the ledproduct-manufacturers are telling the lumens of the LED, at tj.25c. Those are not the lumens when the light is on. Manufacturers like to use also the highest lumens, and not typical lumens. In that kind of investment what we are talking here, i think that manufacturer should have some information about fixturelumens as well.Even better if there is a IES files for it.

And if the lumens are ok, then we can start to talk about the lifetime..Most of my personal testing is been on the lowcurrent leds..results are not very good, at least on for the 5mm led with white colour. I have one E27 with semileds(from U.S.A) inside.5x1w. In 5600hrs my luxmeter is showing 6% less than it was at the start..now im testing CREE XRE 3X1W MR16 warmwhite.But i just started...im waiting about 10% fading in 10 000hrs. But we will see after one year=)


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## liveforphysics (Aug 13, 2008)

Juctuc- That is exactly the sort of info I'm interested in hearing. Tests performed out of the manufacture's lab, and in a real working enviroment.

6% over 10,000hrs for a 5mm white? I'm just taking a wild guess that the spec sheet for that LED lists something like 100,000hr to 80-90% lifespan? 

I love CREE products, and I'm wild about solid state lighting. I also don't feel like they are working to mislead us, but I do think perhaps the testing procedure is not quite perfect. 


This is part of a sample kit given to us by the company. Its a small desklamp. It has a 12VAC trasformer that powers it.

I don't know anything about these emmiters, but they oddly appear to be multi-die perhaps? I took the unit apart and took some photos. 

Again, the lighting for the server rooms is a different product, and CREE Q4 based, this is just a novelty sample they sent out. Any idea's why it appears to have 6 dark areas under the phosphor in each emmiter?

















Thanks!
-Luke


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## Juctuc (Aug 13, 2008)

That 6% is with so called powerled, not 5mm led. 5mm LED will last about 5000hrs, and then it has lost about 50% from the original lightoutput..i think.

I dont know what leds those are in your picture. In the website in this company, who is making these, there is no information what led they are using. Most of the manufacturers will tell if they are using CREE. But maybe this company is starting to use them just now.


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## liveforphysics (Aug 13, 2008)

Wow! 6% from a powerled that was not being overdriven? These experiences are very different than what most LED info would lead you think.



These are the sample T8 replacement LED strips they sent us about 6 months ago. They definately use CREE leds. I don't have one of the newer Q4 based units laying around, or I would photograph it for you. They don't look much different though. Just emmiters on an alumium stick, an LED ballast, and a crazy price tag. 











They seem to do the job just fine for the 6 months I've had them in my office. My concern is fear of dropping 3M dollars on lighting that becomes unsafely dim for this enviroment after a few years.


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## Lunal_Tic (Aug 13, 2008)

Could you not just write lumen maintenance/output into the contract? Something like OSHA's illumination requirements for your application: guarantee x number of foot-candles for a given number of years with the option to replace units that fall below that. I don't really know all the ins and outs but IIRC they have ratings for everything from washrooms to warehouses.

-LT


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## jtr1962 (Aug 13, 2008)

The heatsinking for those strips really doesn't seem adequate to me. I really think power LEDs can exceed 100,000 hours or more, but only if underdriven, and only if run very cool. Neither seems to be the case with that product. You really need a purpose-built LED fixture. I have little doubt the LEDs will still be working ten years from now as the company said, but they will probably be quite dim by then.

Regarding tests, I actually have a Q3J Luxeon which has been running continuously since January 2004 at 350 mA. Although I didn't have a light meter when the test started, it's measured the same for the last 3 years. With over 41,000 hours on it, I'm confident 50,000 hours with >70% initial lumens is easily possible. Indeed, it seems my LED is doing significantly better than that.

Since these fixtures are on 24/7, you might want to look into these. They will run only $5 each in quantity, and have improved color rendering (CRI = 91) plus 34,000 hour life. Note that lifetime for fluorescent tubes is specified with 3 hours on, 30 minutes off. When run continuously most fluorescent tubes exceed rated lifetime by about 40%. This would mean you would get about 48,000 hours in your continuous-duty application. This is over 5 years.


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## Rogerg (Aug 13, 2008)

Reflectors / mirrors overhead and lights lower, out of the way directed upward and accessible. No clue if practical or possible.


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## matrixshaman (Aug 13, 2008)

I'll bet the high price tag has a lot to do with a salesmans' commission. How about trying some of the ones available from various sources in a smaller area and hiring or using your own people to install them? Perhaps do a gradual switchover and over a couple years you will likely be installing more efficient ones and if the older ones degrade than upgrade those also. 

Or hire CPF people to build them for you ongoing for a gradual switchover


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## saabluster (Aug 13, 2008)

WOW! I could design something better in my sleep. I agree with jtr1962. There just seems to be little thought given in how to dissipate the heat. I would stay away from this. What I would suggest is sending one of these to jtr(if he is so inclined) to test and see what how hot the LEDs are getting. This would give us a better idea of the true life span of these. 


liveforphysics said:


> Just emmiters on an alumium stick, an LED ballast, and a crazy price tag.
> 
> 
> 
> ...


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## VanIsleDSM (Aug 13, 2008)

As someone else who dabbles in machining their own aluminum LED fixtures, I'll third the fact that the fixture pictured looks nowhere near adequate to remove the heat.. You'd at least want some vertical fins on either side of the "LED tube" to utilize convection currents to cool it down. Since LEDs become less efficient as they heat up, you're generally looking at 10% less output that claimed by the LED manufacturer at 25C, that's with a good heatsink design with a die temp around 60-70C. With a poor design you'll lose out on another 10-20% output from the LED due to heat, and it will degrade much faster.

I'd test to see what the heatsink temp is as close to one of the LEDs as possible, and then determine the die temp through the typical thermal resistance of a CREE LED of 8C/W to get a real idea of how well these fixtures are removing the heat. 3 Million is a large investment, I wonder how much they are charging for each fixture+driver (16 CREEs in large quantity placed on aluminum extusions).


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## liveforphysics (Aug 14, 2008)

All these LED nuts, and nobody can ID that emmiter? LED museum guy? 

Does anyone know why is appears to have 6 dice?

Thanks!
-Luke


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## LukeA (Aug 14, 2008)

liveforphysics said:


> All these LED nuts, and nobody can ID that emmiter? LED museum guy?
> 
> Does anyone know why is appears to have 6 dice?
> 
> ...



That's a 6-die 1W Nichia device, possibly one of the 083 high-CRI emitters.


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## snarfer (Aug 14, 2008)

Noticed the markings on the PCB so I looked up the company. Supposedly they have "patent-pending" technology for thermal management that conducts the heat from the LED strip to the housing. 

Google patent search did not produce anything relevant name of the company or the CEO/founder in it. So I think that it must be some sort of secret "provisional patent" that allows them to say that it is patent-pending. Or maybe they are licensing it from another unnamed company.

Only problem with conducting heat from strip to housing is that of course the housing is made of steel, which has much less thermal conductivity than aluminum or copper, by about a factor of 10. 

They have listed some photometrics in which they play games with "fluorescent equivalent lumens." What the hell are those? 

[rant]I am pretty sick of all these companies that think they are getting over by making up their own measuring units. Some guys make a panel out of a bunch of crap 5mm LEDs and then say that it is "400 watt equivalent" even though it only draws 35 watts, and a quarter of that is in the resistors. [/rant]

Actually I was doing some tests last week comparing fluorescent lamp from KinoFlo to a project I am working on using multi-emitter LED devices and I was amazed at how inefficient the fluorescent fixtures were. 

We were shooting some color charts, and we set up 72 watt LED lamp with no optics or anything (so it had a very wide spread), at same distance as LED Diva 400, which uses 4 x 55 watt PL-L (essentially T-5 u-tubes). The Diva tubes are rated at 2560 lumens each, so total of over 10,000 lumens, while the LEDs are supposedly 60 lumens/watt so a little over 4,000 lumens. 

I would have expected a difference of a full stop, but it turned out that with the LED lamp just as much light was hitting the chart. So it goes to show you that fluorescent lamp and reflector combinations may not even be as good as 80% efficient. Maybe 50%. And this is with the newer style tubes. T8s are probably even worse!

For a long time now I've been saying that LEDs just can't compete with fluorescent, but after that test I really started to think differently. Maybe it is possible already.

Hey by the way I have also been experimenting with powering LEDs with fluorescent ballasts, and it actually works pretty well. I did a very crude test with a four dollar T8 ballast, a bridge rectifier, 1 mH inductor and 2.2uF /400v low ESR film capacitor. For some reason the ballast took a long time to turn on, maybe because of such a low voltage, but when it did turn on it provided exactly 350 mA of power to the string at various numbers of LEDs. There was absolutely no flicker. I used only two of the four pins. Had to open up the ballast to figure out which pins.

Something else I just noticed in one of their spec sheets (look here). Actually this looks like a different product from the 12 Crees on aluminum that you posted picture of. But in the T8 troffer retrofit brochure they say they are running 30 or 45 LEDs per bar at either 400 mW/LED, 870 mW/LED, or 1400 mW/LED depending on which option you choose. So it looks like they are doing something that makes sense for long life of LEDs.


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## jtr1962 (Aug 14, 2008)

snarfer said:


> By the way I have also been experimenting with powering LEDs with fluorescent ballasts, and it actually works pretty well. I did a very crude test with a four dollar T8 ballast, a bridge rectifier, 1 mH inductor and 2.2uF /400v low ESR film capacitor. For some reason the ballast took a long time to turn on, maybe because of such a low voltage, but when it did turn on it provided exactly 350 mA of power to the string at various numbers of LEDs. There was absolutely no flicker. I used only two of the four pins. Had to open up the ballast to figure out which pins.


That's interesting. I'd guess it would turn on instantly if you used a string of LEDs equivalent in voltage to the fluoro tube. I'm amazed that it works at all, let alone as well as you say. If this is the case, then maybe someone here can design LED retrofits for the original poster. It should be fairly easy electronically if you use the fluoro balllast, and you also avoid the potential liability issues of designing a 120VAC ballast from scratch.


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## snarfer (Aug 14, 2008)

Yes I'm surprised that more people haven't experimented with using fluorescent ballasts for LED applications. Generally those ballasts are extremely efficient, especially for the European market. 

If anyone wants to try it, be sure to use an electronic ballast that switches at high frequency. Also rectification seems to work much better than parallel strings, at least in simulation. I didn't bother actually building the parallel strings design. 

If the original poster is interested in developing an alternative solution, I have some designs that could be useful. Sorry I can't be too specific about it in a public forum as it is the intellectual property of my company.


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## jirik_cz (Aug 14, 2008)

snarfer said:


> I would have expected a difference of a full stop, but it turned out that with the LED lamp just as much light was hitting the chart. So it goes to show you that fluorescent lamp and reflector combinations may not even be as good as 80% efficient. Maybe 50%. And this is with the newer style tubes. T8s are probably even worse!



According to Caliper testing report round 5, T8 in housing with parabolic reflector has 60% efficiency and T12 in "lensed" housing has 72% efficiency.


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## snarfer (Aug 14, 2008)

Thanks. That is an interesting document. I wonder what grade reflector they tested with. Seems like it is possible to order reflector material from 75% all the way up to 99%. Fixture efficiency of 60% would be within the range of what I observed. 

If I was considering this purchase I would definitely get out a light meter and do a direct comparison between the LED replacements and the existing fluorescents at this point.


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## Hooked on Fenix (Aug 14, 2008)

broadgage said:


> 50,000 hours is not that impressive, premium brands of flourescent lamp often last several years of continous burning in good conditions.
> 
> Phillips produce ultra long life flourescents which are claimed to last 63,000 hours, and also claim 90% lumen maintenance at end of life.



I know of no such actual flourescent lamps that will last that long. I think what you are refering to is induction lighting. I saw this technology at the Electric West conventions years ago. Basically it looks like a fat flourescent bulb. It uses a a phosphor coating on the inside of the bulb like a flourescent. It has no electrodes. Instead of using U.V. light to light up the phosphor, it shoots microwaves through the bulb. If I remember correctly, the bulb and ballast were over $200. The most important thing to know about this bulb is that it can't be used indoors. It causes a lot of radio frequency interference so it is only used for outdoor lighting. It's not an option for office buildings. I think the T5 bulbs would be your best bet. L.e.d.s aren't cheap enough yet for that application. You don't want to be kicking yourself in the butt in a few years when they are cheap enough after spending millions on bulbs that have become way cheaper and better.


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## KeithInAsia (Aug 20, 2008)

I think the key is gaining long LED life is a low operating temperature. The vendor should prove that their design will run the LED at the lowest possible temperature.

Also, they need to demonstrate that the driving electronics can be reliable as well. What is the point of the long life LEDs if the electronics are breaking down and possibily destroying the LED at the same time?

That is my 2 cents.


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## rob85635 (Aug 21, 2008)

> I'll bet the high price tag has a lot to do with a salesmans' commission. How about trying some of the ones available from various sources in a smaller area and hiring or using your own people to install them? Perhaps do a gradual switchover and over a couple years you will likely be installing more efficient ones and if the older ones degrade than upgrade those also.
> 
> Or hire CPF people to build them for you ongoing for a gradual switchover


As a newbie to LED technology myself I can only offer some basic thoughts. I really like the idea of researching other sources and maybe having it done internally if you have the staff/expertise. It is certainly a high dollar project and from what I have gleaned in my short time here is that LEDs are great but long term study results are just not reliable enough to judge that many hours into use. 

I am learning so much here and on another electronics forum I belong to. One thing I am amazed by is the amount of members with minds packed full of amazing information. This website is a gift, I only wish I had needed it sooner. I have so much to learn.

Rob85635


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