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-   -   Discussing Life Expectancy of LED Fixtures (http://www.plantedtank.net/forums/showthread.php?t=108740)

gpodio 05-19-2010 10:04 PM

Discussing Life Expectancy of LED Fixtures
 
I'd like to throw this out here for discussion sake...

LEDs have a lifespan rating of several tens of thousands of hours... But will the phosphors that make them emit white light actually last that long?

The reason I ask this is that I've followed very closely the performance of two commercial LED fixtures (one of the best brands at the time of purchase) over reef tanks for the last 24 months. Besides the headache of replacing drivers and LEDs here and there, the overall quality and intensity of the light has greatly reduced over the course of the first year and is below acceptable levels in my opinion over the second year. Now I'm not 100% sure what LEDs were used in these fixtures but I'm guessing they are phosphor coated InGan/AlInGap LEDs just like Philips/Luxeon/Cree uses. I think it's safe to assume we're all using phosphor coated LEDs. My gut tells me that these phosphors, which are practically identical to those used in fluorescent bulbs, may have degraded over time just as they do in fluorescent tubes.

Can people that have been running LED fixtures for over a year chime in and let us know if things look the same today as they did a year or two ago? I'm a big fan of LEDs but when I hear life expectancy claims that go into the 10's of years I start to wonder if the phosphors used will last as long as the diodes themselves.

Thoughts?

Giancarlo Podio

Hoppy 05-19-2010 10:19 PM

My LED fixture has been in continuous use for about 11 months now, with no visible change in output. One would have to do periodic PAR measurements to know for sure if they deteriorate, and I haven't done that. I have never believed that they will last as long as the advertising hype would have you believe, but it will take several years before we will know for sure.

VadimShevchuk 05-20-2010 12:32 AM

Ill wait like 3 years before i buy an LED fixture. So that the technolody could prove thats its worth it.

arktixan 05-20-2010 12:49 AM

I remember learning in school... LEDs should never die/burn out as long as the right amount of voltage is applied, anything higher than X amount burns em out. Then again this was back in Highschool with cheap lil LED red lights for Computers class...

Though one would think, it SHOULD apply to all LEDs. But, I could be wrong. :)

jwm5 05-20-2010 01:09 AM

i believe LED's can get dimmer over time, at least certain types/makes. I have read a few articles on LED TV pictures not being as bright over time, I forget what brand or reasoning, but I have heard that they can dim down over time.

gpodio 05-20-2010 02:51 PM

Quote:

Originally Posted by arktixan (Post 1072481)
I remember learning in school... LEDs should never die/burn out as long as the right amount of voltage is applied, anything higher than X amount burns em out. Then again this was back in Highschool with cheap lil LED red lights for Computers class...

Though one would think, it SHOULD apply to all LEDs. But, I could be wrong. :)

Besides a little loss of intensity, it's true that the actual LED diode can last this long. What I'm referring to here is not the diode itself, but the phosphors that are mixed into the resin lens which are responsible for emitting the white light. White LEDs are usually UV LEDs, in some cases blue or yellow emitting LEDs (monochromatic). Phosphors (a white powder-like substance) are mixed into the resin lens or the lens itself is coated with this powder which emits white light when UV light hits it (think white shirt glowing under a black light in a night club). Just like fluorescent bulbs, they generate UV lighting internally and excite the phosphors which are coated on the inside of the glass tube, these "glow" so to speak white. And it's these phosphors which degrade over time due to UV radiation, heat and other factors. So while our LEDs may indeed continue to work (emit UV) for the rated hours, there's the possibility that the phosphors still degrade as quickly as they do in other applications, meaning the spectrum and intensity of the white light would degrade regardless of the healthy state of the UV emitting diode.

We've tried replacing the drivers on one gang of LEDs with no change in light quality, so we know it's the LEDs that have degraded somehow. Obviously there are other possible causes, perhaps the drivers were operating the LEDs above their rating or a poor job was done in dissipating heat internally... but it still leaves me curious regarding the phosphors and how, if at all, they are able to extend their lifespan to bring them anywhere near the lifespan of an LED diode.

Giancarlo Podio

gpodio 05-20-2010 03:09 PM

Quote:

Originally Posted by jwm5 (Post 1072503)
i believe LED's can get dimmer over time, at least certain types/makes. I have read a few articles on LED TV pictures not being as bright over time, I forget what brand or reasoning, but I have heard that they can dim down over time.

This is correct, and for the exact same reason. LED TV's are backlit by white LEDs, that's the item that looses intensity over time due to phosphor degradation. Plasma TV's are 100% based on phosphorescence, hence their loss of intensity over time and shorter lifespan Vs LCD screes, which can usually have their backlight (LED, ELP or CCFL) replaced as the phosphors degrade. High-end LCD panels use RGB LEDs which don't use phosphors, providing longer lifespan and better color control.

I would think that if someone has developed a phosphor with a huge lifespan we would see this technology in phones, TVs, laptops well before our little niche market. Or do they not want a TV to last 20 years... :icon_wink

Giancarlo


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