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Topic Title: LED lighting
Topic Summary: Light output over time?
Created On: 28 February 2019 07:19 pm
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 28 February 2019 07:19 pm
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Does anyone know if the light output from LED lighting diminishes over time?
 28 February 2019 07:24 pm
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Yes, but the reduction varies from trivial and insignificant to substantial, depending on the quality of manufacture and on the conditions of use.
Worse at higher temperatures, other factors being equal.
 28 February 2019 09:52 pm
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all semiconductors (not just LEDs) rely on regions of a very pure crystal that have been carefully doped with just the right amount of another material to make it more or less conductive, by having a controlled excess of holes or electrons this is known in the trade as 'doping' The dopant has to be something that diffuses in to the material at temperature, becaus that is how they devices are made. BUT, because of this, it does not stop diffusing, albeit at room temperature the process is glacial, so over time an optimum device becomes smudged. This is most serious for devices with small feature sizes, so not much diffusion needs to occur before it is ruined completely, or that run warm, so the rate of diffusion is speeded up.
LEDs, especially white and blue ones, use some fairly easiluy moved dopants, to get the colours, and ones used for lighting are often run quite hot, and tend to have clear plastic bodies rather than metal lids, so the heat has to conduct down the lead frame.
So for real devices the light output may fall to half in 10000 hours or 100,000 hours depending on the running environment.
Try to err on the side of over cooling.

Power supply transistors that are a bit undersized and run hot are another common candidate for early failure.

regards Mike
 01 March 2019 03:34 pm
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I had in mind lights in a commercial environment such as shops.
These would generally have lights on permanently for long periods of time.
Fluorescent fittings just need a lamp change but I guess with LED's the whole fitting would need replacing to maintain light levels.

Assuming 6 day opening for a small high street shop then the lights could be on for around 3500 hours per annum. The temperature in the shop would need to be comfortable, so the lights will be operating in relatively high ambient temperatures. Assuming worst case on Mike's figures, that would give around 3 years before a possible fall in light levels to 50%.
 01 March 2019 03:56 pm
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Hi Mike

if you speak to the luminaire manufacturer, they should be able to give you the L/B figures. Can't remember exactly what they represent but I think the 'L' figure gives the number of hours of operation until the output falls to 70% of the initial output, and the 'B' rating is the expected amount of LED failure at this point.
 01 March 2019 04:17 pm
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LED indicator panel lights were rated at over 100,000 hours, this has now fallen steadily to less than 3000 hours due to the general lighting applications. That's what happens when you turn up the current to force extra light from the semi conductor material. The replacement LED tubes appear to be lasting ok. from waht my colleague says
H&H solid state amplifiers used to have a similar problem in that the output class B stages would fizzle out when the volume was up for a length of time until the temperautre dropped and then would kick in again. I remember too there wasn't sufficient short circuit output protection, but then that's something else.




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