Author Topic: LEDs last a long time, but...  (Read 2829 times)

Danneaux

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LEDs last a long time, but...
« on: May 24, 2022, 08:40:50 pm »
Hi All!

In Andre's recent Forum response on LED lighting ( http://thorncyclesforum.co.uk/index.php?topic=14606.msg109606#msg109606 ), he said LED lamps have a MTBF in the region of 50,000 hours. This is surely correct and we're in full agreement. However, I've noticed some of my older LED lighting no longer seems as bright as it once did and my incident and direct-reading photographic light meters (inherited from both my parents who used separate handheld meters in the days before they were built into film cameras) agree so it isn't my eyes failing or a false impression, they're really dimmer! I dug around for triangulation and found it here...
https://ledlightinginfo.com/do-led-lights-get-dimmer-over-time#:~:text=LED%20bulbs%20will%20get%20progressively,of%20its%20initial%20light%20offering.
This link highlights the importance of the L70 rating, when an LED dims to only 70% of its initial value. There are other ratings, but it seems 70% is the clearly noticeable threshold when dimness is most apparent, though a 20% loss can be readily perceived by the human eye. Based on the above link and others I have read, it seems the dimming comes from five sources:

1) Phosphor fatigue and...

2) ...quantum dislocations within the semiconducting materials (not too unlike what happens to solid-state drives after they've been written to and read from a large number of times).

3) The article at the above link goes into greater detail but it seems heat really is the prime enemy of LEDs, so cooling is paramount to extending usable life. "Usable" in this context means "bright enough for the intended application". Unsurprisingly, (excess) heat also affects overall life (MTBF). Most quality bicycle LED lighting only includes a small copper tab as a heat sink -- if one is included at all. What is really needed is a finned heatsink that extends outside the lamp case where it is exposed to forward airflow while the bike is underway.

4) Though not specifically quantified, the author also mentions vibration is a Not Good thing for LEDs and of course most bicycle headlights are solidly mounted. By way of correlation (keeping in mind that correlation is not necessarily equal to causation and vice versa), it is the lights on my bikes most used on really rough/gravel roads and cross-country that have "aged to dimness" the fastest.

5) Another source of dimming over time is dust collecting behind the phosphor due to poor maintenance. A lot of bicycle lights are open on the bottom and it is possible for dust to enter the light chamber. I remember a post from several years ago in the Dutch-language Weraldfietser ("World Cyclist") forum that showed a photo of an insect on the inside of a Cyo lens. If there's an opening large enough for a bug to enter....

Interestingly, an LED self-dimming over time actually increases its service life, accounting for the phenomenal MTBS rating. However, that MTBF rating as well as the L70 rating is dependent in good part on the quality of the materials. Not all phosphors, circuitry or packaging is of the same quality and this affects lifespan as well. A cheap LED just won't last as long as one assembled with care from top-quality materials. I've swapped some newer, high-quality LEDs into old fixtures and noticed an immediate positive difference. This is common practice on the Candlepower Forums and gives new life to old lights. An LED can also die not only from LED failure but from failures in much shorter-lived subsidiary components like driver circuitry or battery contact corrosion caused by water entry or leaky batteries. If the light stops working, then it is broken and not generally repairable so the end effect is the same as if the LED had failed.

I've also found LED colors shift over time. Some white LEDs become noticeably more blue or brown or yellow, shifting noticeably away from some color temperature of what we call "white". I've also had LEDs develop darker spots in the middle, creating a sort of halo effect that can be very annoying, especially in LEDs used for extended reading.

All the above -- plus the rapid obsolescence/superior replacement cycle of bicycle lighting -- account for my reluctance to spring for a more expensive light like the Edelux which plainly is made of materials far superior to the B&M Cyo series, yet uses the same optics and produces the same lumens. I just can't justify the higher cost of a "lifetime" light I don't plan or want to(!) keep for more than 5-6 years, tops. In terms of value, some iteration of the Cyo is "good enough" for my needs and as we all know, there are already models in the Bumm line that are far superior to even the second generation (II) Edelux.

Here's a few more related links in case you're interested in the dimming aspect and L70 ratings of LEDs. ...
https://www.digikey.com/en/articles/understanding-the-cause-of-fading-in-high-brightness-leds
https://blog.amerlux.com/color-shift-why-it-happens-in-leds-and-the-simple-way-to-avoid-it/
https://hackaday.com/2019/02/05/what-happened-to-the-100000-hour-led-bulbs/

The L70 rating does have relevance to those who wish to keep their LEDs bike lighting on all the time as a safety measure. Sure, it may limit the effective lifespan of the bulb in terms of brightness, but for me the safety aspects far outweigh the replacement cost, which is really not so much over time. As with most electronics, something better or brighter or with a nicer beam will be along shortly.

Relevant to all this, I was recently asked by an LED (wearable) headlight maker to test their latest product. It works very well but uses an embedded rechargeable battery that is not user-replaceable and there is at present no factory recycling program. The 3-year warranty covers only material or manufacturing defects. All other claims are excluded making the effective life of the LED irrelevant past the warranty period and failure most likely dependent on the chargeable lifespan of the battery in the sealed case. Three years' heavy use might not be long enough for the LED to dim noticeably, colors to shift, or for it to fall prey to a primary failure mode. It has, however, already developed a worsening "halo effect" that makes it annoying to use over extended periods. Nothing is perfect!

Best,

Dan.

Andre Jute

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Re: LEDs last a long time, but...
« Reply #1 on: May 24, 2022, 10:37:31 pm »
Wow, what a handsome job you’ve made of that report, Dan. I knew about electronic devices losing potency over time from designing tube amps with vintage tubes, in which high heat is the unavoidable enemy and eventually the killer of the tube, especially in my HIGH-concept designs, operated at high voltages at high currents into high loads. But of course heat dispersal is related among other things to radiating area, so the small radiating area of modern LED bicycle lamps, and the requirement for light weight, means that even tiny LEDs are in danger of having their function degraded by heat. And the halo effect, as the inverse of a hot spot and no doubt equally irritating.* Bit of a downer after the glee of a component that brags 50,000 hours MTBF.

I hope that otherwise you had a good day!

*I found an early instrument lamp from the days when none of the handlebar readouts were self-illuminated and fitted new batteries to it, and lo and behold, it had the beginnings of the halo effect; it saw years of use when I rode almost every night. Then I picked up the kitchen torch, another early LED tool and heavily used to go outside daily to feed the stable cats, and switched it on, and lo and behold, it threw a pure black spot in the middle of a ring of light. Halos everywhere ever since I joked about St Peter driving a Range Rover rather than bicycling. Enough to make you paranoid.

Danneaux

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Re: LEDs last a long time, but...
« Reply #2 on: May 24, 2022, 10:53:39 pm »
Quote
Halos everywhere ever since I joked about St Peter driving a Range Rover rather than bicycling. Enough to make you paranoid.

 ;D

All the best and thanks for the kind words, Andre,

Dan.

martinf

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Re: LEDs last a long time, but...
« Reply #3 on: May 25, 2022, 06:38:52 am »
There is also the fact that the LEDs are only one part of the package that makes up the lamp. It can fail by other means (wiring, switch, components soldered to the PCB inside, this is probably not an exhaustive list).

Switches were the weakest link on earlier B&M headlamps/rear lamps. But so far I have had good luck with their CYO range that use rotary switches.

On the B&M "Line" range of rear lamps the "standlight" capacitor can come apart from the PCB. I have had this happen twice, so far only on Brompton folding bikes, where there is even more vibration because of the small wheels. It is possible to repair this failure, but it means opening the case, which cracks it as it is sealed. I reseal with a "Shoe Goo" type compound, so the lamp looks a bit tatty afterwards, although function is restored.