Opinions on LED spectrum shift

[b4tn]

Well, somehow this posted with no content. Our interweb is acting funny today.

Basicly I have not been able to find any concrete proof that spectrum shifts in LED's . Some have told me that after 4 years you should replace diodes but others have said keep runnig them till they burn out. I have a set of OR T247's going on 4 years now with no burnt out Diodes. When I opened them up the drivers are not driving the LED's to full power so I dont think over driving them would be an issue. What is everyone else opinion? Replace diodes every few years? Or run them till they burn out?

 

[Dana Riddle]

Figure 10 in this paper looks at output of a white LED at about a year (based on a 12 hour photoperiod):
https://ieeexplore.ieee.org/document/7950923
It appears that intensity is more of an issue than spectral shifting. With that said, degradation ('yellowing') of plastic lenses when exposed to UV and violet light is a real issue and will certainly cause a shift in spectral quality.

 

[jda]

The truth is that nobody really knows yet, with regard to actual usage over a reef. There are too few people who have even four year old fixtures running. Most people running older fixtures are using hand-me-downs. PAR meters were not a thing back when these units were new. Anything with phosphors will degrade - diodes too.

This is just anecdotal, but I believe that just like Mercury based bulbs, both output and spectrum will shift exponentially with age. One year? Certainly not an issue. Five? Ten? I have no idea. The good thing with some LEDs is that if it is just output, then most people can turn them up. What you can learn from every other light source is that illumination and effectiveness are not the same thing. 20k Radium, for example, has a 10 year life (about) from the manufacturer for general lighting, but it is good for about one year over a reef.

 

[casper320]

Light Emitting Diodes are fabricated using techniques called bandgap engineering. What this does is it manipulates the energy bandgap of an indirect bandgap material (GaAs instead of Si). The amount of energy necessary for an electron to move from the valence band to the conduction band is known as the energy bandgap. As the electrons recombine into the valence band, energy is lost, this takes the form of either phonons (heat) or photons (light). This is why silicon loses energy due to heat, and LED's emit light as a form of energy loss. The material used determines the bandgap, such as AlGaAs, which in turn determines the frequency/wavelength at which electrons recombine into the valence band and determines spectrum. The depletion region of the P/N junction of the diode will eventually degrade causing lower intensity, but the spectrum should stay roughly the same.

 

[b4tn]

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[b4tn]

I am of the same opinion that if anything shifts it would be intensity. Thanks for the info.

 

[dantimdad]

Light Emitting Diodes are fabricated using techniques called bandgap engineering. What this does is it manipulates the energy bandgap of an indirect bandgap material (GaAs instead of Si). The amount of energy necessary for an electron to move from the valence band to the conduction band is known as the energy bandgap. As the electrons recombine into the valence band, energy is lost, this takes the form of either phonons (heat) or photons (light). This is why silicon loses energy due to heat, and LED's emit light as a form of energy loss. The material used determines the bandgap, such as AlGaAs, which in turn determines the frequency/wavelength at which electrons recombine into the valence band and determines spectrum. The depletion region of the P/N junction of the diode will eventually degrade causing lower intensity, but the spectrum should stay roughly the same.

Welcome aboard @casper320 !

Very good explanation!

I was about to copy and paste an article that said basically the same thing.

 

[oreo54]

Keep em cool....;)
Keep in mind this is old and refers specifically to white diodes and mostly (above excluded) more or less instantaneous color shifts by heating.
https://www.digikey.com/en/articles/techzone/2013/may/thermal-effects-on-white-led-chromaticity

For example, the color of the light emitted from an LED also changes as the device ages, and if the device is dimmed using analog techniques then the device’s color can vary as the output is dialed down (see the TechZone article “Digital Dimming Solves LED Color Dilemma”). These small effects, combined with the temperature-induced chromaticity shift, can add up to a significant change in color – leading to consumer disappointment.

Heat is the LEDs enemy in both the short- and long-term. Not only will high temperatures detrimentally affect the color of the LED, it will degrade both the chip and phosphor, causing a rapid tail-off in luminosity, and eventual failure (defined as the point at which the LED delivers only 70 percent of its luminosity when new).

People "know" about diodes , what they donhttps://www.digikey.com/en/articles/techzone/2012/feb/understanding-the-cause-of-fading-in-high-brightness-leds't know about is fixtures...

In summary

The most common cause of failure for high-brightness LEDs is dimming over an extended period to the point that the light is no longer sufficient for its intended purpose. The primary cause of this loss of output is a reduction in quantum efficiency as dislocations in the chip’s crystal structure increase adding to the number of sites where non-radiative recombination can occur.

LED chipmakers are working hard to reduce the number of defects in the devices when new, but semiconductor manufacturing processes are not perfect and there will always some faults. The most important factor under the control of the design engineer that does influence longevity is junction temperature. Operating the LED according to the manufacturer’s thermal guidelines will reduce the rate of crystal degradation and ensure a long and bright operating life.

Again thermal stability is a current area that is improving..

for Friday fun:
http://pulsarlight.com/world-record-led-installation-longest-continuous-service/