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Blue LED fades to teal/blue-green?!

A project log for today's assorted project ramble "grab-bag"

Assorted project-ideas/brainstorms/achievements, etc. Likely to contain thoughts that'd be better-organized into other project-pages

esot.ericesot.eric 01/12/2020 at 02:2112 Comments

UPDATES At bottom!

I NEED TO KNOW MORE!

This is a $1 Car->USB-power adapter. When powered, the LED is a piercing blue, but when power is removed, the capacitor discharges, I'm almost certain I see the blue LED CHANGE COLOR as it dims, from a piercing/pure blue to a "warm" blue-green/teal.

How can this be?! Isn't the wavelength dependent on the material?

If this LED were supplied with the same average voltage/current, but applied instantaneously via PWM, would it glow blue or teal? Is its color a factor of instantaneous voltage/current, or something to do with "warming up" like a filament bulb? Or is this just an illusion?

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UPDATE;

In this second vid we have the blue->greenish LED now exposed without shining throug translucent housing, and another identical device which to my eyes does not green-shift at all. Oddly, the camera [and/or my phone-screen?] shows a distinct greenshift for that one. The original greenshifting one does, still, appear to my eyes to be doing-so.

I've a vague idea regarding a brief dropout of the switching regulator when power's removed that could cause a brief voltage-spike causing a green-shift that maybe latches until the power drains completely[?], but doesn't increase in brightness since it doesn't recharge the capacitor[?] More on that in comments, below.

Thanks for the thoughts, y'all! This is intriguing, maybe I'll get more sciencey about it!

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UPDATE 2

This article is mostly about white LEDs, but mentions wavelength-shift due to PWM duty-cycle [and concluding due to heat]. Could be relevant-ish, plausibly.

https://www.digikey.com/en/articles/techzone/2014/feb/led-color-shift-under-pwm-dimming

Though, that shift [toward blue, with decreasing duty-cycle/heat] is tiny compared to what I see.

I tried some experiments with a DVD as a prism of sorts. Hard to capture with the camera, but I'm surprised how wide the "spectral width", as to my eye it clearly looks to cover the whole range from green to blue. There's even yellow and red [though dim]. I really thought LEDs were quite a bit more narrow in their spectra.

Still pondering.

Also, still haven't found much regarding peak-wavelength vs. current, *except* in overcurrent conditions.

Discussions

Dr. Cockroach wrote 3 days ago point

Glad your researching that instead of me dude. I have noticed the effect with my white/bluish Leds as well but never gave much thought :-)

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esot.eric wrote 2 days ago point

Not sure if I mentioned; I saw a page claiming white LEDs have a similar phenomenon but for a different reason; apparently the blue light from the LED emitter does double-duty: it both excites the "white" phosphor And shines through it. So, they're designed for a certain brightness where the light produced by the actually red/yellow/green phosphor matches the amount of blue that escapes. Kinda wild, actually. 

And, now that I think about it, maybe a nice discovery, I much prefer my White LEDs 'warm',  but they're harder to find, maybe shifting the PWM duty-cycle or driving current [and brightness, obviously] would allow for that, hmmmm....

Research, heh! I kinda dig the idea of a very subtly two-hue display, if it's possible this way. But, as I've seen, it's very likely to be quite subjective; between my eyes' sensitivity and the camera's and the display's gamma[?]. Seems, though, there could be other uses, e.g. in spectroscopy?

Thanks for voicing the possibility I'm not alone in this observation!

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Ken Yap wrote 5 days ago point

>I'm almost certain I see the blue LED CHANGE COLOR as it dims

Anecdotal and subjective. Spectra vs t plots or it didn't happen.

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esot.eric wrote 3 days ago point

LOL, no problem, I'll dig out my spectrum analyzer after lunch.

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Gerben wrote 7 days ago point

1. Could be over current. Led give a different color when pushed too hard. I've overrun leds myself, and was amazed how long it took before they burned out. Since this leds is only on for short intervals, it might survive being run too hard.

2. It could be that they don't use PWM, but lower the voltage to dim the led. If there is a green and blue die inside the led, the blue dims sooner than the green one (I checked that on one of my RGB leds). That is, the green led is brighter that the blue led at a lower voltage.

3. in the video it appears that the led is behind some white piece of plastic. If might be the phosphorescent in the white plastic that changes the color a bit. At high brightness this might not be as noticeable, but becomes more noticeable at lower brightnesses. Try shining some UV light on the plastic, and see if it phosphoresces.

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esot.eric wrote 6 days ago point

Good points! the funny thing is, this is a very cheap switching-regulator, so I think the LED is powered through a resistor at the 5V "regulated" output [with a capacitor for smoothing]. The effect occurs when removing power to the regulator. So, no PWM *normally*. 

BUT: you might be on to something! My theory goes: as the switching-regulator loses input power, it stops regulating properly, so e.g. the switching transistor may keep the inductor charging longer than usual, then when it turns off the voltage might surge higher than usual, very briefly, causing the green-shift, but not enough stored energy to recharge the capacitor for a *brightness* increase. 

Maybe the "green-shift" remains after the initial surge, until powered-down?

hmmm, green and blue dies could very well be a thing; dunno why they'd do that, but why not, eh? The turn-on voltages differ, so it'd be a voltage-controlled bi-color LED. [Good idea, doable with RGB too... low-V = Red, medium-V = yellowish, high-V = white-ish. Interesting!]

Phosphorescent plastic's a great point I hadn't thought of. There's a new vid I'm about to link that I took off the plastic from one and left it on on another. It wasn't a scientific-methodologically experiment, but may be informative.

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sjm4306 wrote 01/14/2020 at 13:12 point

LEDs don't emit a singular wavelength but have a bandwidth centered around their rated color. My guess is that changing the current affects the predominant wavelength emitted (as such the graphs in an LED's datasheet are specific to the specified test current).

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esot.eric wrote 6 days ago point

yahknow, I don't think I've ever even thought to look up an LED datasheet. Am curious, now!

Am also curious about the predominant-wavelength shift idea; e.g. most displays, I think, use pwm and a specific on-current. Then e.g. gamma correction could maybe be more accurate if the current could be varied. Hmmm

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Florian Festi wrote 01/13/2020 at 20:35 point

I read somewhere a few years ago that LED color can slightly change with current and that's the reason to use PWM as this will keep the color like on 100% current but for any (mean) intensity. AFAIK this is not a temperature thing. I also don't remember is this was about monochrome LEDs or white ones or both. Sorry, for not providing more details or some references.

But it is also possible that this is an optical illusion. Although the human eye has  receptors for blue the green receptors have a pretty wide sensitive range. So it is quite possible that the eye changes the preceived color if intensity levels change dramatically (over saturate).

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esot.eric wrote 01/14/2020 at 03:35 point

Cool, thankya! This is a difficult topic to search for, since color-changing LEDs are a thing [multi-die]. I was able to find something about white LEDs, something to do with the phosphor's ability to respond, so by applying a different current some blue may bleed through, or some amount of blue is expected to bleed through, so under-current may look "warmer" [which, actually, could be quite handy, now that I think of it, thanks!]. Stupidly, I didn't keep a bookmark.

Still wonderin' about monochrome leds, hmm. This dramatic of a color shift would probably be documented in many places :/

And, yeah, illusion makes sense, and camera sensors are probably designed to mimic human-perceivable wavelengths, too.

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One thought was that since this is the output of a switching-regulator, as the power's removed, the switcher circuitry may drop-out, plausibly charging/discharging the inductor differently [unregulated], which might result in the output voltage surging briefly; and I definitely found several sources saying over-voltage/current[?] can result in color-shift [usually to a longer wavelength, like here] but that it's also damaging. [And, it seems mine may've blown]

Hopefully such surges are handled well by phones/tablets/etc.

Thanks for the insight!

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Starhawk wrote 01/12/2020 at 05:38 point

Eric, my friend, you find the most gloriously interesting things, now, don't you...? ;) the last time I saw an LED change color like that, I was watching a DVD burner drive die spectacularly -- a victim of the eBay-issue "molex power supply" it was being driven by. About 90min of TLC with a utility knife later, I was able to find that the supply had blown an electrolytic cap inside and probably put a rather "shockingly" high voltage to the poor drive... whose read/write LED had glowed visibly red through the plastic cover of that part of the drive as it died what I suspect was a rather tortured and unpleasant death indeed. (At least it didn't last very long... 5sec or so.)

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esot.eric wrote 01/12/2020 at 06:46 point

ah yeah! Overpowering LEDs is pretty interesting! I've seen yellow from red LEDs; but these things kinda make sense, things be *hot*, atoms/electrons be over-excited, materials maybe even melting, or even molecular changes...

But, this guy... I can't explain.

Imagine RGB LEDs that only have two pins, one die, mind blown.

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