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