Highest efficiency LED?

[Nca78]

@NeverDie said in Highest efficiency LED?:

flashing an LED for 5 mS to 10 mS is quite noticeable.

I checked in an old sketch for a board that was using a 0603 green led (cheap one so not even bright light) and I had a 20ms on time. But this was producing quite a strong light flash, easily noticeable in daytime through 2mm of white PLA (battery was CR1632 and resistor I think around 2K).
I think it's a combination on "on time" and the current the led receives, so you will need to experiment with the leds you buy, the "problem" is not really about a duration but of a quantity of light you manage to get from the LED compared to ambient light.

I don't know if you saw the answers to this question, but the experiment is quite interesting.

[NeverDie]

@Nca78 From what I've read, the eye averages the amount of energy it sees over some period of time. That's how the whole persistance of vision thing works with moving pictures. So, it may turn out that it doesn't matter so much whether I use high power for incredibly brief period or lower power for longer. Maybe it's all equal. I'm sure this has been well studied, but so far I haven't been able to google up the answer. I guess I'll just order a few different types and try some experiments. In the meantime, if somebody reading this knows the answer as to which approach is a more efficient use of energy, please do post. I'm wagering whichever green/lime LED has the highest lumens/watt will be the winner, and that everything else is secondary.

The guy in the post that @Nca78 linked to above said he could see a flash from a very ordinary LED that lasted just 1 microsecond, but with rather special conditions like cupping his hands around it to make a dark contrast and expecting the flash and staring right at it. So, maybe a much brighter LED (say 150 lumen) pulsed for one microsecond wouldn't require those special conditions. That would be pretty amazing, because then the energy consumption would be tiny. Somehow I'm both hopeful and skeptical at the same time.

[NeverDie]

It turns out there are two different types of peak eye sensitivity: scotopic at 507nm and photopic at 555nm:
https://en.wikipedia.org/wiki/Scotopic_vision#Wavelength_sensitivity
So, I need to get that sorted as to which one I need/want, so that I can buy some LED's centered on the appropriate wavelength. Or maybe I should split the difference and get something that overlaps both? Or perhaps actually get both and only flash the one that matches the circumstances. I think I'll pick photopic, because I probably care more about daylight vision than dark adapted vision.

[zboblamont]

@NeverDie Not so scientific, but am acutely aware even with peripheral vision when my nodes flash blue up to 4m away indoors even in artificial light, duration 5ms.

[NeverDie]

@zboblamont Do you happen to know the make and model number?

[zboblamont]

@NeverDie Sorry no information as not the constructor, it is on a pre-built board, now 3 years old.
The LED is a 1mm wide SMD driven on 3.3v via a 220R, but exceptionally bright compared to a second yellow LED on an identical circuit, probably from the same manufacturer.
My intended point was that blue may an option worth examining alongside the suggested green.

[NeverDie]

@zboblamont I've read that bright blue LED's can be harmful to the eyes, and that you can pretty quickly damage your eyes unintentionally. I think the explanation offered was that since the eye isn't particularly sensitive to blue, your eyes can suffer damage before the pupils close. Or, maybe because blue is so close to UV, and so you might get extra invisible dosing that way? Not sure.

Anyhow, I haven't read any similar warnings regarding green, so maybe they're safer.

[zboblamont]

@NeverDie I can't think why any particular tone in the visible light spectrum should harm the eye, else LEDs might be issued with a Health Warning ;)
Hopefully your experiments will identify what you're after...

[NeverDie]

@zboblamont Thanks. FWIW, I first became aware of the issue from reading CREE datasheets. At some point CREE put out an advisory: https://www.cree.com/led-components/media/documents/XLamp_EyeSafety.pdf

Cree classifies its own XR-E royal blue LED's as RG-3 High Risk, which it defines as "Hazardous even for momentary exposure".

"To date, the testing shows that Cree’s blue and royal blue LED components (450-485 nm dominant wavelengths) pose a higher potential eye safety hazard than its white LED components. Other colors of LED components, such as green and red LED components, do not pose as significant of an eye safety risk. Regardless of LED color, Cree advises users to not look directly at any operating LED component. "

In the context of this thread, it's that last sentence that's tricky. I suppose for an indicator common sense suggests it's just plain safer to stick with a lower power LED and run it for a longer duration. So, the challenge will be to find high lumens/watt but at not too high absolute lumens.

[NeverDie]

Closing the loop, I finally found the information I was looking for: "Also note from Figure 17 that — if the flashes are separated by at least 20mS — the brain ‘sees’ the individual flashes at full brilliance if they have a duration of 10mS or greater, but sees them at diminishing brilliance at durations below 10mS (a 2mS flash appears at roughly 1/5th of true brilliance; the perceived brilliance falls off rapidly at durations below 1mS). The perceived duration of a 20mS flash (30mS) is only 50% greater than that of a 10mS flash (20mS)." https://www.nutsvolts.com/magazine/article/practical-led-indicator-and-flasher-circuits

:)

[NeverDie]

Reporting back:

Testing some cheap aliexpress 3mm red or yellow through-hole LED's, I found they required around 10ua to have decent visibility. Other LEDs that I got from amazon required several hundred microamps to be equally visible. However, running this 25 cent CREE green LED at 1ua at around 2.5v was sufficient to be dimly visible: https://www.mouser.com/ProductDetail/941-C503BGASCB0F0792
Even 500na was enough, though at less than that it becomes dodgy. Part of the credit goes to narrowly focused optics.

Also, 10ms was enough. So, 1ua at 1Hz with a 1% duty cycle means just 10na on average. Not bad! The pulsing actually makes it easier to see than it just being constantly turned on, I presume because of better contrast.

I haven't yet tried it at night, but so far this is already better than what I was expecting a priori. Putting it at the bottom of a black tube might also be interesting to try: you'd get improved contrast, and with the narrow optics maybe little light would be sacrificed to the blackness of the tube

Anyone here found an LED that's more efficient than this one? Or is this already about as good as it's likely to get?