Blue LEDs - Amazing Fact

[SharpEars]

I just put some voltage into a blue LED and got it to glow. "So what?" you say.

Here is the amazing fact: The glow is visible at a current of a mere 500 nA, that's right as in hundreds of nanoamperes (at approx. 2.26 V of forward voltage). In fact, in a dark environment, even less current will make the thing visibly glow. I find the fact that it can produce a visible glow at potentially less than a microwatt of power to be unbelievable. I was at the border of my measurement equipment's capability just trying to measure the current requirements of the darned thing.

Don't try this with a red LED (you'll be lucky to see glow at hundreds of microamps - a full three orders of magnitude higher).

[ataradov]

Cree RGB LEDs for outdoor displays (http://www.digikey.com/product-detail/en/CLV1A-FKB-CJ1M1F1BB7R4S3/CLV1A-FKB-CJ1M1F1BB7R4S3CT-ND/1987488) are also lighting up at about the same current. They are extremely bright when driven properly, but as bright as normal indication LEDs at very low currents.

[ajb]

I was wiring up a little test fixture a while ago with some random red and green LEDs I had on hand.  From 5V, I think I finally settled on something like 1k for the reds and 47k for the greens to get them to a reasonably even non-blinding brightness.  That's around 40uA for the greens, I'm not sure how low they would go, but it's really ridiculous how much light you can get with such little current.

[georges80]

I have a nice 1 li-ion torch with an XML in it. I have a tritium vial on the outside to help locate it at night. A few years ago I installed an 0805 sized blue LED into the head so it can be 'seen' next to the XML. The blue LED is direct driven from the li-ion cell through a 100k resistor. Works great. Total drive current is <10uA so effectively no issue in terms of long term runtime issues

cheers,
george.

[firehopper]

dont forget that our eyes are very sensitive to green, so that may be why green leds seem to be brighter than other leds.

[SharpEars]

dont forget that our eyes are very sensitive to green, so that may be why green leds seem to be brighter than other leds.

... and are very insensitive to blue, making the <500 nA of drive current even more incredible.

[amyk]

Mike says he could see a red LED down to 500nA and a white one to 15nA(!) in this video:

https://www.youtube.com/embed/CIwlyy_KpXo

...and then shows a photomultiplier detecting the light at tens of pA.

[mikeselectricstuff]

dont forget that our eyes are very sensitive to green, so that may be why green leds seem to be brighter than other leds.

... and are very insensitive to blue, making the <500 nA of drive current even more incredible.

A white LED may be more visible due to the better wavelength for the eye, though the more diffuse source due to the phosphor may counteract this to some extent. 

[pullin-gs]

Off-topic (sort of) factoid:
Do you know why the telescope operators use red LED lights to read their maps/etc when they are out with their telescopes at night?
True, our eyes are more sensitive to green (thus you would think green is preferred).
However: They use red because the human eye's iris reacts the least to red....thus when the light is needed to read a star map or find an eyepiece, the light will not cause their "night vision" sensitivity to detail to be compromised.
It can take 30 minutes or more for a telescope operator's eyes to totally acclimate to the dark.   

You would totally be schlonged (Dont you love the election season?) in that would would have to wait for 30 minutes to catch a good view of a DSO such as the Messier 101 pinwheel galaxy (or other DSO) if you used a green light to find that 8mm eyepiece.

v/r,

P

[SharpEars]

Mike says he could see a red LED down to 500nA and a white one to 15nA(!) in this video:

https://www.youtube.com/embed/CIwlyy_KpXo

...and then shows a photomultiplier detecting the light at tens of pA.

I should have probably mentioned that I used a regular non-superbright wide viewing angle diffuse blue led (i.e., not clear) which even at a full 20 mA is not all that bright compared to modern bright LEDs.

[calexanian]

I have observed this same thing. some of the LED bulbs I have in my house stay luminescent for several minutes after turning them off from what I assume is the residual charge on the filter capacitors staying just barley above the voltage of the forward voltage drop of the led.

[CatalinaWOW]

At low light levels, blue colors are at the peak of human vision response, while reds are nearly invisible.  Check the attached curve.  The key is the Scotopic curve, which applies at low illumination levels.  (Think moonlight or less).  At normal illumination levels the Photopic curve applies, and blue doesn't do so well.

[matseng]

At low light levels, blue colors are at the peak of human vision response, while reds are nearly invisible.  Check the attached curve.  The key is the Scotopic curve, which applies at low illumination levels.  (Think moonlight or less).  At normal illumination levels the Photopic curve applies, and blue doesn't do so well.

Now that(!) is an amazing fact.... 

[T3sl4co1l]

Let your eyes adjust to darkness and you'll be able to see 10x lower current, or thereabouts.  Which I believe is down in the endothermic range for most LEDs (i.e., it gets ever so slightly cooler as it produces light, because Vin < h*c / (e*lambda)).

Tim

[German_EE]

Years ago a friend of mine built some crystal sets that used an LED as a detector. When tuned to a strong signal he described the LED as "like the cigarette of a sentry glowing in the dark"

[dom0]

I have observed this same thing. some of the LED bulbs I have in my house stay luminescent for several minutes after turning them off from what I assume is the residual charge on the filter capacitors staying just barley above the voltage of the forward voltage drop of the led.

I noticed this, too, with some power LEDs. I think it is some kind of fluorescence of the phosphors and not related to residual currents, because I was able to observe it on the lab bench with the power LED inputs both grounded :-)

It also is dependent on how bright and how long the LED was on.

[Zero999]

I have observed this same thing. some of the LED bulbs I have in my house stay luminescent for several minutes after turning them off from what I assume is the residual charge on the filter capacitors staying just barley above the voltage of the forward voltage drop of the led.

I noticed this, too, with some power LEDs. I think it is some kind of fluorescence of the phosphors and not related to residual currents, because I was able to observe it on the lab bench with the power LED inputs both grounded :-)

It also is dependent on how bright and how long the LED was on.

I've also noticed this.

It's definitely the phosphor because I tested it with an LED which hadn't been used for days. I excited the phosphor with a blacklight and it carried on glowing when the light was turned off.

[Alex Nikitin]

One more interesting fact about blue LEDs - the light capabilities at low currents will suffer and the light output will be noisy if the LED sustains a static damage. I've designed in the past a very high dynamic range (over 100000:1) controlled light sources for PMT calibration with a blue LED and a closed-loop photodiode control for the light output. You have to handle these LEDs with all anti-static precautions, otherwise they may become unusable for such an application, very noisy and non-linear at low currents.

Cheers

Alex

[SharpEars]

At low light levels, blue colors are at the peak of human vision response, while reds are nearly invisible.  Check the attached curve.  The key is the Scotopic curve, which applies at low illumination levels.  (Think moonlight or less).  At normal illumination levels the Photopic curve applies, and blue doesn't do so well.

This is a bit misleading. The scotopic curve is actually centered around green, not blue.

The green curve in the following diagram is the scotopic one:


Look at the visual spectrum to see the actual colors represented by the wavelengths. The blues in the 450-465 range are pretty low on the above curve, compared to the greenish peak:

[Fungus]

Here is the amazing fact: The glow is visible at a current of a mere 500 nA

500nA is still over 1,000,000,000,000 electrons per second... each one is a possible photon.

[CatalinaWOW]

At low light levels, blue colors are at the peak of human vision response, while reds are nearly invisible.  Check the attached curve.  The key is the Scotopic curve, which applies at low illumination levels.  (Think moonlight or less).  At normal illumination levels the Photopic curve applies, and blue doesn't do so well.

This is a bit misleading. The scotopic curve is actually centered around green, not blue.

The green curve in the following diagram is the scotopic one:


Look at the visual spectrum to see the actual colors represented by the wavelengths. The blues in the 450-465 range are pretty low on the above curve, compared to the greenish peak:

What is probably more misleading is that some manufacturers call an LED at 505 nanometers "greenish blue" which is where I got the long wavelength end of the blue range.   Most people would probably call that a green with a touch of blue.   It is still true that under low light conditions many "blue" LEDs will get a stronger eye response than many "red" LEDs.  at 460 nanometers, what I would call a true blue the low level eye response is about 60%, while the red response is somewhere around 1 or 2 percent.  What most people would call green is at about 540 nanometers and is not at the peak under either high or low lighting conditions, but is very high on the curve.  In the intermediate, or mesopic range the eye response is some sort of blend of the two, and green is very near the peak of that response.

All of this is very dependent on the exact LEDs used, the viewing conditions, and how long the eye has accommodated to the light level.  It helps to put color spectrum on same scale as the eye response plot when visualizing all of this.

[T3sl4co1l]

Still, if that curve is in linear scale, then that's only -3dB, a very modest visual difference.

Speaking of cheating colors, there's "high efficiency red", which most people would still call red, but they're noticeably orange, and thus that little bit brighter visually.

Tim

[T3sl4co1l]

Here is the amazing fact: The glow is visible at a current of a mere 500 nA

500nA is still over 1,000,000,000,000 electrons per second... each one is a possible photon.

At about a 20% probability (hence the typical efficiency of LEDs!); it would be 100%, except that:
- Some electrons are shunted through impurities or defects (especially at low currents and with shitty dice)
- Some recombine (spontaneously, or more likely at impurity sites) without emission
- Many photons get stuck inside the crystal (which, remember, has quite a high index of refraction, and the photons are created in all directions)
- And a few get stuck in the package/lens, or reflect back to the die

The first one is very apparent in circuit.  Very cheap dice/emitters from China contain bargain-bin or rejected parts, often due to high leakage.  These defects typically behave ohmic (~ohms to Mohms), so prevent emission at low currents (the voltage isn't high enough), but are not so great that operation at rated current is severely impaired (i.e., emission might be half or worse, but you probably already guessed that from the sticker price, riiiight?).

This is why, in a series-parallel array type emitter, you can get different intensities within a given string, at low current.  If the dies were good quality, all the parts in a string would light evenly, as you would expect (you'd still expect variation between series strings though).

Tim

[Zero999]

Ignoring the sensitivity of the human eye, the efficiency of LEDs is greater at either extremes of the visible spectrum. Deep red and blue/indigo LEDs are more efficient than orange, yellow and green LEDs.

Higher efficiency yellow and green LEDs which use blue LEDs and phosphors are being developed.
http://phys.org/news/2009-07-yellow-gap-full-conversion-blue.html
http://www.compoundsemiconductor.net/article/91299-cranking-up-the-efficacy-of-green-leds.html
https://www.osapublishing.org/view_article.cfm?gotourl=https%3A%2F%2Fwww.osapublishing.org%2FDirectPDFAccess%2F27C75349-BD7A-D82E-896CD0A86BEB6A6F_199457%2Foe-18-11-11063.pdf%3Fda%3D1%26id%3D199457%26seq%3D0%26mobile%3Dno&org=

[BradC]

It's definitely the phosphor because I tested it with an LED which hadn't been used for days. I excited the phosphor with a blacklight and it carried on glowing when the light was turned off.

The fluoro over my desk does the same thing to the screen on my scope. Eeerie to see it glowing after I turn out the lights.
Yes, all the diffuse led bulbs in our house have the afterglow too.