LED Apparent Brightness

1. Human eyes respond to average illumination above the flicker rate.

1. LEDs have a most efficient current; past that peak, they're less efficient.

John

John's right. At 80 mA, you get way less than 4X the instantaneous luminosity. There's no mystery about it.

Now, if you kept the current at, say, 20 mA, and pulsed it, then the luminosity should decrease proportionally, but our eyes have a logarithmic response (pretty much the same as all of our other senses), so "half as bright" could be very much different from "half the power".

Hope This Helps! Rich

RST Engineering (jw) wrote: (snip)

Yes. They tell you how to drive the LEDs with pulses, not to increase the perceptible brightness, but because that technique is necessary to multiplex LEDs to save driver hardware when lots of LEDs must be driven. Think large screen TV and message boards. These days, the best you will see about brightness is that pulse driving up to some multiplexing factor does not degrade the brightness by more than X%.

At well above the frequency at which flicker is perceptible, the eye averages the brightness. So, if the LED is linear and has no internal resistance (ie. to a first approximation), it's a wash.

dance as

say

time

In one of the agilent app notes they say it is always more efficacious to drive their leds with a puredc current rather than modulate it.

Not to change the subject (never!) but here in San Francisco, at least half of the LED traffic lights seem to be failing. It's almost always the green, but I did just see a bad red. Strangly-shaped blobs/rows/polygons within the array go black, often accompanied with other regions that flicker weakly. On Van Ness Avenue, maybe 70% of the green lights are defective.

As a cost saving measure maybe they should use incandescents.

John

Don Klipstein wrote: (snip)

I wish they would lengthen their life by dimming them in proportion to the ambient light level. The green ones around here (central Virginia) are very annoyingly bright at night. If it is foggy or drizzling rain, they light the mist on the windshield so bright you can't see past it.

There is a bit of a myth that the human eye to some extent is a peak detector even if the flicker rate is high enough for the light to appear continuous. This is not true. If it was true, then there would have been plenty of effort put into pulsing fluorescent lamps, mercury lamps, metal halide lamps, etc.

What is true is that many LEDs are nonlinear, and back in the 1980's it was noticed that the type of LED chip mostly used in digital displays then were much brighter with high instantaneous current than with low instantaneous current with the average current being the same in both cases. A display with a few digits and 7 segments per digit and two or a few LED chips per segment eats a lot of current, even with only a few mA per chip. It was noticed that with only a couple mA average current per chip, they were brighter when pulsed with higher instantaneous current than when fed steady DC at the same low average current. Many people believed that a quirk in human vision was responsible, but actually the reason was that these LEDs were less efficient at low instantaneous current.

They actually recommend pulsing some of their LEDs to achieve instantaneous current of at least 10 mA if average current is less than 10 mA.

Some of their LEDs with a particular chemistry have degradation rate not only a function of junction temperature, but also a function of duty cycle, proceeding only while current is flowing (or maybe if voltage is across the junction). I suspect some sort of electrochemicaly motivated diffusion. I also suspect that some of their LEDs may have minor flaws, probably mainly as a result of aging, causing excessively low efficiency (or maybe no output at all) when instantaneous current is low.

Another thing: If you have LEDs with adjustable brightness, especially in an application involving color mixing, it is often recommended to vary duty cycle rather than current. This is because LEDs have performance specified at a particular instantaneous current, and variations in color and in efficiency from one lot to another or from one piece to another can be greater when current is greatly different from that which performance is specified at. Some LEDs have color varying with instantaneous current. In addition, different chemistries have different nonlinearities

- most InGaN blue LEDs have efficiency maximized when current is somewhere around 10-20% of maximum rated, while most red LEDs have efficiency maximized when current is at least 30% sometimes over 50% of maximum rated.

- Don Klipstein ( snipped-for-privacy@misty.com,

Or find out what green units are used in Upper Darby, PA. They have gone a few years already quite well. I have not noticed any failures.

I have seen a few reds in Philadelphia with partial failures, but they could be less-good models (Philadelphia tried several different models, at least in pilot programs on a few streets) and also most of these could be about 10 years old already.

A modern red or green LED traffic signal consumes typically about 12 watts maybe a bit less nowadays, and the incandescent it replaces is probably a 92 watt or 116 watt one.

- Don Klipstein ( snipped-for-privacy@misty.com)

Instead of trying again, when your preconcepts are not met, wouldn't it be better to drop or modify your expectations?

If you would have listened to the advice all this unnecessary experiments would have been saved. Can you see how your belief (in an urban myth) completly overrides even the outcome of your own experiments? You seem to be unable to work scientifically. An intelligent person continuously questions his beliefs, in fact he is clear about where his opinions come from and doesn't identify with either side. Truth is so powerful, that it will win ultimately, so why extend this period unnecessarily?

Even if you meant that just as an empty phrase, it hits right on the spot.

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ciao Ban
Bordighera, Italy

Excuse me? I got three or four answers in s.e.c., none of which spoke directly to the question. This morning, a week and a half after I asked the question, Don Klipstein (thank you, sir) answered the question with some intelligent comments, which I take to heart.

Perhaps your newsreader got some posts mine didn't. Or perhaps you had best lay off the schnapps before posting. I'm not sure which. I got nothing of benefit from s.e.c..

No, it meant exactly what I said. I did an experiment. It didn't turn out like what I was led (you should pardon the pun) to believe would happen. Klipstein explained it elegantly. Case closed.

Jim

I've been a regular on this ng for about 8 years and don't remember this subject being discussed, at least not in detail.

I perused Don's site for about two hours before I asked these questions. I just went back and reread every word on the page you referenced below. Perhaps you would be so kind as to point out the error in my reading and give me a couple of words from the paragraph that answers my question directly. I'll go back and see how I could have missed the subject twice.

It is only an urban myth if you don't know any better. If you are designing something and it doesn't come out the way you expected it, it is a mystery. I don't mind solving my own mysteries, but I also choose not to reinvent the light bulb.

Jim

Which speaks directly to white LEDs, but not to LEDs in general. And what is "slightly" more efficient? If I can put in an average 25% more power by pulsing and the LED goes 1% less efficient, I've achieved a 24% gain with no more battery power consumed.

Same argument. Back to the question of what "slightly" means.

"efficiency" can be measured in a whole bunch of ways. If lamp output goes down a couple of percent in efficiency with more current, but the current can be delivered more efficiently, then total efficiency can be discussed as lamp output as a function of total power taken from the driving source. That was my original question, but Klipstein elaborated to the point that I think I understand that lamp efficiency falls faster than source efficiency when you climb above nominal current. At least that's what I THINK he said.

Then we get into the question of what "nominal" current means and how it is specified. I don't think I've seen that number on any of the data sheets I've perused, be it HP, Kingbright, Toshiba ... or Mouser. THere are a lot of "recommended" currents, but what is the basis for recommendation?

Too many questions, not enough answers.

Jim

Jim, this theme has been discussed to death in this NG, always the same urban myth, maybe it was not you, I also didn't check your posts in other NG. Don maintains his patience and he also maintains this nice site where he explaines the second myth about higher efficiency

Good source of information from an unbiased technical person.

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ciao Ban
Bordighera, Italy

RST Engineering (jw) wrote: corrected top posting:

"Most such white LEDs will be slightly more efficient when moderately underpowered and will be less efficient when overpowered."

"Another significant difference is that incandescent lamps operate much less efficiently when the filament is not as hot, while most LEDs used in flashlights ususally operate slightly more efficiently than normal at reduced power."

These two sentences say the highest efficiency is measured with a little less than the nominal current, so when you try to increase the current the efficiency goes down. Maybe you didn't understand it this way? There is a lot of useful information on these pages, but not written in spectacular but rather technical manner.

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ciao Ban
Bordighera, Italy

Perfect. Thanks.

Jim

Two come to mind without much thought:

1) A means of varying apparent brightness without changing supply voltage or limiting resistor. 2) A means of achieving similar apparent brightness with a multiplexed drive scheme as compared to steady state.

and the page referenced below was:

That one has no direct link to my page that mentions pulsing and this business about it supposedly making LEDs appear brighter. My page that discusses this is:

My lede.html page has a link to my LED top page (ledx.html) which has a link to ledp.html, but this is roundabout and I do consider it possible to spend 2 hours reading the other stuff (including linked pages) before getting that far unless you read all of ledx.html before following any links.

- Don Klipstein ( snipped-for-privacy@misty.com,

AKA

Huh?

Sorry, your logic sucks. Electrical Power is current multiplied by voltage.

Have you really looked at a data sheet? Maybe you should read again Dons pages how efficiency is defined.

All answers are there if you would have understood the basic principle of efficiency.

--
ciao Ban
Bordighera, Italy