Author Topic: LED visibility optimization at very low currents  (Read 10912 times)

0 Members and 1 Guest are viewing this topic.

Offline jmajaTopic starter

  • Frequent Contributor
  • **
  • Posts: 296
  • Country: fi
LED visibility optimization at very low currents
« on: April 20, 2017, 08:48:22 pm »
What is the best way to drive a LED to be visible in the dark? What kind of LED should be chosen for this purpose? The target is to be able to clearly see is the LED on or off from about 5 m distance. And only during night when you can't see a mechanical output. The device is targeted to consume less than 50 uA and it will have four LEDs than can possible be all on. The user can only see two at the time (red and green).

Is it possible to reach this goal with max 10 uA/LED? Probably the LEDs could be blinked at ~1 Hz with 25-50% on time, which would help saving current at the same visibility.

Is it better to use PWM or a constant current drive for better visibility at the same average current? PWM may be a problem, since the CPU needs to sleep.

Does the size of the LED change the visibility? Is higher lm/W always better? lm/w is given at much higher current typically. Narrow viewing angle would help, but rather wide must be used.
 

Offline Audioguru

  • Super Contributor
  • ***
  • Posts: 1507
  • Country: ca
Re: LED visibility optimization at very low currents
« Reply #1 on: April 20, 2017, 09:03:04 pm »
With LEDs you get what you pay for.
My Philips Lumileds Luxeon Superflux LEDs are very wide angle and are very bright at low currents. The LEDs in my cheap Chinese solar garden lights have a very narrow angle and are not bright with the high current they use. I salvaged a few strings of outdoors LED Christmas lights and they have a wide angle and are bright at low current.   
 

Offline GeorgeOfTheJungle

  • Super Contributor
  • ***
  • !
  • Posts: 2699
  • Country: tr
Re: LED visibility optimization at very low currents
« Reply #2 on: April 20, 2017, 09:15:09 pm »
What hp did to save energy 40 years ago was to put a lens in front of very tiny leds: the bubble 7 segments displays google.com/search?q=bubble+7+segments+displays&tbm=isch

WRT the max 10 uA/LED, that indeed sounds like a very little amount of energy... not sure about that.

WRT "lm/w is given at much higher current typically" back then people used the LM3909 for that.
The further a society drifts from truth, the more it will hate those who speak it.
 

Offline wraper

  • Supporter
  • ****
  • Posts: 16796
  • Country: lv
Re: LED visibility optimization at very low currents
« Reply #3 on: April 20, 2017, 09:18:45 pm »
What hp did to save energy 40 years ago was to put a lens in front of very tiny leds: the bubble 7 segments displays google.com/search?q=bubble+7+segments+displays&tbm=isch
I don't think that lens had anything to do with energy saving. By using it, they could use one tiny LED die as a segment.
 

Offline mikeselectricstuff

  • Super Contributor
  • ***
  • Posts: 13695
  • Country: gb
    • Mike's Electric Stuff
Re: LED visibility optimization at very low currents
« Reply #4 on: April 20, 2017, 09:27:25 pm »
For maximum visibility, (Cool) white, small size, narrow angle if your viewing angle . match angle to required viewing angle. If you want to stand out from other lights, maybe deep green, as this is at the peak or the eye's sensitivity.
Where you want "noticeability" for emergency type applications, strobing can help as the eye is more sensitive to movement.
In principle, for a steady brightness, PWMing doesn't give any advantage in terms of perceived brightness, but efficiency at low currents (< about 5% of nominal current) can vary a lot, particularly with white, so if only for consistent appearance, a higher current pulsed at a low duty cycle can be beneficial.
   
Youtube channel:Taking wierd stuff apart. Very apart.
Mike's Electric Stuff: High voltage, vintage electronics etc.
Day Job: Mostly LEDs
 
The following users thanked this post: Someone

Offline jmajaTopic starter

  • Frequent Contributor
  • **
  • Posts: 296
  • Country: fi
Re: LED visibility optimization at very low currents
« Reply #5 on: April 20, 2017, 10:19:13 pm »
The  LEDs need to be green and red. Not an emergency application, just sensor information to the user.

The viewing angle must be wide, maybe 150 deg.

Is efficiency  better or worse at low current? Say 10 mA 0.1% PWM = 10 uA vs. constant 10 uA. Is there a big difference between these in lumens?
 

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 19345
  • Country: gb
  • 0999
Re: LED visibility optimization at very low currents
« Reply #6 on: April 20, 2017, 10:22:16 pm »
For maximum visibility, (Cool) white
Actually for maximum visibility in the dark, you need 498nm, the peak sensitivity of scotopic vision. Unfortunately LED efficiency (I'm talking about power in vs radiant power out here not luminous efficiency) drops, towards the middle of the visible spectrum, with a minimum around the greenish-yellow region, so it might be more efficient to use a slightly shorter wavelength or an indigo LED with cyan phosphor.
https://en.wikipedia.org/wiki/Scotopic_vision



The  LEDs need to be green and red. Not an emergency application, just sensor information to the user.
Red could be a challange, since the dark adapted eye is less sensitive to red, than any other wavelength.

Quote
Is efficiency  better or worse at low current? Say 10 mA 0.1% PWM = 10 uA vs. constant 10 uA. Is there a big difference between these in lumens?
Much worse with PWM. LED efficiency is generally poorer at higher currents.
« Last Edit: April 20, 2017, 10:26:46 pm by Hero999 »
 

Offline GeorgeOfTheJungle

  • Super Contributor
  • ***
  • !
  • Posts: 2699
  • Country: tr
Re: LED visibility optimization at very low currents
« Reply #7 on: April 20, 2017, 10:25:36 pm »
Is efficiency  better or worse at low current? Say 10 mA 0.1% PWM = 10 uA vs. constant 10 uA. Is there a big difference between these in lumens?

Yes because @10µA constant current it won't even lit I think. That's why the LM3909 exists.
The further a society drifts from truth, the more it will hate those who speak it.
 

Offline mikeselectricstuff

  • Super Contributor
  • ***
  • Posts: 13695
  • Country: gb
    • Mike's Electric Stuff
Re: LED visibility optimization at very low currents
« Reply #8 on: April 20, 2017, 10:29:20 pm »
The  LEDs need to be green and red. Not an emergency application, just sensor information to the user.

The viewing angle must be wide, maybe 150 deg.

Is efficiency  better or worse at low current? Say 10 mA 0.1% PWM = 10 uA vs. constant 10 uA. Is there a big difference between these in lumens?
Test it and see. If this is something for production, do NOT run the LED at 10uA as you have no guarantee of performance at this current level. 1% at 1mA or 0.1% at 10mA will be inside the range that the manufacturer characterizes and tests at, so brightness will be more consistent from batch to batch. I
For indication it looks like you will have the opportunity to strobe it as well to save power - maybe 7-20Hz at perhaps 5-15% duty - again experiment to see what works best. 

 
Youtube channel:Taking wierd stuff apart. Very apart.
Mike's Electric Stuff: High voltage, vintage electronics etc.
Day Job: Mostly LEDs
 
The following users thanked this post: Someone

Offline mikeselectricstuff

  • Super Contributor
  • ***
  • Posts: 13695
  • Country: gb
    • Mike's Electric Stuff
Re: LED visibility optimization at very low currents
« Reply #9 on: April 20, 2017, 10:30:33 pm »
That's why the LM3909 exists.
Existed. Long obsolete.
Youtube channel:Taking wierd stuff apart. Very apart.
Mike's Electric Stuff: High voltage, vintage electronics etc.
Day Job: Mostly LEDs
 

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 19345
  • Country: gb
  • 0999
Re: LED visibility optimization at very low currents
« Reply #10 on: April 20, 2017, 10:32:27 pm »
Is efficiency  better or worse at low current? Say 10 mA 0.1% PWM = 10 uA vs. constant 10 uA. Is there a big difference between these in lumens?

Yes because @10µA constant current it won't even lit I think. That's why the LM3909 exists.
The LM3909 came out a long time ago, before efficient LEDs existed. It's no longer made but wouldn't surprise me if you can still get clones and new old stock on ebay.

It's true that bright flashes will be more easily noticed, than a constant dim glow, so in that respect, pulsing can be useful. However, as the frequency increases to the point where the pulses merge together to form a dim glow, due to the persistence of vision, it's better to use use a tiny constant current, rather than short pulses.
 

Offline GeorgeOfTheJungle

  • Super Contributor
  • ***
  • !
  • Posts: 2699
  • Country: tr
Re: LED visibility optimization at very low currents
« Reply #11 on: April 20, 2017, 10:40:55 pm »
Existed. Long obsolete.

Oh dear, oh dear  :palm:
The further a society drifts from truth, the more it will hate those who speak it.
 

Offline james_s

  • Super Contributor
  • ***
  • Posts: 21611
  • Country: us
Re: LED visibility optimization at very low currents
« Reply #12 on: April 20, 2017, 10:48:12 pm »
The LM3909 used an extremely low duty cycle in order to reduce power consumption and make the individual flashes relatively bright. Using PWM to get something resembling constant illumination is unlikely to have much benefit, you also have to factor in the power consumption of the PWM circuitry.

It would be easy enough to do some experiments, get a bunch of LEDs, a multimeter and some pots and test them out at different currents in a dark room.
 

Offline mikeselectricstuff

  • Super Contributor
  • ***
  • Posts: 13695
  • Country: gb
    • Mike's Electric Stuff
Re: LED visibility optimization at very low currents
« Reply #13 on: April 20, 2017, 10:50:03 pm »

It's true that bright flashes will be more easily noticed, than a constant dim glow, so in that respect, pulsing can be useful. However, as the frequency increases to the point where the pulses merge together to form a dim glow, due to the persistence of vision, it's better to use use a tiny constant current, rather than short pulses.
Better how ?
Unless the LED datasheet specifies output at low currents, you have no guarantee of brightness.
White LEDs in particular can show huge brightness variations below a few hundred uA.
Some OSRAM lighting LED datasheets actually state that they should not be run below 10% of their rating for this reason.
Youtube channel:Taking wierd stuff apart. Very apart.
Mike's Electric Stuff: High voltage, vintage electronics etc.
Day Job: Mostly LEDs
 

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 19345
  • Country: gb
  • 0999
Re: LED visibility optimization at very low currents
« Reply #14 on: April 20, 2017, 10:58:17 pm »

It's true that bright flashes will be more easily noticed, than a constant dim glow, so in that respect, pulsing can be useful. However, as the frequency increases to the point where the pulses merge together to form a dim glow, due to the persistence of vision, it's better to use use a tiny constant current, rather than short pulses.
Better how ?
Unless the LED datasheet specifies output at low currents, you have no guarantee of brightness.
White LEDs in particular can show huge brightness variations below a few hundred uA.
Some OSRAM lighting LED datasheets actually state that they should not be run below 10% of their rating for this reason.
That's because the response of the phosphor in a white LED is often non-linear. It isn't just the brightness which can change but the spectral output and colour, hence manufatures are hesitant to specify their products beyond what they've themselves tested them to. In this case the user is not interested in white LEDs, so that's irrelevant. Whether or not the datasheet specifies it or not, luminous efficiency of the LED junction itself improves at lower currents.

The LM3909 used an extremely low duty cycle in order to reduce power consumption and make the individual flashes relatively bright. Using PWM to get something resembling constant illumination is unlikely to have much benefit, you also have to factor in the power consumption of the PWM circuitry.
Yes this is true.

For short pulses, the eye will not be able to determine the length of the flash. A 20ms pulse will appear to be twice the brightness of a 10ms pulse, with everything else (intensity wavelength etc.) being equal. Thus, if it's flashes you need, you're better off using longer, lower current pulses, than short high current.
 

Offline CatalinaWOW

  • Super Contributor
  • ***
  • Posts: 5172
  • Country: us
Re: LED visibility optimization at very low currents
« Reply #15 on: April 20, 2017, 11:13:53 pm »
Since your color choices are fixed and there is really no magic operating point for LEDs (other than not at the high end of their current capability) your only real flexibility is to combine persistence of vision/attention and human optical sensitivity with required data rate to reduce duty cycle.

First, using an efficient LED find the lowest current/time pulse which can be detected by your operators in their environment.  The eye is amazingly sensitive and if you can put your operators under a light hood long enough to dark adapt, and they have no other instruments or displays providing light you will be amazed how low it can go.  Even moonlight and a complex scene can make a huge difference in this detection level.  The message here is that your environment is critical to the question.  You need to define the worst case from your design standpoint.

Can your information be conveyed with one flash every second? Every 10 seconds?   50 seconds?  If the information rate is low enough you might be able to get average current down where you desire.  Persistence of vision is measured in tens of milliseconds so I am sure your average power desired cannot be achieved based on a duty cycle that makes the light appear constantly on.

You will probably have to do some human factors testing in an area that I haven't seen much on.  There will be interactions with things like human attention span, so something like three flashes at 0.5 second intervals with 10 second rest time might end up being your best operating point (those are made up numbers to illustrate the concept).  Things like how well the operator has to localize the light will affect all of this.  I am sure that the operators will not like the settings with the lowest usable powers.  You will have a trade between operator happiness and power consumption.
 

Offline jmajaTopic starter

  • Frequent Contributor
  • **
  • Posts: 296
  • Country: fi
Re: LED visibility optimization at very low currents
« Reply #16 on: April 20, 2017, 11:34:51 pm »
I did some tests with a very old (90's?) red LED I happened to have by the bench. It is very clearly visible with 100 uA average current using 7.5 mA PWM. With 300 uA constant current it was much dimmer, maybe equal to 25 uA average PWM. Even at 5 uA PWM I could see the LED from 5 m, but not that clearly.

I have to test with modern LEDs, but I was hoping to get some help selecting a good candidate. There are so many of them and I don't quite know what to look for in the specs.

I'm hoping the new LEDs are much more efficient and thus may be as visible with 10 uA. Maybe even with constant current. I trying to find out is PWM an option or will the cortex-M0 take too much power while powering PWM during standby.

The state of the LEDs should reflect the sensor output with around 1 s maximum delay. So flashing the LED needs to be around 1 Hz or faster. The processor will likely wake up 2-4 Hz so 25 or 50% slow PWM would be OK by just bit banging at wake up and no need to keep the fast PWM working.

The user will be in the dark for a long time, so adapted to darkness. The LEDs need to be visible whenever light level is too low for seeing the sensor. So also in moonlight and in the evening. There are some displays and distant lights, but they are not bright in the dark.
 

Offline mikeselectricstuff

  • Super Contributor
  • ***
  • Posts: 13695
  • Country: gb
    • Mike's Electric Stuff
Re: LED visibility optimization at very low currents
« Reply #17 on: April 20, 2017, 11:48:38 pm »
I have to test with modern LEDs, but I was hoping to get some help selecting a good candidate. There are so many of them and I don't quite know what to look for in the specs.
High brightness and/or specced for low current ( few mA). but things like angle will make the biggest difference. Clear package rather than diffused.


Youtube channel:Taking wierd stuff apart. Very apart.
Mike's Electric Stuff: High voltage, vintage electronics etc.
Day Job: Mostly LEDs
 

Offline mikeselectricstuff

  • Super Contributor
  • ***
  • Posts: 13695
  • Country: gb
    • Mike's Electric Stuff
Re: LED visibility optimization at very low currents
« Reply #18 on: April 20, 2017, 11:54:34 pm »
The state of the LEDs should reflect the sensor output with around 1 s maximum delay. So flashing the LED needs to be around 1 Hz or faster. The processor will likely wake up 2-4 Hz so 25 or 50% slow PWM would be OK by just bit banging at wake up and no need to keep the fast PWM working.
That would probably work fine. Assuming a 4Hz wake-up you may want a much shorter pulse than 250mS, but you could do this using a simple capacitor charge-pump arrangement - maybe arrange it so you get a bright flash for maybe 50mS, which then tails off to a lower constant-ish current, That way the flash attracts attention, but the tail-off preserves visibility for a quick glance.
Youtube channel:Taking wierd stuff apart. Very apart.
Mike's Electric Stuff: High voltage, vintage electronics etc.
Day Job: Mostly LEDs
 

Offline LaserSteve

  • Super Contributor
  • ***
  • Posts: 1270
  • Country: us
Re: LED visibility optimization at very low currents
« Reply #19 on: April 21, 2017, 12:31:22 am »
Green and yellow for the photopic and scotopic / mesoptic
Peaks. Blink with short flashes at 0.5 to 3 Hertz for maximum attention gathering factor. Using 100 uA
From a current source at low duty cycle and flashing will be more noticble then 10 uA CW. You have other options such as double flashes and flash order to convey other meanings. Ie  RG, GG, RR,. Angle does matter. Look at the flash patterns on light aircraft for examples, as they are designed for maximum recognition against a city or dark background.

S.

« Last Edit: April 21, 2017, 12:38:05 am by LaserSteve »
"What the devil kind of Engineer are thou, that canst not slay a hedgehog with your naked arse?"
 

Offline GeorgeOfTheJungle

  • Super Contributor
  • ***
  • !
  • Posts: 2699
  • Country: tr
Re: LED visibility optimization at very low currents
« Reply #20 on: April 21, 2017, 12:40:28 am »
I did some tests with a very old (90's?) red LED [..] It is very clearly visible with 100 uA average current using 7.5 mA PWM [..]

Then you're done if there's a LED with 10x better lumen-per-watt efficacy than the old one you're using.

https://en.wikipedia.org/wiki/Light-emitting_diode#Efficiency_and_operational_parameters
The further a society drifts from truth, the more it will hate those who speak it.
 

Offline Cerebus

  • Super Contributor
  • ***
  • Posts: 10576
  • Country: gb
Re: LED visibility optimization at very low currents
« Reply #21 on: April 21, 2017, 03:23:06 am »
Back at Christmas I wired two very ordinary 3mm red LEDs (random picks from the parts bin) to two 3V lithium coin cells. One with a 330K dropper resistor and one with a 1M dropper resistor. The purpose of all this was to add a marker light to where the door handle is, as we'd hung a blackout blind on the window and now at night we get proper 'country' darkness rather than the perpetual demi-light us city dwellers have become accustomed to. Toes were stubbed. I wasn't sure what would be enough current so I had two tries and left them to run side by side.

Both have been sitting next to my bedroom door running continuously since. In night-time darkness with the blind down both are clearly visible from 3 metres away and 40 degrees off axis. I'm not saying that's their limits of visibility, that's just where I can clearly see them from in bed. So that's one at around 5 uA, and one at around 2 uA; visually there's a difference in brightness but it's not instantly noticeable, you have to consciously look to see which is the brighter.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

Offline Dijital

  • Contributor
  • Posts: 18
  • Country: us
Re: LED visibility optimization at very low currents
« Reply #22 on: April 21, 2017, 05:00:09 am »
I did an actual empirical test for ya

Using a Cree XQE (XQEROY-00-0000-000000L01) royal blue (465nm) I was able to see it in the dark from ~5m at 10uA directly driven (around 2.45V). It is very dim, but it stands out enough against the dark. However I feel that even in partial moonlight it may be too hard to see. I would shoot for green as it has the highest luminous flux of the bunch.
 

Offline mjs

  • Regular Contributor
  • *
  • Posts: 117
  • Country: fi
Re: LED visibility optimization at very low currents
« Reply #23 on: April 21, 2017, 07:30:54 am »
If you can live with shorted flashes, use those with 2-4Hz repetition rate on a modern green LED. Note that modern bright InGaN LED have Vf higher (2.8-3.8V) than the older GaP ones.

I did a daylight visible (in shadow, not direct sunlisght) blinker with 2N6027 running with 9V battery at about 5uA, about 0.5Hz rate. Forgot that completely and found it still flashing after 9 years!
 

Online Someone

  • Super Contributor
  • ***
  • Posts: 4510
  • Country: au
    • send complaints here
Re: LED visibility optimization at very low currents
« Reply #24 on: April 21, 2017, 11:48:41 am »

It's true that bright flashes will be more easily noticed, than a constant dim glow, so in that respect, pulsing can be useful. However, as the frequency increases to the point where the pulses merge together to form a dim glow, due to the persistence of vision, it's better to use use a tiny constant current, rather than short pulses.
Better how ?
Unless the LED datasheet specifies output at low currents, you have no guarantee of brightness.
White LEDs in particular can show huge brightness variations below a few hundred uA.
Some OSRAM lighting LED datasheets actually state that they should not be run below 10% of their rating for this reason.
That's because the response of the phosphor in a white LED is often non-linear. It isn't just the brightness which can change but the spectral output and colour, hence manufatures are hesitant to specify their products beyond what they've themselves tested them to. In this case the user is not interested in white LEDs, so that's irrelevant. Whether or not the datasheet specifies it or not, luminous efficiency of the LED junction itself improves at lower currents.
I'm siding with mike on this, LEDs (even within the same batch) can vary wildly at low drive currents. And the luminous efficacy can drop as the current decreases..
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf