lumen vs current

Discussion in 'General Electronics Chat' started by bug13, Sep 21, 2012.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi guy,

    I was wondering if a led's lumen is portational to the average current supplied? Provided the voltage is constant, and the voltage and current supplied is within the rated limit.

    Thanks
     
  2. Dyslexicbloke

    Active Member

    Sep 4, 2010
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    Not relay ... at least its not a linear relationship.
    The voltage is a product of the junction properties and the current it also varies slightly with temperature.

    The output is controlled by the current which must be set independently of the forward voltage.

    Output rises with current but the closer you get to the maximum rating the less effect a change in current has.

    It is actually quite a complex subject particularly when consider that many LED's are controlled with PWM signals.
    That gets you into average current as opposed to constant current and worse still you start thinking about perceived brightness as opposed to a constant specific lumen output.
     
    Last edited: Sep 22, 2012
  3. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Thanks Dyslexicbloke

     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    No. It's highly non-linear and you get progressively less light increase as you increase current.
     
  5. bertus

    Administrator

    Apr 5, 2008
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  6. bwack

    Active Member

    Nov 15, 2011
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    Here's a graph of lumens (in percent) versus current for a XM-L Cree. It is not linear, but that unlinear depending on who's looking at it.

    [​IMG]

    Do also note that the junction temp is held at a constant 25 degree Celcius in the lab, but you do get to see what kind of shape this curve gets independent of temperature change. Highly unlinear is too harsh to say I think.
     
  7. chrissyp

    Active Member

    Aug 25, 2008
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    note that the lumen output on instruments is not the same as you will see.Depending on the wavelength of the led's you are observing .For example a wavelength near 400nm (ultra violet) will be more uncomfortable to observe with a very small increase in intensity,but at 700nm (near infra-red) there will need to be a large increase in intensity before it is noticed . Green is good as it is straight down the middle of you visual spectrum.
     
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  8. Wendy

    Moderator

    Mar 24, 2008
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    This is why PWM with LEDs is so popular, that is very linear.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Thanks! I would say that is quite linear, apart from sone % drop in efficiency as current goes up.

    The really non-linear thing with LEDs is people's eyes! ;)
     
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Read this. The images are from a dig camera, not human eye. As you push higher current, you get less and less brightness increase for your investment, hence it is non linear.

    http://led.linear1.org/how-is-led-brightness-related-to-current/

     
    Last edited: Sep 22, 2012
  11. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Thanks guys, that's a lot of useful information for me, even I asked the wrong question.

    The mistake I made was, I assumed that the lumen = perceived brightness, now I found out I am wrong.

    what I really want to know is, the relationship between the perceived brightness and the average current (PWM).

    by the replies in this post, I assume it is something like the human's ears to sound? something like a function of log?? but, I may make a wrong assumption again.

    PS1: I learned a lot from this post, thanks bertus
    LED Brightness VS Power/Current

    PS2: thanks bountyhunter, your link has some very useful information, that's what I am looking for
    http://led.linear1.org/how-is-led-br...ed-to-current/

    PS3: to the link bountyhunter provided, I have done some similar test but with PWM, result are similar, but Bill_Marsden pointed out, PWM is very linear(I have to say, I only "test" the brightness with my and another students eyes), is anyone else done some PWM testing already?
     
    Last edited: Sep 22, 2012
  12. Audioguru

    New Member

    Dec 20, 2007
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    Of course it is.
    Your hearing's and your vision's response to loudness and brightness is logarithmic if you ignor vision closing the iris to reduce very bright light.

    You can hear a whisper far away and hear an extremely loud jet airplane up close.
    You can see in dim moonlight and see in bright sunlight.

    2 speakers playing are only a little louder than 1 even though the power is doubled.
    2 lights create only a little more brightness than 1 even though the intensity is doubled.

    The response is logarithmic so they can deal with the huge range.
     
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  13. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I can tell you that when I buy white LED flashlights and take them apart to soup them up, I have to crank about 6X current through the LED to make the light look about twice as bright. That pretty much follows what the log scale would predict.

    YMMV
     
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  14. Audioguru

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    Dec 20, 2007
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    PWM on an LED causes dimming. if the average current is half then the dimming is barely noticeable. If the average current is 1/9th or 1/10th then the perceived brightness is half.

    Look at how much light is produced by an LED when its average current is almost nothing.
     
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  15. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Does it mean I can save 50% of energy but only loss little brightness percive by humans eyes? If I do a 50%PWM?
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    Yes. Try it to see.
     
  17. Wendy

    Moderator

    Mar 24, 2008
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    PWM is not the same as varying current, it is totally different. PWM is the ration of on time to off time, when the LED is on it should be at full brightness.

    The biggest effect from this is you can go from no light to full light. You can't do this with a variable resistor. It also creates the perception (basically from the human side) that you have complete variability. In any system, be it optics or audio, humans are likely to be the least linear component.
     
  18. chrissyp

    Active Member

    Aug 25, 2008
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    The brightness of leds vs current is linear.cameras will see it that way,however the human eye will not.The perceivable light level is dependant on the application,ie are they facing you like a visual display or are they facing away like a flashlight, is it used in daylight or at night.
    Beware when using PWM drives with camera lighting that you do not get frequency clash as the camera may pick up the flashing leds.
     
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