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The Millicandela Rating System - How does it work?

Discussion in 'General Electronics Chat' started by Cyclone, May 15, 2004.

  1. Cyclone

    Cyclone New Member

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    Hi

    I am trying to buy some really bright leds for a project I’m working on. I have been to various sites that sell leds. Digi-key was the first place I checked. They have these leds that have a high mcd rating. For example this white led was rated at 11000mcd(441-1009-ND). I bought some of these leds and when I powered them up they weren't bright at all. I have some white 2300mcd leds that seem a lot brighter. Digi-key sells other colours of leds produced by the same manufacture with extremely high mcd ratings as well. I also got some of these leds and they didn't seem that bright despite the high mcd ratings.

    So my question is how is the brightness of leds measured and how does the View Angle of the led affect the brightness? I have seen a 11,600mcd green led with a view angle of 15 and a 7200mcd green led with a view angle of 30. Which one would look brighter?
     
  2. jbeng

    jbeng Member

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    Merriam-Webster defines the candela as the base unit of luminous intensity in the International System Of Units. (See the rest of the definition here: http://www.m-w.com/cgi-bin/dictionary?book=Dictionary&va=candela&x=16&y=15 ). A millicandela is simply 1/1000th of a candela.

    Regarding your other question, I expect the 11,600mcd w/15 degree viewing angle would be brighter. The mcd rating is higher than the second one and the light energy is more concentrated into a 15 degree "beam".
     
  3. grrr_arrghh

    grrr_arrghh New Member

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    just a thought, maybe you didn't supply the LED with enough voltage/current?
     
  4. dave

    Dave New Member

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  5. Cyclone

    Cyclone New Member

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    Thanks for the info. :)


    Hmm so the lower the view angle of the led is the more the light is concentrated onto a smaller surface area. So lets say I have a 10,000mcd led with a view angle of 8degrees. The light it emits would be really intense if you looked directly at the led and the led would cast a small area of light onto a surface from a greater distance then if you had a higher view angle value. Now lets say I have a 10,000mcd led that had a view angle of 30degrees instead of 8. The light the led emits would be more scattered and wouldn’t be so intense if you looked directly at the led. The led would scatter the light more and onto a larger surface area but wouldn't cast a small area of light like an 8degree led would. The 30degree led also wouldn't shine light as far either as the light is more diffused.


    Am I making any sense?
     
  6. gerty

    gerty Member

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    Yep..
     
  7. Oznog

    Oznog Active Member

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    Nope. At least I don't think so.

    The definition of candela there was an amount of light per unit of angle. Thus a 10,000 mcd over 7 deg would look as bright as a 10,000 mcd light over 14 deg, IF you're looking straight at it from the front.

    Candela isn't the total amount of light put out- only the amount put out in one direction. Thus focusing comes into play.

    On the other hand, the one which emits 10,000 mcd over 14 deg will definitely put out much more light power overall. I don't know how to do this with polar coordinates, but it's over twice as much, something like 4x I think.

    So in answer to your question:
    "I have seen a 11,600mcd green led with a view angle of 15 and a 7200mcd green led with a view angle of 30. Which one would look brighter?"

    That depends on the testing conditions. In terms of total luminous power, I do not have the formula to compare them. The 11,600 LED will make a 1 ft spot 10+ ft in front of you appear around 50% brighter than the other, so it generally makes a better flashlight, hands down.

    Someone correct me if I'm wrong...
     
  8. gerty

    gerty Member

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    I believe the last question was which angle (wide/narrow) would have a spot more visible at a distance, not on the surface of the led.
     
  9. Oznog

    Oznog Active Member

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    If it's only a small spot the LED is pointing straight at, say it captures 1 deg of the LED's light, then a 10k mcd will do the same thing regardless of whether it's 7 deg or 14 deg. The mcd rating is luminuous flux per unit of angle, not total luminous flux.
     
  10. Cyclone

    Cyclone New Member

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    Oznog, so are your saying the view angle of an led doesn't really affect its brightness at all? hmm thats confusing.

    So if I had a choice between two leds that had the same mcd rating I would always choose the one with the higher view angle?
     
  11. Oznog

    Oznog Active Member

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    Brightness has different meanings. Some people are concerned with total power regardless of angle, others making directional flashlights only care how bright the center of the beam is. The mcd is how bright the center of the beam is. Actually, they average what each mcd is for the brightest parts of the center of the beam, and then give that and an angle that the mcd rating was taken over. So you could say it's basically 5k mcd over 7 deg. Put a tiny calibrated photosensor anywhere within the 7 deg range, adjust the reading for however far you've place it from the emitter, and you'll find it's 5k mcd +/- 15% or so anywhere within that 7 deg area.

    mcd is flux per unit of angle. If you have the same mcd rating, the one with the higher angle produces more overall power.
     
  12. laroche73

    laroche73 New Member

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    luminance

    A cross question for this thread and another on LED multiplexing. I've read that the human eye perceives a multiplexed LED at reduced duty cycle and increased current (say 4 times the current @ 25% duty cycle) to be brighter than a non-multiplexed LED driven with the same average current. Why is this so? - Claude
     
  13. chrishilton@onetel.com

    chrishilton@onetel.com New Member

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    Basically, another question!!

    If I am looking for max luminous power for money then shouldn't I forget the complexities of luminousity ratings and simply calculate milliwatts per pence or suchlike.
    I've noticed that eBay sellers have placed all their emphasis on ambiguous mcd ratings which vary from 6000mcd for a 25mA forward current 3mm white LED to 110,000mcd for a 25mA forward current 10mm LED.
    SURELY, these LED's emit the same amount of light!!
    Or is there some vast difference between the efficiency of different LED's.
    I suppose that if the 10mm led has a nice big 'lens' then perhaps the light focuses through a point. So if I was trying to get a nice big mcd rating wouldn't I measure intensity at the led focal point [if there is one].
    PS. If anyone is looking for an excellent low power, high efficiency, directional and genuinely white light source - then luxeon 3watt make fantastic spotlights.
    PLEASE can someone answer my question about power rating versus intensity i.e. IS IT THAT SIMPLE?
    Regards Chris.
     
  14. TekNoir

    TekNoir New Member

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    The quote is from http://www.superbrightleds.com/led_info.htm

    Maybe it will help...

    (PS: The site linked to in the quote - http://led.linear1.org/ - has a huge amount of general information on using LEDs for those interested.)
     
    Last edited: Aug 8, 2007
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I use a 30 degree angled LED in my LED flashlight so I can see where I am walking. It is very bright and lights my path.

    I have seen cheap LEDs with a 7 degree angle and they shine only a tiny dot of light on the ground, not wide enough to see anything. Their focussing makes their cheap old technology LED seem as bright as a newer wider angled LED.
     

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