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  1. #1
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    So which is it? 228 lumens or 242 lumens?

    While doing some calculations for a new bike light, I ran into a problem. One set of calculations showed a 3 Q5 CREE light with 726 lumens while another showed 684 lumens.

    Here is the calculations for each:
    228 lumens at 1A * 3 LED's = 684 lumens

    110 lumens * 2.2 (from Relative Intensity vs. Current Graph) * 3 LED's = 726 lumens
    110 lumens is the average for 107 min and 114 max.

    So we have roughly a 42 lumen discrepancy. So which is the correct output?

  2. #2
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    Using a light meter measuring a 3 X cree XRE-Q5 light build with a spot lense, I get only 440 lumens from about a metre away.

  3. #3
    fuggansonofahowa
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    It's all "paper" numbers anyway....kinda like computer networking speeds...100mbit will never be measured as such. Just call it 700 and smile when you temporarily blind your riding buddies.

  4. #4
    GMF
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    A couple things:

    1) you aren't including the heat effects, which could reduce your light output by as much as 30%.
    2) you aren't including optics losses which are another 10%-15%
    3) i'd bet good money that you wouldn't tell the difference between 42 lumens at that light level and aren't worth fretting about. A couple hundred, on the other hand, is more worth noting.

    Also, rottweiler, i'd be surprised if your light meter was actually measuring lumens. Light meters typically measure a spot intensity (lux), and lumens are a measure of total light output. You likely measured 440 lux, which you can not extrapolate into lumens.

  5. #5
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    These are what I consider "raw" Lumen counts. Does my Wilma produce 830 Lumes outputwise with its 4 LEDs? Maybe not becaus ethat may be a "raw" number (kinda like the HP of a motorcycle at the crank versus the wheel) but it's very bright and I love the way it works. I'm not sure whey it would be a "problem" if each LED produced a couple dozen more or less Lumens in theory.

  6. #6
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    Ok guys, we are talking about pure theoretical numbers here but still there is a discrepancy in CREE's own documentation. Heat effect, optics, converter loss, etc... doesn't mean anything at this point.

    IMO, this is a very important calculation (this is not a measurement) to compare several different builds I am considering. 2 CREE @1A or 3 CREE @ 800mA? I certainly don't want to build one configuration, measure the light output, then build the other configuration, measure the light output, and then decide which one I want to use.

    BTW, I have decided to average the two numbers instead.

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