# Thread: So which is it? 228 lumens or 242 lumens?

1. ## 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. 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. 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. 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. 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. 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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