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Are Lumens Additive?

  1. May 21, 2012 #1
    I've looked for this answer around the web, and I've gotten mixed results, so I'm asking here. If I have 2 light bulbs that produce 1k lumens each, will it be the same as 1 light bulb producing 2k lumens?

    And on the topic, if I have a light source and I put a lens between it and a wall, what measurement will tell me how bright the image will be when it hits the wall. Thanks!
  2. jcsd
  3. May 22, 2012 #2


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    I believe that they do add with each other.
  4. May 22, 2012 #3


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    mattloto, get yourself educated in the units of visible radiation. Actually, there are a lot of them, some derived, like the lumen. Once you have the definitions down correctly you can always communicate with others accurately. You might start here:
    http://en.wikipedia.org/wiki/Lumen_(unit [Broken])
    Last edited by a moderator: May 6, 2017
  5. May 23, 2012 #4

    Andy Resnick

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    Lumens are a photometric version of radiant flux, the radiometric units are Watts. Because flux (power) is an integrated quantity, it does not have any directional dependence. So yes, the lumens will add.

    However, because 'lumens' takes into account the spectral sensitivity of vision, you have to be careful when converting watts to lumens- two identical wattage bulbs, if the spectral output is different, will have different lumens.
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