Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Relationship between Cadella and Wattage?

  1. Jun 16, 2009 #1
    Is there a general relationship that can be used for LED lights tying togethor Cadella output and Wattage required?
  2. jcsd
  3. Jun 16, 2009 #2


    User Avatar
    Science Advisor

    Yes, you see it in the maker's specifications for the lamp.

    They sometimes give a figure of X candelas per watt.

    The more candelas per watt, the more efficient the lamp is.

    A candela is roughly equal to the old candle power unit.
  4. Jun 16, 2009 #3


    User Avatar
    Science Advisor

    More commonly, LEDs and other lighting products, are rated in units of luminous efficiency in terms of lumens per watt, a more useful metric for most lighting applications because while candelas tell you the total amount of light output, lumens tell you how much light what you're illuminating will get--how bright it will appear. You can derive that from candelas, too, of course but you have to use a little math and know the radiating angle of your light source.
  5. Jun 16, 2009 #4


    User Avatar
    Science Advisor

    I've seen both, but lumens per watt is more common, for sure.
    The candela is probably a better unit because it doesn't depend on the angle of light exit from the device.
    With highly directional devices like LEDs it is getting harder to compare them in terms of lumens.

    There is a good comparison chart on Wikipedia for Luminous Efficiency:

    For comparison, here is a bit about the relationship between Lumens and candelas:

    a light source that uniformly radiates one candela in all directions radiates a total of 4π lumens. If the source were partially covered by an ideal absorbing hemisphere, that system would radiate half as much luminous flux—only 2π lumens. The luminous intensity would still be one candela in those directions that are not obscured.
  6. Jun 17, 2009 #5
    Well, thanks for the info.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook