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Help ! Converting Lux to Watt

  1. Mar 1, 2012 #1
    I need a little help. Here is my problem,
    I have a light source and a monocromator that seperates the white light to to its components of different wavelenght. I want to measure the power of the output light which has a single wavelenght. I am using a Lux meter to measure the illumation of the light but I dont know how to convert it into watt.

    Could you please help me about this? ı am reading stuff about lux, watt and lumen but im totally confused.
  2. jcsd
  3. Mar 1, 2012 #2


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    Lux is lumens per square meter, where lumens in turn is a weighted function based on the photopic response of the human eye. To convert from lumens to watts (or watts to lumens) you need to use the photopic efficacy relation, which can be used to define lumens/watt as a function of wavelength.

    I have an SPIE field guide which has a small table with discrete values for the photopic efficacy. For example, at a wavelength of 560 nm the photopic efficacy is 680 lm/w.

  4. Mar 1, 2012 #3
    Thanks for your help. :)
  5. Mar 4, 2012 #4
    I have finally found a way of converting lux into watt. Just for being sure can you tell me whether its tru or not?
    1- First i divide the the value of light illumation which i measured in Lux by 6830.
    2- Then I devide the result i found by the eye sensivity coefficient which is different for each wavelenght.
    The result gives me mW/per sequare cm. Do you think there is a wrong operation?
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